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Don’t Let Your Attic Suck - Power Attic Ventilators Are a Bad Idea


power attic ventilator building science infiltration air leakageTrade shows can be both educational and infuriating, and there’s one product I see at a lot of trade shows that fits easily into the latter category - power attic ventilators. The variety of this device that’s been popular lately is the solar-powered attic ventilator. Using the Sun to make the electricity for this fan, however, makes it only marginally better than its grid-powered cousin.

Oh, yes, power attic ventilators will probably keep your attic cooler, and that means you’ll have less conductive heat transfer across your ceiling. The problem is that a significant portion of the cooling in your attic will be provided by your air conditioner. So, you spend money to buy the fan, to run the fan if it’s not solar, and then your air conditioning bill goes up, too.

How can that be, you ask? Isn’t it supposed to pull that blazing hot air from the attic and send it outside, replacing it with much cooler outdoor air that gets pulled in through the soffit and gable vents? In marketing theory, yes. Building science shows a different result, however.

What really happens is that when that power attic ventilator runs, it’s going to pull air from wherever it can find it. Since air takes the path of least resistance, some of it will most likely be coming from the conditioned space in your home. So basically what you’re doing is air conditioning your attic. The longer the fan runs, the more conditioned air it pulls into the attic.

If you have a perfectly air-sealed ceiling, you’re not going to have this problem, of course. The reality, however, is that few ceilings are leak-free. Since air needs only a pressure difference and a pathway to move, and your ceiling probably has plenty of pathways, it’s best not to enhance any pressure differences that will increase air movement into or out of your home.

In other words, don’t install that power attic ventilator. If you have some installed already, disable them so they never run.

Power attic ventilators can cause problems even without air conditioning. One potential problem would be sucking moist, moldy air up from the crawl space intoPower attic ventilators waste money, make your home less comfortable, and can be a health and safety problem by pulling conditioned air from your home. Solar-powered attic ventilators are only marginally better. the house. Another would be backdrafting a water heater and putting carbon monoxide in the house. These are real problems from real houses that have really happened.

If you’re tempted to buy one because it’s solar-powered and won’t increase your electricity bill, go back and read what I just said. These things probably won’t save you any money. Even if they’re solar, they’ll still suck the conditioned air out of your house and make your bill higher, not lower.

Peter Yost over at Green Building Advisor wrote about solar-powered attic ventilators a couple of years ago and laid it out perfectly. He also gave a great quote by Dr. Joe Lstiburek:

“In order for the fan to work the air needs to come from the outside and not be pulled from the house so this means that the attic ceiling needs to be airtight. If the attic ceiling is airtight you don’t need the fan. Your money is better spent on something else.”

If you want to get into the nitty-gritty research data about attics, you can download this paper (pdf) from the Florida Solar Energy Center reviewing the research not only about attic ventilation but also about sealed attics with insulation at the roofline instead of at the flat ceiling.

The bottom line is that in most cases, power attic ventilators are a waste of money. In some cases, they can be dangerous because of backdrafting.


Related Articles

The #1 Reason Power Attic Ventilators Don't Help

Power Attic Ventilators Banned by New Georgia Energy Code

Oooh, Shiny Stuff! - Radiant Barrier Fundamentals


NOTE: Comments are moderated. Your comment will not appear below until approved.


Good article describing the best conventional wisdom on these fans. I like the quote "Air is stupid, it goes wherever it wants". 
Riddle me this: These fans are sold with the idea they pull "bad" air out of the attic and it gets replaced with ambient outdoor air. They talk piously about having adequate soffit vents so that's where the air comes from. All the problems are dependent on the idea of depressurizing the attic. If these fans were installed in pairs and one were reversed to force ambient air *into* the attic, would that not eliminate the #1 objection from knowledgeable building scientists?
Posted @ Wednesday, June 08, 2011 6:15 AM by M. Johnson
Mark: Yes, using fans to put positive pressure in the house would eliminate pulling conditioned air into the attic, but if the ceiling has leaks, you've now turned the problem from exfiltration to infiltration and are pushing hot attic air into the house. There's also still the problem of using all that power for something that's not necessary. As Joe said, if the ceiling is perfectly air-sealed, you don't need the fan. If it isn't, it causes problems.
Posted @ Wednesday, June 08, 2011 6:21 AM by Allison Bailes
My riddle was if they were installed in *pairs*, which would be pressure *neutral*. The resulting temperature difference would reduce heat gain through ductwork and the ceiling plane. This leaves all the pro arguments, are there any cons besides the cost of buying in pairs? 
Joe probably has thought thru all aspects of his argument, however he did not explain them here. When he says all the advantages of attic ventilation relate to holes in the ceiling plane, that is not obvious to me. 
I did not really regard this idea as extremely out of the box, it just seems the simple and obvious way to overcome the #1 objection to these fans.
Posted @ Wednesday, June 08, 2011 6:35 AM by M. Johnson
Mark: Sorry I missed your point about trying to keep the pressure neutral in the attic. That would indeed solve the problem of increasing air leakage and cooling the attic with conditioned air from the house. But the cost would not be warranted.  
First of all, the heat is getting into the attic by radiating downward from the roof deck, so trying to solve that problem reducing heat convectively will be of limited effectiveness. Any exposed framing, ductwork, air handler, attic stairs, or other materials will absorb that radiation and transfer some of it downward. Fans will remove only the part of the heat that gets into the attic air. 
Second, if the ceiling is well-sealed and insulated, you don't need to worry about cooling the attic a few degrees. A good building envelope solves that problem. 
Third, high attic air temperatures don't kill shingles, so you don't need fans to prolong shingle life. UV radiation is what kills shingles. 
Fourth, just installing fans in pairs wouldn't guarantee neutral pressure. Nor would it guarantee that no air gets sucked up (or pushed into) the house. Do you really think the roofers who install the fans are going to get up there with manometers and ensure that the system works properly? And if it doesn't, how will they tune it? 
I just see more problems and expense than are worth the trouble with this, Mark.
Posted @ Wednesday, June 08, 2011 6:49 AM by Allison Bailes
I am a home energy auditor, and I DO go around with a manometer checking pressures. 
Everyone who has posted that air-sealing the ceiling of the upper floor / floor of the attic is the right answer is 100% correct, but that still leaves thermal conductivity taking place.  
Lowering the temp of the attic will help that problem. That's why the fan is there to begin with. 
If the attic floor is NOT air-sealed (98% of the cases), and there is an exhaust fan (70% of the cases), what is the simplest answer? 
The "pair of fans" is not a bad idea, but it has to be balanced.... In most cases, we are starting with one fan already in the attic roof. 
I can use my manometer to do a pressure differential reading between the attic and the upper floor, and then turn on the attic fan to see what difference the fan makes. 
Then (in an ideal world, of course), a "positive flow" fan (the other side of the "pair of fans") can be installed with a flow rate that can balance the depressurization that the fan which is already there causes. 
It can get installed in an already existing gable vent, so not a difficult job. 
Any opinions on this "fix"? 
Posted @ Wednesday, June 08, 2011 7:17 AM by Ira Eisenstein
Ira: In theory, that sounds like a good idea. I've already addressed the reasons why I wouldn't recommend it in my second answer to Mark above. Regarding conduction, a lot of that heat that's conducting downward is the result of radiation, and you can't eliminate that problem with fans (convection). 
Also, what happens when you've got the positive and negative pressure fans installed and balanced perfectly for neutral pressure and then one or more of them go out? It means finding out that this has happened and then trudging through the attic to find and fix the culprit(s). It just sounds like way too much solution for the problem at hand. 
To answer your question about the simplest method for dealing with an attic that already has power attic ventilators, just disable them. When I was working on houses, that's what I did. Get an electrician to disconnect the wires and 'safety them off' with wire nuts and electrical tape.
Posted @ Wednesday, June 08, 2011 10:50 AM by Allison Bailes
I agree with your comments about attic fans. I'm wondering your position on what are referred to as "whole house fans"? 
The idea of using outside cool air in the morning and evening (especially in a moderate climate) to reduce the need to use AC seems logical. 
I'm new to your blog, so you may have covered this topic in another post. But I'm curious as to your opinion on whole house fans.
Posted @ Wednesday, June 08, 2011 11:02 AM by Tim Anderson
Mark, as Allison said, two fans *might* avoid depressurizing the attic, but if one fan isn't cost effective, adding another makes even less sense. People who install or recommend attic fans are focused on a problem (cooling loads) without stopping to consider how much energy these fans consume vs. how much they might save. I put together a thumbnail analysis in a comment over at HVAC-talk...
Posted @ Wednesday, June 08, 2011 11:47 AM by David Butler
Tim A.: No, I haven't written on whole house fans yet, but since I'm on a roll with fans this week, I was thinking I need to hit that topic soon. I want to get back to energy security this week, so the whole house fan article might have to wait till next week. Subscribe to the blog by entering your email address at the top right of the page to get notices of all our new articles. 
David B.: Thanks for the link to your other post. That sums it up well. (For those who don't want to copy and paste, you can click here to go to that post.)
Posted @ Wednesday, June 08, 2011 1:55 PM by Allison Bailes
Tim: We don't have air conditioning, so we close our brick house up during the day and open all of the windows at night and turn the "whole house" fan on to pull the cool air in and through the house. It's not the most energy-efficient way to go, tho: it's difficult to control humidity, and it is, after all, a giant hole in the house. It does help us save on our energy bills in the summer, and if feels good not to run the a.c. But we need to keep working to make our house tighter. 
I would assume a whole house fan would be far less likely to backdraft. I only know enough to be dangerous, tho -- someone else should weigh in on this.
Posted @ Wednesday, June 08, 2011 1:59 PM by Melanie Grove
Let me try that again. That attempt to put in the link didn't work in my last comment.  
For those who don't want to copy and paste, you can click below to go to that post.  
David Butler's post on power attic ventilators
Posted @ Wednesday, June 08, 2011 2:03 PM by Allison Bailes
Tim, whole house fans can save a lot of energy when used in the appropriate situation. They work best in semi-arid and arid climates with large diurnal temperature swings. Otherwise, you need to keep an eye on the dew point. My rule of thumb is to operate a fan (or open windows in general) when outside dew point is lower than 55F (RH is useless for this purpose). If you allow the fan to add moisture to the home, it will lead to higher cooling bills when you later close up the house and turn on the AC. 
Unfortunately, most areas of the country don't have enough hours with favorable conditions to make a whole-house fan attractive. 
That being said, if you can leave your house open for most of the summer, then the added moisture becomes a non-issue (like the way houses worked before we had AC). 
BTW, the 'hole' in the envelope created by the fan must be thermally insulated during the winter. Tamarack makes a couple of models with R38 automatic folding doors.
Posted @ Wednesday, June 08, 2011 2:32 PM by David Butler
David: My husband just got BPI certified and we've been looking at something better than do-it-yourself insulation on our fan. We have a lot of work to do on our old, leaky house. Physician, heal thyself!  
Allison, I look forward to your take on the whole house fans, too.
Posted @ Wednesday, June 08, 2011 7:36 PM by Melanie Grove
Great info, as always! I do hope this will be a wake-up call to designers and builders to start designing homes that actually work properly from the get-go. 
We know so much about how houses use energy, but rarely put that knowledge into practice. 
Attic ventilators are an example of how we've come to rely on mechanical solutions to problems that could be prevented through good energy-efficient design.
Posted @ Thursday, June 09, 2011 5:55 AM by Richard Taylor, AIA
I started my company, About Saving Heat, in Denver CO. in 1975, 34,000 retrofits has taught me a lot in our relatively dry climate, with cool nights. Although i did grow up in Manhattan (I have to tell my Kansas clients it is the other Manhattan) When we add a roof vent to an older Denver home, without any existing roof vents, on a sunny day, (we have 300 of them) it is like a volcano, dust and debris shoots out, Most soffits are not tight, (we do add soffit vents too) power attic fans is a waste of energy. Hot air wants to rise (to the occasion!) Solar attic fans are a waste of PV (Passive Voltage). I do agree Tamarack or Air-Scape fans with R-38, motorized attic doors are great compliments to AC, in a well insulated and tight home. In dry climates, (Colorado) evaporative cooling works even better, putting the whole house and most attics (connected attics) under positive pressure, with cool filtered air at 10-20% of the power of AC. and actually contributes to global cooling! I bring in neighborhood 95 degree air, cool it to 70 degrees, by evaporating water it enters my basement, pressurizes my house, and goes out my 2nd story windows, (i can leave open all summer) at 75 degrees, after picking heat from my home. So neighborhood air at 95 in and 75 out = Global Cooling!
Posted @ Sunday, June 12, 2011 2:18 PM by Dennis Brachfeld
You really should do more research before you post blogs like this. I agree that there is not a need for an attic fan in a home that is built correctly. However, some of your statements of ill effects are founded on unrealistic situations and are just plain incorrect!
Posted @ Monday, June 13, 2011 11:55 AM by Steve Meyer
Allison, in your article you presented the argument against attic fans as A, B, and C. When you finally replied to my suggestion, you abandoned those arguments and jumped to X, Y, and Z. It's poor logic to move the goalposts in order to win a score. I am disappointed in you.
Posted @ Monday, June 13, 2011 12:05 PM by M. Johnson
Richard T.: Yep. Good design is definitely the place to start. Even in poorly designed homes, though, power attic ventilators rarely offer any real benefit. 
Dennis B.: Passive ventilation is the way to go, no doubt. 
Steve M.: I've actually looked into this quite a bit. Turns out that most people in the field of building science feel the same way about these devices. So, which statements in my article are unrealistic and 'just plain incorrect'? Inquiring minds want to know.
Posted @ Monday, June 13, 2011 12:08 PM by Allison Bailes
M. Johnson: I've reread the article and my responses to your comments above, and I don't see what your A, B, C, X, Y, and Z are, so it's a bit hard to respond to your latest comment. I don't see any inconsistencies or moving of the goalposts, as you say.  
Perhaps it's a bit confusing because I did add one more reason not to use them in the comments. Power attic ventilators are justified by the people who promote them for two main reasons: 
1. They keep the attic cooler, which makes it easier to cool the house because the temperature difference between attic and house is lower. 
2. They keep the roof cooler, thus increasing the life of the shingles. 
In the article, I talked mainly about #1, and I brought up the #2 reason briefly in the comments. Perhaps this is the root of your discontent? 
The truth is that power attic ventilators are a big waste of money for most houses. There may be some situations where they could help, but the consensus in the world of building science is that they don't improve home performance, and they waste energy.
Posted @ Monday, June 13, 2011 12:21 PM by Allison Bailes
A minor note in addition to all these reasons, in the winter, it would be best not to run a PAV at all, but they generally will run regardless as few are going to go to the effort to disable them seasonally.
Posted @ Monday, June 13, 2011 1:30 PM by Steve Byers
Allison, you respond once again that you don't understand what I say. I am not trying to speak in riddles, am (was) making an effort to intelligently discuss the reasoning behind this general wisdom. It is so very hard when the discussion partner has his mind made up and will not listen. I feel you are shunning a logical conversation, and that is what disappoints me most. 
"... I don't see what your A,B,C, and X,Y,Z are..." 
A -- "...The problem is that a significant portion of the cooling in your attic will be provided by your air conditioner... Since air takes the path of least resistance, some of it will most likely be coming from the conditioned space in your home." 
B -- "Power attic ventilators can cause problems even without air conditioning. One potential problem would be sucking moist, moldy air up from the crawl space into the house." 
C -- "Another would be backdrafting a water heater and putting carbon monoxide in the house." 
X -- "First of all, the heat is getting into the attic by radiating downward from the roof deck, so trying to solve that problem reducing heat convectively will be of limited effectiveness... Fans will remove only the part of the heat that gets into the attic 
Y -- "Second, if the ceiling is well-sealed and insulated, you don't need to worry about cooling the attic a few degrees. A good building envelope solves that problem." 
Z -- "Third, high attic air temperatures don't kill shingles, so you don't need fans to prolong shingle 
life. UV radiation is what kills shingles. 
Z2 -- "Fourth, just installing fans in pairs wouldn't guarantee neutral pressure." 
Allison I had hoped that the monologue would not be your preferred mode of discourse. 
Regards -- Mark J.
Posted @ Sunday, June 19, 2011 10:57 AM by M. Johnson
To spell out my concern further, arguments A,B, and C were all that was mentioned in the original article. It is not insignificant to hypothesize an idea which nullifies these arguments. 
Innovation comes from understanding lines of logic and seeing whether they are indeed flawless. It was formerly my hope that we could discuss these issues in some depth rather than just advocating a conclusion. 
Regards -- Mark J.
Posted @ Sunday, June 19, 2011 11:13 AM by M. Johnson
The thing about the attic fans I have seen is the 3,000 square feet of attic area per fan requirement from the manufacturer. Not cubic area, but square area. I framed houses for 25 years and we would put one, two, or three of these fans in an attic to really move the air. At least all the fans i installed are dead now. 
The direct relation to the RPM that the motor turns to the CFM the fan blade generates at that RPM is the unspoken argument.  
So in theory a roof fan installed in an attic with 3,000 square feet of attic space and either a 2" continuous soffit vent, or properly spaced 8"x16" around the eave would supply enough CFMs so the fan would not have to suck more air out of the living space. 
My question is how many times have you seen 3,000 sq.ft. in an attic and if it was that large there are generally more than one fan. 
It is the same problem with oversizing the HVAC. There is simply not enough cubic area to feed the CFM requirement for the blower. This result is a lot of infiltration of unconditioned air in the building envelope which is full of moisture in the SEC resulting in mold, mildew, bugs, etc. in the building envelope. 
To fix almost any HVAC problem in a building, reducing the RPMs to the blower reduce the CFMs it will move. This starts the road to using the HVAC system to CIRCULATE the air inside a structure and properly ventilate said building instead of the gasping for any infiltration available to feed the CFM needed by the blower/motor combination. 
Posted @ Tuesday, June 21, 2011 9:36 AM by Gene Wilhoit
I was looking into installing a fan in my attic because I have to do electrical and its outrageously hot up there. But common sense told me what this article is saying. Wouldnt a fan simply draw air from the house? 
While I am willing to pay to airconidtion my attic while I woprk in it I certainly have no need to AC my attic all the time. Since the attic is uninsulated at the top that certainly would make energy bills rise.  
My attic is killer hot like most. I climb up for 5 mintues and its like I took a shower I'm so wet. Not a good situation since I have a resperator as well. Last thing I want is to pass out up there. 
Posted @ Tuesday, July 05, 2011 10:02 AM by Tim Conway
As a home owner that has very little education/knowledge in HVAC/attic/air flow/etc I am in need of assistance! 
I live in Kelowna, BC, Canada where we have very mild winters and hot summers. 
My home is a walkout with many windows. I have many south east facing windows. 
The home is 3700sq feet with a partially finished basement, a Goodman 3ton AC. 
Temp outside = 72 
Temp inside home with AC running = 76 
I was told to add an attic fan as the house is just heating up from the sun.... this blog makes me think otherwise? I have had 3 HVAC guys out and they all aren't really sure what the issue is...
Posted @ Wednesday, May 16, 2012 12:16 PM by Darrel
Sorry for the awkward post.. 
Note: this is a vaulted ceiling that seems to trap all the air
Posted @ Wednesday, May 16, 2012 12:25 PM by Darrel
i have a 1100 sq ft home.outside today was 90 .attic was as hot as a volcano.inside the house we can maintain 70-75 degrees with the AC going but it seems the AC has to switch back on very frequent.will cooling the attic help in making the AC work less?i have vents on the ends of the house and the vent thing going along the whole roof of the house .the house also has the inlet vents along the back side and from of the house for air to get inside the attic.i just changed 7 of the 13 inlet vents as they we so old and covered with paint i do not thing much air was passing through.should i see how changing these inlet vents does before i try anything else?? feel free to email me direct
Posted @ Friday, May 18, 2012 10:27 PM by john
To some degree the article is true. What is fail to mention is that if you have a attic fan with a thermostat and keep a constant temp oh lets say in the summer time around 95 still pretty warm, where if you didn't have it it might be up in the 110 to 120 range even as high as 130. It's not so much for keeping your A/C from working as much as it is allowing your shingles to last much longer. Some of the new vented roofs being applied is for this very reason. If the attic fan vendor is trying to tell you it will cut cost, well it will on having to replace shingles in 10 to 12 years. How I know this? I have been in the roofing business a long time and even with the best shingles heat will absolutely destroy them. I have been back to houses that do not have the attic fans or a vented roof and in usually about 12 to 15 years we are replacing or repairing the shingles. Where as I have been back to houses for check ups every five years and the ones that use their fans usually are pushing the 25 to 30 year mark on the roof. Just what the shingle manufacturer exclaims.  
Just my two cents worth. This can go both ways. You just have to weigh out your options and what fits your bill.
Posted @ Thursday, June 21, 2012 8:43 AM by MS
MS: No, actually it doesn't go both ways. I appreciate your experience as a roofer, but the science says otherwise. Here's a way that might help you to grasp the real issue: If you lie on a table out in the summer sun with lots of skin exposed, you're going to get a sunburn. The UV rays are what's damaging the skin. If you set up a fan to blow air under the table, will that prevent you from getting a sunburn? No. Same with shingles. It's UV that kills them, not heat from below.
Posted @ Thursday, June 21, 2012 8:53 AM by Allison Bailes
Excessive heat is one of the primary causes for blistering of asphalt roof shingles, according to the Canadian Asphalt Shingle Manufacturer's Association (CASMA). Obviously, as the part of your home that's exposed to the sun, your roof is going to get hot, but that in itself is not the problem. Heat produced inside your home, as in a poorly ventilated attic, can cause the shingles to puff up in places. The same thing can happen when areas of your roof that normally have been shaded are exposed to the sun -- for example if a sheltering tree is removed. Blistering that is small and relatively superficial is termed "rash blisters." Blisters that extend to the felt layer of the shingle are called "tent blisters." These can lead to more serious roof damage.  
Posted @ Thursday, June 21, 2012 9:58 AM by MS
MS: Thank you for illustrating that my point is correct. Your example of the formerly-shaded shingles suddenly having problems when exposed to the excess UV shows perfectly what's going on. It's the UV, not the attic temperatures. Besides, that heat inside the attic wasn't generated in the attic. It came through the roof.
Posted @ Thursday, June 21, 2012 10:07 AM by Allison Bailes
Never said you was wrong about the UV so why make your self out being right. And I never mention where the heat was coming from. Obviously it has to come from the sun. “Building codes” in all states require certain amounts of venting and that they be divided between high and low. Ridge venting, which is a continuous vent running along the ridge of the roof line, is optimal, as it takes advantage of the highest point in the home. High venting may also be achieved using mushroom vents or cans. These tie into the shingles of a roof and cover a hole cut in the roof. Low venting is almost always the result of vents located in the overhangs (or soffits). Soffit vents allow for air movement from the lowest part of the attic space. 
Attic venting allows fresh air into the attic space and lets older air cycle out. Soffits are most suitable for allowing air in, which will travel up through the attic space as it warms and escape through the high venting. Attic ventilation keeps stale air from sitting (and heating) in your attic. It also retards moisture caused by the hot, damp conditions that occur in an unventilated (or improperly vented) attic. 
By reducing temperature swings, attic ventilation will help to increase the life of shingles and other roofing materials. An improperly ventilated attic becomes very hot, and this heat can damage wood and, especially, shingles. In the winter, a hot attic will cause snow to melt at higher points of the roof. Water will run down to the eaves and gutters, refreeze and create ice dams. Ice dams will back up and often lift shingles, causing leaks and maybe even interior damage. 
Perhaps the most important benefit of proper attic ventilation is that it keeps moisture from developing in the sauna-like condition that arises from poor ventilation. Moisture can damage building materials and any items stored in the attic. Of even more concern, moisture will promote mold growth that can become a health hazard. 
So before you really start to worry a lot of people about how it is bad to have a attic fan or that they really don’t need to vent do a little more on your part and be considerate it goes a whole lot deeper than just saving money on your heating and cooling bills, it’s designed to save your whole home. That is why it is very important that you do seal up the bottom half and have good insulation in all exterior and interior wall including floors.  
Posted @ Thursday, June 21, 2012 10:19 AM by MS
MS: There's not much evidence in the world of building science for your claims. For example, if you have an attic that not only isn't vented, but has insulation at the roofline, the shingle temperatures will be about as high as they can get because you're minimizing the conductive heat flow into the attic. Forget about what flows back from a hot attic because it mostly stays right there on the roof. The Florida Solar Energy Center has done a lot of research on roofs, and I wrote about it a while back. The average temperature rise will be about 2° F, with a max of about 9° F in the middle of the day. 
If you really want to understand this topic, I encourage you to read the article and download the FSEC report: 
How Hot Is Your Roof? - Insulated Rooflines and Shingle Temperature 
Also, venting attics was started to prevent condensation problems and ice dams in cold climates, not to keep attics cool in summer. The moisture generally isn't much of an issue in hot attics because it doesn't find surfaces at or below the dew point.
Posted @ Thursday, June 21, 2012 10:37 AM by Allison Bailes
Okay for one how many houses have you built?  
How many roofs have you put on?  
How many times have you been up in a attic in a house that has insulation right up next to the roof line? Not many I promise ya! 
I had also already mentioned the ice dam effect that can happen from winter. Do you think that when it starts to cool off in the afternoon that with the air being cooler outside and the attic being hotter that condensation will not form. "What" really. The temp doesn't have to drop to much for dew to start. And it doesn't have to be freezing either. I have measure temps as hot as 130+ inside a attic and work til midnight and temps drop to maybe around 80 that is enough for expansion and contracting to do harm alone. You just are not getting it lady that it's not just the cooling effect we are after here. Why don't you talk to real life builders, contractors that deal with this every single day. Instead of science. You want science go out in the field and you will get your science. I have been doing this for almost 45 years. And I promise you one thing theory is just that. It would be correct if you was to put a barrier there to stop it from filling space but that just isn't logical or cost effective. It make no sense at all. And for something else when ever FSEC and any other solar research companies start to make and sale building materials let me know. If you won't real life proof go to the ones that make it. I'm sure they have spent a hell of lot more money into research for their own product than any of those so called scientist you keep mentioning. It's one thing to lead a horse to water. If you know what I mean. Common Sense!!!! is in the building world, you really ought to check it out sometimes. Not located in your local Barnes and Nobel. It's what built this great Nation.
Posted @ Thursday, June 21, 2012 12:43 PM by MS
Sorry meant to say Mister at the lady part.
Posted @ Thursday, June 21, 2012 12:52 PM by MS
MS: You might want to take a look at my bio and read a bit more here in this blog before making more assumptions. Also, in my experience, people who use the 'common sense' defense are often making excuses for their incomplete knowledge. I'm sure you have a lot of practical experience, but that experience will make a lot more sense to you if you understand some of the science that you belittle. Most of the folks I know in the field of building science combine both the practical and the academic and are not the one-dimensional caricatures you're attacking.
Posted @ Thursday, June 21, 2012 1:19 PM by Allison Bailes
No not attacking and damn sure am not making excuses. You want to throw stuff out at us. Here ya go. 
As a member check this out. 
B. R. Stewart 
Agricultural engineer-environmental control ~ Agricultural Extension Service ~ Texas A&M University 
Ventilation of the home attic is important for two reasons. During the summer, excess heat that builds up in the attic during the day results in high energy costs for cooling. Also, moisture produced within the home may move into the attic if ceiling vapor barriers are not used. If this moisture is not exhausted from the attic it can condense and cause insulation and construction materials to deteriorate. Thus, temperature and moisture control are the major reasons for providing attic ventilation. 
Ventilation Quantities  
How much attic ventilation is required to provide proper temperature and moisture control? A number of studies sponsored by federal energy funds are under way to look at ventilation rates and methods. However, several studies previously made can help with this decision. The maximum ventilation rate is required to remove heat during the summer cooling months. Attics can reach temperatures of 150 to 160 degrees F during a summer day, although outside air temperatures are only 95 to 97 degrees F. The cooling load for a home air conditioner depends on the difference in temperature between the inside and outside air, and reduction of attic temperatures from 155 degrees to 105 degrees F will result in a significant reduction in cooling load. In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost. With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost. Thus, high attic ventilation rates are most important for poorly insulated ceilings. A poorly insulated ceiling is one whose R rating is less than 14 or one with fewer than 4 inches of fiberglass, rockwool or cellulose insulation. 
Attic temperature depends on the amount of solar radiation, construction details and the rate of ventilation. Calculations indicate that on a July day in Texas, a ventilation rate of one air change per minute for a typical attic using 95-degree F air will lower the peak attic temperature to about 101 degrees F. Providing half air change per minute will lower the temperature to about 106 degrees F. Thus, the first half change per minute is most effective and a doubling of this rate only achieves about 5 degrees F additional cooling. Studies indicate that further increases in ventilation are not effective in significantly reducing attic temperatures. 
Winter attic ventilation must be sufficient to remove moisture vapor moving from the living space to the attic. In general, ventilation adequate for summer cooling is more than adequate for winter ventilation. Winter rates need not be more than about a tenth of the summer rate. 
Calculate the required summer ventilation rate by determining the volume of attic space and dividing by 2. This will be the cfm (cubic feet per minute) of ventilation air needed. The volume is determined approximately for a rectangular house by multiplying the height from the ceiling to the peak/ridge (H) times the width of the house (W) times the length (L) and dividing by 2 -- ( H x W x L / 2 ). For a gable roof, this will be reasonably accurate. For a hip roof house, the volume will be overestimated but adequate. 
Ventilation Methods  
Attic ventilation can be accomplished by gravity ventilators, wind assisted ventilators or power ventilator.. Regardless of the method used, the purpose is to provide uniform ventilation of the attic for proper temperature and moisture control. 
Natural ventilation is the most common and energy-efficient method of achieving attic temperature and moisture control. This method takes advantage of two principles. First, as air is heated it becomes less dense and rises. Second, wind movement over and around a home creates areas of high and low pressure. If a space has high air outlets in conjunction with low inlets, ventilation occurs as the air within the space is heated. The greater the vertical distance between the outlet and inlet, the greater the ventilation rate will be. Thus, roof outlets should be at or very near the ridge, and inlets should be under the roof overhang or near the ceiling line. 
Ventilation caused by wind pressure differences requires less vent area to achieve the same ventilation rate as ventilation by gravity. One problem with wind ventilation is that the areas of high and low pressure change with wind direction, causing difficulty in locating inlets and outlets so that ventilation will take place regardless of wind direction. The best system is one in which the outlet is near the ridge and the inlets are in the soffit area. The hip roof is best suited to this system, since there is soffit area on all sides of the house. Outlet vents can be provided by roof louvers, gable end louvers, turbine ventilators or continuous ridge vents. These should be constructed so that rain and snow cannot enter. Water leakage is more likely with the turbine ventilator and gable end louver than with fixed roof or ridge ventilators. 
Inlet vents may be of the manufactured type, either slotted or perforated, and installed as individual units or in a continuous strip. The slotted opening seems to resist clogging by fibers and dust better than the perforated opening. Screen wire is sometimes used for soffit vents, but again clogging is a problem. In some localities frequent cleaning may be necessary with either type.  
Providing for Natural Ventilation  
Natural ventilation caused by a chimney effect or by wind movement is the most economical ventilation method. The quantity of ventilation air depends on opening size, temperature rise and wind movement. Tests have shown that effective natural ventilation can be achieved by providing inlet and outlet open vent areas of approximately 1 square inch per square foot of attic area, when roof slopes of 3/12 to 5/12 are used. This vent area should be the net open area rather than gross vent size. Some prefabricated vent materials may have only 60 percent net open area. Thus; if a vent area of 10.4 square feet is required, the gross vent area would be 10.4 / 0.6 = 17.3 square feet. [National standards appear to require only 1/2 this amount of open vent area. The open vent area calculated by these formulas is to be divided roughly equally between inlet and outlet.] 
Suppose a 1,500-square-foot home requires the above soffit vent area. If the total soffit length of the home is 100 feet, the vent width for a continuous soffit vent would be (17.3 x 12) / 100 = 2.1 inches. Ventilators should be purchased on the basis of net ventilation opening. 
Roof overhangs of 12 inches or more provide ample space for inlet (soffit) vents. Outlet vents should have the same net area as inlet vents. This is easily provided by continuous ridge vents or individual roof vents for gable-roofed homes. The roof ridge length may not be long enough on a hip roof to use a continuous ridge ventilator. In this case, several individual roof vents, well spaced near the ridge, should be used. 
Power Ventilation 
[For a safety and other reasons some authorities discourage the use of power ventilation. -- jim] 
Power ventilation can be accomplished in two ways. In homes not mechanically cooled (air conditioned) the temperature can be controlled to some extent by the use of attic fans. These fans are usually ceiling mounted in a central hallway so that outside air is pulled through open windows and exhausted through the attic. Sufficient outlets must be installed in the attic to exhaust the air without creating high pressures against which the fan must operate. The net area of attic outlets should be 1/800th of the rated fan volume in cfm and 1/8 inch static pressure. The outlets should be distributed uniformly. 
Air conditioned homes can use power attic ventilators by installing an exhaust fan through the roof or in the gable. Inlets for the ventilating air should be at the soffit, or the opposite gable, when no roof overhang exists. 
Power ventilators have the advantage of providing good ventilation even when there is no wind. They also provide limited attic temperature control when installed in conjunction with a thermostat. For well-insulated ceilings (i.e. insulation levels of R-19 or above) it is doubtful that a power ventilation can be justified economically.  
However, there are some attics which cannot be ventilated by gravity or wind-assisted methods, and in these the power vent is necessary for moisture and temperature control. 
For areas of Texas where blowing dust and sand are a problem, consideration should be given to the use of a power ventilator with automatic or motorized inlet louvers which close when the fan is not in operation. Ventilation can be shut off during a dust storm to prevent the buildup of dust and sand in the attic.
Posted @ Thursday, June 21, 2012 1:30 PM by MS
MS: As fun as this is, I really need to get back to work. If you'd like to learn some building science, we'd love to have you in one of our HERS Rater training classes. Our next one is in August. I'll even give you a $100 discount* if you take the August class. We could go out into some Atlanta attics together!  
*That's on top of the Early Bird discount if you register early.
Posted @ Thursday, June 21, 2012 1:46 PM by Allison Bailes
It's been fun Allison and thanks for the offer. I will not be able to make it then. I will be flying out to Jamaica for a mission trip. Don't get me wrong here. I understand where you are coming from as well. I love science myself and never belittle it. Just sometimes it gets in the way of just doing it. It would be great if all houses could be green. Just not practical because of the way the economy is going these days. And for people who make materials for us home builders and scientist to test they have families also, too feed and support. So I say, instead of bashing home ventilation sales cause a blog like this has, it would be better if it's something that has to be in the home help make it run better and efficiently. 
Thanks for your time. 
Posted @ Thursday, June 21, 2012 2:03 PM by MS
Attic fans, just like any other product, do benefit when properly installed in proper conditions. 
If air leakage through the ceiling is an issue, fix the leaks. If not having enough air to feed the attic fans is available from outside, add soffit, ridge or gable vents. 
It is not an absolute truth that if the ceiling has no leakage then attic fans are not necessary. Ceiling insulation only retards the transfer of heat from the attic to the interior of the house. The hotter the attic temperature, the greater the difference is between attic space temperature and the temperature of the home. The greater the difference in temperature, the faster the rate of heat transfer. 
Any we look at it, as long as the ceiling has no leakage and enough outside air is available for attic fans, attic fans reduce the amount of heat transfer into the house without taking away from the cooling of the house.
Posted @ Sunday, June 24, 2012 4:26 PM by Robin Boyd
I read the article and comments with keen interest. Although I have not extensively studied the draft Florida Solar Energy Center report from 6 years ago, it appears to be mostly a derivative of other original works conducted long ago. That does not make the report invalid. Perhaps though the report is not the cornerstone or repository of all human knowledge on the subject. 
You seemed to have looked down upon posters who have tried to contribute to the conversation while leveraging over 100 years of real world, non-derivative experience. Don't bother reading the post which incorporated the work from Texas A&M - what do they know when you are such a world player and gate keeper of science. 
I'm still gathering information to draw my own conclusions. I am not even sure yet I disagree with your premises. However, you strike me as the global warming type. That is, the science is settled in your mind and everyone else who has questions, counterpoints, or alternative experiences must therefore be non-scientific and mistaken. 
Hey, who needs discussion, other points of view and debate when your dealing with science right? 
Week work Allison. 
p.s. I misspelled 'weak' so can get of a quick zinger about grammar or punctuation. Keep up the good work. 
Posted @ Tuesday, June 26, 2012 12:37 AM by L. Albertson
Posted @ Tuesday, June 26, 2012 12:41 AM by Luke Albertson
Robin B.: If you've done a great job air sealing the ceiling plane and you've got lots of insulation, a power attic ventilator may well keep the attic cooler. It will do so at a cost, though, and the relatively small reduction in temperature will not justify that cost. 
Luke A.: I've done lots of air sealing in attics. I've done lots of Blower Door testing of homes. I know how difficult it is to get a perfectly air-sealed ceiling plane. Doesn't that count as real-world, nonderivative experience, too? 
I do consider what people have to say, but I haven't read anything in the comments here that makes me think, "Yeah, I guess I might put one in my home after all." To recap: 
1. PAVs can be dangerous by causing backdrafting that could put carbon monoxide in the home. 
2. If you do a great job with the building envelope to minimize the chances that the PAV will cause health and safety problems, the PAV becomes unnecessary, providing perhaps only a small benefit, but at a cost. 
3. The top building science guys in the world — Joe Lstiburek, Peter Yost, John Tooley, John Proctor — who all have tons of real-world, nonderivative experience themselves, say PAVs are dangerous at worst and only slightly helpful at best. 
PS If you look through everything I've written, here or elsewhere, you won't find a single instance where I've hurled 'zingers' at anyone for spelling or grammatical errors. Sorry. I don't do that.
Posted @ Tuesday, June 26, 2012 6:23 AM by Allison Bailes
My situation - home is 25 years old and we had an attic fan for the first 22 years. All was fine and comfortable. We replaced our roof, they took out our attic fan and we now have a ridge vent (we were told it was better). It is almost unbearable to sleep in our 2nd floor bedrooms now without fans as it is way to warm. You can feel a huge heat difference as you reach the top of the steps. This has been since we got the new roof. I can only assume the difference is due to the lack of an attic fan. I did not notice and change in our electric bill without the attic fan. Even if using one did raise our electric bill, we have now added fans to the bedrooms and I was considering getting a portable AC to use on the 2nd floor, so wouldn't it be better to install an attic fan?  
Could not having an attic fan have created the extra warmth in our upstairs? Thoughts?
Posted @ Saturday, June 30, 2012 10:17 AM by Kathy
Kathy: Without going into your attic and seeing what else might have changed when the roofers were working, I can't really give you an answer. Somme possibilities: 
* Lots of old roofing material fell into the attic, compressing insulation. 
* Workers damaged the duct system or air handler (if you have those in the attic). 
* They didn't provide enough passive ventilation area.  
There are plenty of homes with attics that get very hot and 2nd floor living space that stays quite comfortable without power attic ventilators. I wouldn't jump to the conclusion that you need PAVs to keep your home cool.
Posted @ Saturday, June 30, 2012 8:38 PM by Allison Bailes
Thanks for the quick response! We just had shingles replaced but they took out the attic fan. There is no roofing material on any of the insulation nor do we have an air handler up there. Not sure what they would have had to do in order to add the ridge vent - assume they might have to open up the roof some to give it a ventilation area? Is there something we can look for? We will need to check that. There was an 8-10 degree difference between our 2nd floor and 1st floor areas last night. Thanks for your thoughts!
Posted @ Saturday, June 30, 2012 9:26 PM by Kathy
What if you have the ventilation attic fan running when the following criteria is met: 
1. Temperature of outdoor is cooler than inside the house. 
2. Indoor windows are opened while fan is running 
3. AC is not on. 
With this setup, this would increase the time it takes for the home to cool down without running the ac at night. Especially when outside temperature is ideal already.  
Would this requirement justifies for an attic fan?
Posted @ Sunday, July 01, 2012 4:24 AM by Alvin
OK. I don't have time to read all of these posts, but I get the general idea. However, I hope some of the experts here will enlighten me. Right now it is 105 degrees outside in my southwestern Illinois town. Last Thursday I was outside and noticed a screeching noise, looked up and could tell the noise was coming from my roof. The only possible sources were the two vents for my attic fans. I ran to the breaker box and shut them off - noise was gone. My house was perfectly cool before, even though my air-to-air heat pump was running a lot, it was keeping up. Now the last few days with 100+ temps, and my attic fans not running, my house will not cool. It is 78 degrees with the thermostat set at 75. It HAS to be because the attic fans are not running. They are automatic and I often heard them kick on. I don't care about pressures and that stuff, I just want a cool house. Right now I'm fuming because my power bill will be enormous and I'm not keeping cool! My house is fairly new - only 7 years old and well insulated. Don't you agree that the reason my house is hot is that the attic fans are not running? Has to be! An electrician is coming on Monday to check it. In the meantime, I put up with the heat.
Posted @ Saturday, July 07, 2012 5:04 PM by Caroline
No Caroline, science clearly says your house is not hotter with out powered roof vents. I can't beleive you even posted this!  
Funny science indeed. While being a scientist myself I also deal in common sense. My 50 year old attic gets really hot, 60 degrees above ambient, it has big openings on the gables but nothing that makes the air move, no ridge or soffit vents. Until we installed a solar powered fan. It cost $330 (qualifies for a tax credit) and took a few hours to install. Now it comes on, lowers the attic temps 30 degrees and the AC system will keep whatever temp you set. Before It would run constantly and only get down to 75 on a 90+ degree day. Science says air moves in the path of least resistance. That means air comes in those big gable openings in the attic, not a few 1/8 inch cracks in the moulding here and there from the house. I suppose instead of a $350 diy job I could have spent thousands adding ridge and soffit vents but at this rate it will pay for itself in a year or two. It has a five year warranty on the motor and 20 years on the solar panels so even if I had to buy a new one every 5 years I am ahead money wise while the cool living in a heat wave is priceless!
Posted @ Saturday, July 07, 2012 6:58 PM by Bryan Hersman
Still trying to look into why our 2nd floor is so hot since getting a new roof with ridge venting and taking out our attic fan. When we got a new roof they took out our attic fan and put in a ridge vent. If we do not have soffit vents is the ridge vent able to do its job appropriately? Thanks!
Posted @ Saturday, July 07, 2012 9:17 PM by Kathy
Caroline: If your ceiling is properly air-sealed and insulated, power attic ventilators probably wouldn't make such a big difference in your home's temperature. If I were you, I'd call someone who knows building science rather than an electrician. 
Bryan H.: Actually, I discussed the science in the article above, and it doesn't say what you claim. 
Kathy: The best thing you can do is find a local home energy pro to come out and take a look.
Posted @ Tuesday, July 10, 2012 10:32 PM by Allison Bailes
You have to have soffit vents for the ridge line vent to work. Where is the cooler air coming into your attic from if the hotter air is going out the ridge line vent? It's not.
Posted @ Wednesday, July 11, 2012 11:15 AM by Mike King
I live in Northern MN. I have blown in just over 1 foot of cellulose (more where I get ice dams). When the contractor re-did my roof a ridge vent was installed and I have 3 small gable vents but they covered my soffet vents so I get no air flow from them. Does my current situation provide proper attic ventilation? Still get massive ice dams in a bump out ridge. Thanks to all reply.
Posted @ Tuesday, July 17, 2012 1:14 PM by Tracy
Update! It turns out that when the group took out our attic fan when they put on our new roof with a ridge vent they DID NOT vent our soffits. Nor did they cover our gable vents (which was probably a good thing since they didn't vent the soffit) as I understand is also supposed to happen. Thus, the reason for all the heat upstairs. They have been apauled at their work and have sent project managers to take a look to find the best situation for our home. Thanks for all the help and advice. 
Posted @ Tuesday, July 17, 2012 1:31 PM by Kathy
We lost our central air this season and will have to save our pennies till we can replace. 
One thing we discovered this hot summer is that our home 
appears to be much cooler if we don''t run the attic ventillators. Your article confirms confirms our suspicions. 
Thank you for this information. 
Allentown PA 
Posted @ Tuesday, July 17, 2012 4:23 PM by mike
You're completely wrong about how a power ventilator works. You may know a little about the theory of heat transfer and the vaccuum these things create, but wrong about the reality of what the ventilator does for an AC unit and how it affects the homeowners bill. You're also wrong about where the vaccuum will be finding it's source for replenishment (path of least resistance? Nice electronics cliche)... a well designed soffet system will offer the fan more than enough supplemental air and a sealed roof access will keep the cooled air in the main part of the home from escaping into the attic. Attic heat works like a heater sitting on top of the home, and don't try telling these gullible folks that hot air rises, therefore it can't affect the temperature in the main living area... it fights the AC all day long at appx 150-165 degrees Farenheit just on an average 90 degree summer day, radiating and heating the cool air down down down until the whole house it noticeably warmer. I challenge you to a battle of applied knowledge in this field any day, it's people like you who make people who need an attic fan second guess themselves out of speaking with a true professional.
Posted @ Friday, July 20, 2012 6:01 PM by Howard Mason
Howard M.: Did you just throw down the white glove? I love a good challenge!  
Fact #1 Yes, in a house with enough soffit vent area, the power attic ventilator (PAV) may not draw much from the conditioned space. Most houses don't ceilings air-sealed well enough that I'd recommend one.  
Fact #2 If the house does have plenty of soffit vent area and a well air-sealed and insulated ceiling, you don't need a PAV.  
Fact #3 In homes with atmospheric combustion appliances, PAVs have caused backdrafting that put carbon monoxide in the house and made people sick. That's the most dangerous kind of applied knowledge.  
Fact #4 The state of Georgia banned PAVs because they're bad.  
Fact #5 Attic air temperatures get up to about 130° F max. See the link I've provided elsewhere on this page about that.  
Fact #6 Plenty of houses with no PAV stay perfectly cool and do so with low energy bills. How do you account for that?  
Hmmmm. Why is that I seem to keep repeating what I've already written in the article above and in other comments. This isn't that hard to grasp.  
Do you by any chance sell power attic ventilators, Howard?
Posted @ Friday, July 20, 2012 6:20 PM by Allison Bailes
Wow, I just stopped by to see why attic ventilators are a bad idea. Perhaps if my home were an earth contact, with ground temps hovering around 70F, an attic fan would be out of the question. But since I'm sitting under an oven, and even my A/C unit uses ambient (outside) air to "cool" my house, it makes sense to us laymen that turning the oven down will help keep the air down here a little cooler.  
Science? I've got something for all that science - it's hot, and the cooler the air is around the living area, the cooler the air will be inside same. Air leaks? So-called perfectly sealed attics? I'm from Missouri, show me.  
You've got a point, I won't argue it - if the living area of my home is sealed up like a beer cooler, it won't matter what I put on top of it. But it ain't that way in real, it's almost too much to ask for. There are far too many variables for balanced air flow, balanced air pressure, and proper heat transfer (back outside) while trying to remain cautious about backdrafting your furnace or hot water heater. Not to mention the shifts that will invariably alter the way a fixture sits atop its foundati... listen to me, I've gone all book stupid.  
Okay, so attic ventilators aren't the thing to have. How 'bout if I stay with the presently installed wind-driven turbine and existing ridge vents, and just clean the soffits - will that help the a/c run a little more efficiently? By the way, why DO they put all those vents up there to begin with? 
Thanks, JD
Posted @ Wednesday, July 25, 2012 7:46 PM by Jack Dyle
Wouldn't the amount of conditioned air that the attic fan sucks out of your house depend on how much air can replace it from the outside? So if you have good seals on your windows and doors, wouldn't this effect be minimized? 
BTW, my attic fan was out for half the season, and now that it's back, I believe that my temperatures inside stay as much as 2 degrees lower on very hot days when my a/c can't keep up.
Posted @ Friday, July 27, 2012 5:22 PM by BJZ
I've read many, many of these comments and I'm happy to say that I've SOLVED this problem in my own home.  
1) Is it possible to patent a specific process that provides a legitimate cost savings in attic cooling?  
2) If so, would I be able to profit from it financially every time a builder wanted to incorporate my patented idea? 
I know this isn't your specialty and it would probably be easy to make subtle changes to my design so I wouldn't benefit from it...But I am an American and a true capitalist at heart. I've never patented anything in my life, but I know how much this simple idea has benefited my own home and electric bill. So in sharing it with others, I would hope I could at least send my kids to college on it, if you know what I mean... 
Thank you in advance for your response.
Posted @ Monday, August 06, 2012 10:21 PM by DB
I live in inland San Diego California and am in the process of constructing an addition with PV solar. The original house has a hip roof and soffit vents, no ridge vents. Needless to say, it gets really hot in my attic (150 F). I am having a few gables installed so I can get some effective passive attic ventilation, but want to maximize its effectiveness, that is what brought me to this article. It appears from the comments that people are confusing effectiveness with economy. It is logical that an attic fan would actually do a better job of cooling the attic than passive ventilation, however, it is more expensive from an energy usage perspective than other means. Is that a reasonable conclusion?  
So how do you feel about whole house fans or Quiet Cool fans?
Posted @ Saturday, August 11, 2012 1:08 PM by Barry
Correction, 120F, not 150F
Posted @ Sunday, August 12, 2012 5:10 PM by Barry
Posted @ Friday, August 31, 2012 2:24 PM by Chester Shannon
I was looking for just this sort of discussion. Two fans wouldn't work most of the time. Voltage in power to the house fluctuates up and down all during the day but within a range that is not normally noticed. Fans motors are all slightly different even of the same make and model. The real difference shows when voltage is varied in that some slow down more than others. With fans of different manufacture or age or both and the difference can be quite significant. Unless the fans EACH have flow rate monitoring coupled with auto speed adjusting, it would fail.  
As far as asphalt shingle failure goes - Yes, UV is tops followed closely by weather, especially hail and wind in excess of the rated shingle speeds. I don't remember the low end, 60 or 70 mph for rated protection that is used for most roofs except for FL when code calls for 120 if I remember right. 
Posted @ Wednesday, September 19, 2012 12:25 PM by Tom
I guess we should consider the opinion of a PHD in Physics, even though no mention of what school he attended, or that his resume show him working with a general contractor wonder why you need a Physicist to build homes? And lets also not for get that the DOE the Department of Energy suggest attic ventilation. Oh yes and one more point this guy is the one selling snake oil his business is based on finding problems and selling his solution. Does he show any evidence on this site where is "educated" ideas have saved a single dollar like before and after bills, and maybe he is right but Attic Ventilation is also about Mold and Rot also not just heat. So follow this guys suggestions with no evidence or follow the building code and the DOE your call.
Posted @ Thursday, November 15, 2012 11:42 AM by Hamlin
Hamlin: If you'd click over to my bio in the About Us section, it's pretty easy to find out what school I went to (University of Florida, 1998). I'd also suggest you go back and read this article because I didn't say attic ventilation is bad. I said power attic ventilators can be bad. Sounds like maybe you're in the business of selling PAVs.
Posted @ Thursday, November 15, 2012 12:47 PM by Allison Bailes
Attic Ventilation works. My 2,400 square foot house before installing fan cost about $250 to cool in the hottest months. After the fan was installed, the highest my bill ever got did not exceed $180. Just like comment above, attic was 150 degrees. Fan's thermostat was set for 100. I think the temp was typically around 110-115 around 4:00. That is a delta of 40 degrees not radiating heat into the structure. Attic ventilation works but you do have to have a tight ceiling. Make sure there is plastic between the gyp. bd. and the trusses. Then check for foam insulation pads between the light fixtures and back boxes of all electrical penetrating the ceiling.Any other "leaks" will be so minimal, the gains of the ventilator will far offset any added air conditioning being pulled into the attic.
Posted @ Wednesday, November 28, 2012 10:39 AM by Alan Wieskamp
Alan W.: Yes, it's certainly possible that in some cases, power attic ventilators help. The point of the article, and the many times I've repeated it in the comments, is that the risks are not worth it.  
Now, to some of your mistakes.  
1. I doubt that the air in your attic gets close to 150° F. Even in Florida, attic temperatures generally max out at 130° F. That means the delta (ΔT) you referred to would be more like 5 to 20° F, not 40° F. 
2. PAVs don't affect the radiant temperature.  
3. Putting plastic between ceiling drywall and framing does not work in all climates and can cause serious problems.  
4. "Any other 'leaks' will be so minimal..." Really? Are you saying an open chase or a whole house fan or, as I saw in one house, a .7 square foot hole to vent the fridge would be minimal? 
I sure hope you aren't building or designing homes with such a tenuous grasp of basic building science.
Posted @ Wednesday, November 28, 2012 1:28 PM by Allison Bailes
My problem is in trying to achieve a balanced passive system when the 12/12 hip roof on the housse really limits the exhaust area available. I also think my soffit intakes are too restrictive. Is it better to have excess intake than not enough? Any products you can recommend to increase exhaust area without power ventilaton.
Posted @ Sunday, December 02, 2012 10:10 PM by Juan S.
Several issues become evident in reading the opinions here. Many repeat portions of either tradition-based rules, or new "science-based" recommendations, without taking several steps back to assess the big picture. 
First, several aspects of trade advice seem sound enough, except actual practice reveals very different results. Ridge vents have in recent years been derided, because of the ways most provide incredibly restricted actual airflow, despite their technically adequate measurements. Soffit vents similarly suffer from restrictions of design, as well as real-world constraints, perhaps the worst being attic insulation blocking most to all effective airflow. The silly panels some add along the roof line may appear adequate to control insulation issues, but the narrow cross-section allows only a fraction of intended volume flow. The actual severely restricted cfms of proper attic ventilation, may be the greatest common problem in the average home, even newer ones. 
On the other side, concerns about inappropriate air migration, especially in tight homes, where strong flows created by attic fans could create backflow down furnace or water heater exhaust vents is real, and of great concern. 
Practical thinking requires a sound grasp of the main scientific issues, but must reconcile the average consumer's financial need to balance spending against future, and long-term gain. A real world example is a friend who built his albeit modest dream home with the latest concepts of the day, only to realize a loss of scores of thousands of dollars because no buyer was willing to pay for his altruism, or the loss to repair high-tech systems. I also visited a green home opening, and was astonished to see inside the control room a maze of wires and tubing like a nuclear subs. A steady drip suggested a minor leak, but only "Bob" knew the system enough to locate and fix it. Good luck, without Bob as your neighbor. 
Describing attic solutions which are going to require prohibitive retrofit expense, like barrier sealing from the living space below, and then claiming that attic fans cost way too much is comparing boilers with reactors. Our attic is fan-vented but we open the access hatch so that the air flow from incoming swamp cooler then cools both living and attic space. Even so, attic fan will run for hours after dark, until its 85 deg. setting is met. Where's the UV at that point(or do you mean IR) except stored as hot wood, etc. Even by itself, the fact that the attic fan keeps the home at least 5 degrees cooler disproves the "science" that dictates that such fans cannot affect UV-only attic heat. If your science cannot adequately address common observed results, it is either flawed or incomplete. 
As to condemning a paired set of attic fans, one for inflow and one out, because the notion that very minor differentials between two fans is a deal-breaker is totally disingenuous, and reveals a bias favoring new, high-tech solutions. Fans and electric bills cost far less than a new roof; you simply must always factor in the total airflow picture and adjust for every change.  
Explain clearly after-construction processes of adequately "air-sealing" a ceiling. It is likely nearly impossible, as the inter-wall spaces provide a large conduit for direct crawlspace to attic airflow. "Someone who knows building science" I fear is likely to be under 30, with little to no actual construction experience. I know a lot of builders aren't especially prone to the latest stuff, but they also have likely seen the last 30 years of bad investments on solar systems that died within 5 years, never paying for their installation, etc. I know, today's are different. Right. 
Even when modifications are sound, and justifiable, the cost differential between new construction and later retrofitting can be prohibitive. 
There is an oft derided, but nevertheless essential principle in home construction, that of "grandfathering". Young know-it-alls would certainly love to get rid of that concept - except almost no one would be able to afford an older property, if they were forced to meet every modern standard as a purchase requisite. In addition, history has not always been kind to every aspect of code which once met muster, whether it's the infamous aluminum wiring debacle of the 60's into the 70's, or the PEX-type plastic water tubing that failed, or the Chinese drywall, or push-connect outlet wiring, and on and on. 
Generally, over time, codes get better, homes get tighter and more economical to heat and cool, and safer, in regard to fire prevention, inhibition or detection. But it's never perfect - mistakes will be addressed, corrected, and so we move on. The code does not say a modern home is going to be cheaper, and it is presumptive to claim that every new requirement is always going to save money in the long run, because often they don't. We are entering an age where information is cheap, but technology is not, so much. The sophistication and upkeep on a home may become unjustifiable, from a good-old Return on Investment analysis. Anyone willing to pay an effective up-front surtax for saving the world is welcome to do so, but the majority is still stuck in a world driven first by economics. For better or worse.
Posted @ Monday, December 10, 2012 8:15 PM by Eric L.
I installed an attic fan in my neighbors house. We had the correct amount of eave vents to match the amount of air being pulled out of the attic by the attic fan. The results were awesome. For the few pennies it cost to run the fan the temperature of the second floor dropped 10 degress. There was no air being drawn from the interior of the house, checked with a manomometer. When I installed an addition on my own house the roofer installed a ridge vent. My attic was hot as hell. I installed an attic fan and now my attic is the same temperature as the outside air, reducing the load on my air-conditioner.
Posted @ Sunday, February 23, 2014 7:48 AM by Frank Humr
If you stayed in a perfectly sealed house you would get sick and die. People need oxygen. Attic Fans installed properly don't suck any more air out of your house than a normal bath fan. If the gaps in your ceiling are so big as to allow too much air through then fix the holes. But don't go crazy, you need oxygen anyway. It will be cooler upstairs without having to really crank down the temperature on your thermostat. Helping to equalize the high temperature upstairs to the cold temperature in basement alone could stop knee-jerk reactions of homeowners who have no other choice but to crank down the temperature to get enough cooling upstairs. So get a Jet Fan, Worlds Best (and most efficient electric) Attic Fan.
Posted @ Tuesday, February 25, 2014 11:53 AM by Jeff Tideman
Wow. I don't read much online, but found this "discussion" intriguing to say the least. I am focused primarily on the battle between what appeared to be the author and a contractor. I am a contractor myself and pride myself on fixing what others screw up and doing the job correctly the first time. This discussion seems a lot like the ones I have with my wife. Each person vigorously trying to prove themselves correct without really listening to what the other is really saying. I'm sure I'm oversimplifying things, but feel that although you both are on opposite sides of the fence, you are saying the same thing. One is saying that attic fans are bad because they allow conditioned air into a unconditioned space causing mold, ice damming and energy waste. The other is saying air flow is needed to help keep the non conditioned space a more consistent temperature throughout and reduce condensation, mold, etc. . Air flow is good in conditioned spaces and those outside of our conditioned spaces for all of the reasons many of you have previously listed above. The flow of conditioned air into these spaces is bad. Warm meeting cold and cold meeting warm causes expansion and contraction as well as condensation, mold, etc. Its not easy, but insulate and properly seal the conditioned space from the non conditioned and create thorough airflow in both spaces to stay in those spaces, not intruding the other. I felt like you were both saying this, with a few sidebars. That's my two cents.
Posted @ Thursday, March 13, 2014 8:06 PM by Will
I have a 5 year old home and considering a AHEA solar attic fan. By reading this blog it attic fans can suck cold air through the ceiling and make the AC work harder, is this true. Has anybody heard of this type solar fan sold by AHEA (American Home Energy Audit)?
Posted @ Monday, April 14, 2014 9:11 PM by Bob R
I just had a power ventilator installed in the attic - apparently before reading this. A wind storm blew off the cap and rain water was leaking in the attic. I do not believe the fan ever worked to begin with, so decided to replace it. Now I'm wondering if that was the best idea. I am going to set the fan's thermostat steting to 120 or whatever the max is so it doesn't pull as much out, but I guess it was a waste of money. I was reading somewhere that ventilation is required by code, but the roof already has 4 vent holes on the top of the roof with soffit vents. The attic does get to 95 degrees when it's 75 outside, and during the summer I know it's a sauna up there (probably 140-150 F on a 105 degree day!) - but from reading this article I guess it doesn't matter how hot the attic gets as long as the house is fine!
Posted @ Wednesday, April 23, 2014 10:38 AM by f00dl3
This must have been written for the Electric companies, the soffit and ridge vents allow a slow exchange of air as long as there is enough wind to do so the powered vent is only to ensure the air is moving through the attic. 
This process allows you to keep the attic cooler and therefor you get less heat transfer between the ceiling and indoor temp. People will read and believe anything the big companies put out there but when you read up on the construction of homes and the attic ventilation you will learn there is enough to allow the air exchange.  
Only if your house has the insulation attached to the roof and not blown insulation then would it be questionable to install an attic fan. 
I know it gets up to 160 F in my attic and that is also where the A/C evaporator is located there, by reducing the temp. I also reduced my electrical cost was in summer about 300 and is now only 200. This is because the wind must work harder blow through the attic, the fan when properly done lets the wind blow easier through the attic. 
Also your logic is flawed, the minor leaks you talk about in the ceiling could only be by the lights in the ceiling and the A/C vents. The rest would not leak unless you have a hole somewhere that you need to repair. No house is leak free because if it was you would have problems breathing because the air would become stale and there would be too little oxygen for people to live. 
Posted @ Monday, April 28, 2014 12:01 AM by Billie
I appreciate your knowledge-so by asking a question I am wondering how to best use that knowledge-so please forgive my ignorance. If forcing hot air out, pulls cool air alone with it-can you turn the fan around and simply keep air flowing within the attic. Moving air is cooler that stagnant. I am planning on lining my attic ceiling with board insulation to keep that heat at bay.  
Posted @ Friday, May 02, 2014 9:54 AM by Robin
Here is something which has not been considered in the discussion. We have a house with an attic fan. After finding my 8" plastic insulated ductwork destroyed, my insulation soaked with urine and 30 lbs of feces in my attic, I learned that a racoon had pushed through my (dead) attic vent. The vent motor lasted just a few years. My contractor says this is a common problem around my area. $1000 to replace attic ducting damaged by the animal tearing it up, and $425 to trap and remove the raccon and install a cage around the vent. I will not be replacing my attic fan any time soon, as this issue along with the widespread understanding that they are simply not cost effective from an energy standpoint, argue against it.
Posted @ Wednesday, May 07, 2014 11:06 AM by aurum79
Attic Fans are GREAT if you use them correctly - We live in Colorado where many, if not most homes have no air conditioning. Our home is a bermed-in passive solar home. In summer, it is great until about 4:00 PM because we have double-cell shades that block the light out during the heat of the day. Our house is designed with a large 18 foot high ceiling in the main living area that slopes up to the 4 large solar windows across from a full brick wall on the bermed in side. The rest of the house,bedrooms, baths, laundry etc is normal 8ft ceilings and an attic. The temperature goes up to as much as 90 degrees by about 8:30 PM on the hottest days of the year. WE were spending a lot of electricity on multiple fans. We have changed all bulbs to LED bulbs and I am building large mirror solar ovens for cooking to replace the electric stove/oven for long cooking and use a microwave for most daily cooking. We have cut our electric use from an average of about 920 KWH/mo to 467 KWH this month, before getting the solar ovens going and installing the 1000 Watts of off-grid electric solar panels I am installing to separate lighting, refrigerator, (switchable to grid)microwave and gas-furnace Fan operation as necessities if power ever goes out. The fans around the house have been our biggest drain on summer electricity. To-the-point at hand, I am installing a fan drawing air from the peak of the 18 foot high ceiling - blowing it's air out of the high ceiling room into the adjacent attic area and putting a shutter at the far end of the rest of the house. I am putting 140 feet of underground ventilation tubes that should feed the air into the house near ground level about 15 to 20 degrees below the ambient outside temperature in the summer and recycle our inside air at bout 50-56 degrees underground in the winter (as low as -10 degrees outside) in the winter, keeping the gas furnace from heavy gas use at night and using the solar heat during the day. I fully expect my utility bills to show an amazing overall drop with 20 years of electric/gas/water bills and statistics in my files and computer. The attic fan from inside the house high point exhausting out through the attic peak at the other end and drawing from the underground ventilation system should make it the ideal example of an attic fan success. using electricity from my necessities-only, off-grid solar panels should complete the system. Whether you are a tree-hugger or looking for energy security in this rapidly changing world,it's not the money that counts most - but an extra hundred dollars a month will help pay off bills that will suck up some of that massive inflation coming from our $17+ trillion dollar national debt and hundred trillion+ unfunded liabilities.
Posted @ Thursday, May 08, 2014 9:47 AM by John Davis
Mr. Davis, your house sounds very interesting and I would love to read an article about it and see pictures. 
You have a special case environment, where there is very little priority on sealing the house against infiltration... at least during the warm months when the attic fan is to run. I hope everybody keeps in mind how very different that is vs. the hot-humid summer climate in Houston, Atlanta or similar places.
Posted @ Thursday, May 08, 2014 11:30 AM by M. Johnson
@John, whole house fans are ideal for passive solar homes and can be a real energy saver in dry climates. But this article is about attic ventilation fans.
Posted @ Thursday, May 08, 2014 12:29 PM by David Butler
I have a ridge vent with an attic fan. I am also in the process of getting a new roof. One roofer suggested installing a PAV with a humidistat& thermostat and eliminating ridge vent . Good idea? Thank you.
Posted @ Wednesday, May 14, 2014 5:12 PM by Brian C
If you insulate the under-side of your roof (preferably with spray foam), and the gable walls, you seal up the soffit vents and you seal the top-plates (framing part that goes across the top of the wall studs) so no air from the attic can get "down" the inside of the wall,  
you don't need a ridge vent or soffit vents or power attic exhaust vent fan (or PAV). The attic becomes semi-conditioned space, and shouldn't get very hot (because you insulated underneath the roof decking). 
You should seal any holes in the 2nd floor ceiling (like recessed lights, etc) also.
Posted @ Wednesday, May 14, 2014 9:36 PM by Ira Eisenstein
I installed a roof fan(1200 cfm) 2 summers ago, immediately I noticed upstairs temperature was 3-4 degrees cooler. my a/c costs are also about 5%cheaper. I am an HVAC contractor and recommend these to all my builders. They all like the idea and ask why no one has mentioned this before. After reading a few of the posts here, I will advise them that it is wise to make sure all ceiling penetrations etc are well sealed as well
Posted @ Saturday, May 17, 2014 11:32 PM by ken
Ken, I am always glad to hear people's direct experiences, even if it seems contrary to conventional wisdom. But in such a case, examining the details may reveal something logical rather than contradictory. First of all I wish everyone would post what climate they are in. Some are more amenable to outside air infiltration than others. 
Where I live in S. Texas, the summers are hot humid and monotonous. Overnight temperatures never get cool enough to lower humidity, even at 6am it is normally a mistake to open the windows. I am pretty sure your results would not be duplicated in my region. 
I have measured AC costs for a decade now and a 5% difference simply cannot be measured in my experience. It just gets lost in the weather/vacation/rate_change etc. noise.
Posted @ Sunday, May 18, 2014 9:29 AM by M. Johnson
@Ken, in addition to what M.J. said, I would add that this: In a particular home, if a PAV truly reduces cooling costs enough to pay for itself (don't forget to consider the energy the fan consumes), then that tells me there are issues with ceiling insulation and/or attic venting. 
In new construction you'll get more bang for the buck by reducing thermal bypass at the ceiling (e.g., better insulation, closing/sealing chases and fur-downs, etc.), and by moving HVAC and ducts into conditioned space. As an aside, if there's one thing building scientists and engineers agree on, it's that ducts in a vented attic = design flaw.
Posted @ Sunday, May 18, 2014 12:50 PM by David Butler
So I may not be a scientist in the sense of home heating and cooling but I did take many classes in thermodynamics and studied heat transfer in depth. I use it on my job and study and engineer fire safety related things.  
First off the article is correct only if you do not have enough ventilation coming through the attic. I installed an attic fan and also installed more than enough vents for the fan. Air like any fluid will take the path of least resistance, so giving enough ventilation creates the least resistance so you don't have to worry about it pulling indoor air. 
My electric bill has gone down around 20% when I run the fan. I recorded temps of over 160 degrees and staying above 120 hours after the sun fell down and the outside temp dropped below 70. Thats what made me realize something had to be done. Now when its running I can on the hottest of days (direct sunlight) bring it down to a more reasonable 110 degrees. Then at night after the sun isn't hitting it bring it down to around the temp outside. So think what you want about the article but a proper installation WILL save you money. 
The only thing I recommend is installing a extra loud smoke detector up there just in case the motor seizes and could create a potential fire hazard. I am always home when its running because I wouldn't trust a motor like that away from the house.
Posted @ Saturday, May 24, 2014 7:43 PM by john C
I live in San Diego and my house gets really hot in the Summer from heat in the attic. It has two 12"x24" attic vents, one on the East and one on the West ends. Would it be wise to put two solar fans in these vents, one blowing into the attic from the West and one blowing out of the attic to the East? I also have inadequate insulation and no radiant barrier. What would you install first?
Posted @ Saturday, May 24, 2014 9:36 PM by Michael McClure
John C. I'll start by quoting what David Butler said in the comment right above yours: "In a particular home, if a PAV truly reduces cooling costs enough to pay for itself (don't forget to consider the energy the fan consumes), then that tells me there are issues with ceiling insulation and/or attic venting." 
In addition I'll say that I have difficulty believing your numbers. For an attic to maintain a 50° F temperature difference to the outside "hours after the sun fell down" doesn't seem possible unless you have a heat source in the attic or a heck of a lot of thermal mass on the roof. 
Michael M.: Air sealing and insulation should be the first things you do. Skip the fans. They won't be worth the cost.
Posted @ Sunday, May 25, 2014 7:19 AM by Allison Bailes
Wow. Always amazes me how "heated" a discussion can become over powered attic fans. That said, I had an elephant in my attic that I, two years ago, told him with no minced words he had to go. His name was long; "Excessive radiant heat gain due to highly absorptive roofing materials" but that is not why I evicted him. Well, at least not the only reason. 
My attic has HVAC ducts running throughout. And the elephant with the stupidly long name kept crashing into them, crushing their ability to perform. This wasn't the first house I had lived in with an elephant of this breed, but after a hail storm damaged our dark asphalt shingled roof, I was determined this house would no longer tolerate such a comfort and money sucking squatter living overhead. 
Turns out this elephant just can't stand white reflective standing seam metal roofing, so he packed up and left without an eviction notice. He did, however, like many tenants in arrears on rent, leave a few things behind in the attic, such as unsealed fur-downs, pocket doors, and metal duct seams and fittings. So, while I achieved my initial aim of evicting an overweight freeloader, I'm still cleaning up some loose ends he left behind. Even so, I now marvel at an attic that daily tracks at or near outdoor ambient temperatures in warm to hot weather, without any forced ventilation other than two whirlybirds and soffit venting. I also was awed last week when my data logging Ecobee thermostat revealed a three degree temperature rise in interior temperature over eight hours during a sunny day with the a/c off the entire time, and it never reached the higher "unoccupied" temperature that would have triggered the a/c on. Outdoor temps peaked at 90 on that day, as did the attic. 
So farewell to the elephant that would not have been tamed much by a powered attic ventilator. My indoor mean radiant temperature levels celebrate your departure every day!
Posted @ Sunday, May 25, 2014 9:34 AM by Cameron Taylor
To quote a previous comment: "However, there are some attics which cannot be ventilated by gravity or wind-assisted methods, and in these the power vent is necessary for moisture and temperature control." 
This is exactly why my home needs a power attic vent -- my issue was 2-fold: 
[1] The prev owner "finished" parts of the 2nd floor, making them into closets. There is 5/8" board and insulation, but the walls are only 2x4, hence only 4" insulation can be used. They tried to compensate with many random pieces of 1/2" foam board, but it didn't provide "air seal". 
[2] The prev owner added an enclosed porch to the rear center of the house. Not only did they add a very large section of roof to soak up heat into the attic, they also deteriorated the airflow from the rear soffit. Hence, there is a build-up of heat where the original ventilation has been compromised. 
My solution was 3-fold: 
[1] correct the improper insulation on the finished closets. 
[2] Add a single 1600cfm attic fan (roof-type) to make up for the alterations made to the rear of the house that cramped air flow. 
[3] I replaced my spent 7yr old furnace (cheapy Gibson that was over-sized) with a variable speed blown Trane. This enabled us to improve/optimize the HVAC flow to/from the 2nd story. 
Now when I walk upstairs, I no longer feel that "heat wave". 
Powered attic vents aren't for every situation, but there are some outlier cases out there (like mine) that will benefit from one. 
No house is built perfect.
Posted @ Tuesday, May 27, 2014 10:51 AM by Dan
Ok... I'm going to say it because no one else will... 
Allison Bailes, you're an ASS!! :-)
Posted @ Wednesday, May 28, 2014 1:09 PM by Brian
Brian: Oh? Did I burst your bubble? Are you having a little cognitive dissonance? Or are you always so clever and witty?
Posted @ Wednesday, May 28, 2014 2:25 PM by Allison Bailes
I,ve had my solar attic fan for 2 years now and each month when i look at my power bill in thw summer like now i can see a lower bill then before i had the fan put in so i guess it just matters which side of the road you live on.
Posted @ Thursday, June 05, 2014 1:52 PM by phil
Allison, your article is interesting, but can you please provide me with a works cited to the science that is against PAV's? I understand you are getting your information from Building Science, I presume, so any form of specific reference that supports your information would be most helpful, such as names/titles/authors of reports, studies, tests, ect. Thank you for your time. Links to this info is fine as well.  
Posted @ Wednesday, June 18, 2014 6:43 PM by John
This article is speculation at best. 
Every attic has ventilation. Ridge, soffit or gable. any draw from the house itself will be minimal if any. the path of least resistance is this vents. I have installed a hundred of these and never once had a negative feed back. all say it helps and no increase in cooling costs. One other big benefit is reduced attic heat will (scientifically proven) prolong the life of roof shingles by as much as 20% if installed when singles are new. Attic fans do indeed help. The cost to run an attic fan is pennies per month.
Posted @ Thursday, June 19, 2014 6:25 AM by Dave
@John, this issue was put to bed 20 years ago. Here are some additional resources: (scroll down) 
@Dave, your comment re: 'path of least resistance' is misinformed. Air will always move through *all* available paths. It's just a matter of degree. Of course, the larger the attic vents, the less the pressure difference caused by the fan. But *any* negative differential between attic and house will pull air from the house (depends also on fan flow rate and how well attic plane is sealed).  
Keep in mind that ceilings are typically the largest single leakage path (see 4th paragraph on page 4: 
The impact on shingle life is greatly exaggerated. When you consider the cost of the PAV and the energy it consumes over the life of the shingles, I'd be very surprised if it comes close to breaking even. Studies have shown that single color has a bigger impact on life than roof deck temp.  
Folks, it's better to seal and insulate the ceiling to begin with than to apply what amounts to a mechanical band-aide.
Posted @ Thursday, June 19, 2014 2:47 PM by David Butler
To Dave: 
I appreciate your real world experience, I think that will always outweigh any scientific evidence. But I am big on scientific evidence, so we shall see how good this evidence really is. 
To David Butler: 
I appreciate the quick response and your supported evidence. This all looks good, but I would need actual reports, on the type of equipment they used, make/models, the type of houses they tested on, the insulation used in those homes, ect..ect..this should all be documented in their report if it exists, and I would like to read it. The information you provide explain that such a study exists and refer to it, but they don't make it avail to view. Without such a detailed report, then anyone can say anything, but a report can give the public something to view and to determine for themselves the evidence laid out before them. And it seems like the evidence comes from a study of 8 different homes. Is this enough to have 100% assurance that this happens to anyone who has an attic vent? If there is a report, there should be a rationale to why only 8 homes were tested, and why those homes were specifically chosen. The bottom line, without any actual report, words are just words, and even then, that report would need to be analyzed to determine its authenticness and reliability.
Posted @ Thursday, June 19, 2014 8:09 PM by John
@John, here's a link to the Davis/Tooley report: 
And here's a paper by FSEC on attic ventilation in general (see pp4-5 for more studies): 
And here's an article on PAV's @ GBA that includes some additional resources: 
And here's what Joe L has to say on PAV's: (#10) 
BTW, if you don't like the results of a study, you're free to do your own. But throwing stones at a study because you don't believe it, or you disagree with the methodology or general applicability, are truly the empty words.  
The problem is that nearly all PAV's are installed by folks who don't understand or acknowledge the potential downsides.
Posted @ Saturday, June 21, 2014 2:16 PM by David Butler
Thanks David Butler, I haven't had a chance to read those reports, but once I do I'll reply. A study is a study, I wouldn't have a problem with the results of any study, it's the process of that study that I may disagree with, that's the most important part, thus this would be the methodology. Disagreeing with a methodology is only empty words if the methodology proves to demonstrate a proper scientific process, otherwise the methodology will always be flawed. Will be in touch, thanks for your time.
Posted @ Saturday, June 21, 2014 5:35 PM by John
I read this report. I do like how they measured the differences in air and gas pressure and did actual testing, but I just had a few questions. The report states that carbon monoxide can get dispersed through the home, doesn't carbon monoxide vent through the roof? How is it possible to get carbon monoxide into the home when the carbon monoxide is being vented through the roof ventilation pipes? It talks about carbon monoxide backdrafting as the cause? If backdrafting is the cause, can you please explain to me how the carbon monoxide gets backdrafted into the home? 
As far as the differences in air pressure in the home going into the attic, a test with a manometer was used to determine the amount of air being sucked into the attic. I'm still trying to grasp how this is possible. I could understand air seeping through tiny cracks and unsealed lights, but it's hard to imagine a lot of air would be lost through these small openings. Are there other means of travel that air would take in order to get into the attic from inside the home? Or are these means of travel the only way air from the home gets into the attic? 
In either case, couldn't someone who was concerned, go buy a manometer and buy a carbon monoxide meter, then do this test themselves? And in general, if the carbon monxoide issue was really an issue, wouldn't homes all across the US experience their carbon monoxide detectors going off? 
A lot of questions, but if you wouldn't mind answering, I'd appreciate it.
Posted @ Sunday, June 22, 2014 9:55 PM by John
John: David has done yeoman's duty answering questions here, so I'll give him a little break. Knowing him, though, I think he'll probably follow up with more explanation. 
You have good questions, so let's take them one by one: 
The report states that carbon monoxide can get dispersed through the home, doesn't carbon monoxide vent through the roof? How is it possible to get carbon monoxide into the home when the carbon monoxide is being vented through the roof ventilation pipes? It talks about carbon monoxide backdrafting as the cause? If backdrafting is the cause, can you please explain to me how the carbon monoxide gets backdrafted into the home? 
When the pressure in the home is lower than the pressure in the flue, the exhaust gases won't go up the flue. They come into the home. That's backdrafting. It's also when more carbon monoxide is likely to be created because the combustion process changes as the flame may not get enough oxygen, resulting in incomplete combustion.  
As far as the differences in air pressure in the home going into the attic, a test with a manometer was used to determine the amount of air being sucked into the attic. I'm still trying to grasp how this is possible. I could understand air seeping through tiny cracks and unsealed lights, but it's hard to imagine a lot of air would be lost through these small openings. Are there other means of travel that air would take in order to get into the attic from inside the home? Or are these means of travel the only way air from the home gets into the attic? 
Well, all those little holes can add up to a lot of leakage. Take top plates, for example. The 1/8"-1/4" gap at the edges of all the top plates runs for hundreds of feet in a typical home. That could add up to a total hole size of 0.5 to 1 square foot or more. See Mind the Gap - Air Leakage at the Top Plates. Then there are the medium- and large-sized holes. And there are a lot in a typical house. 
In either case, couldn't someone who was concerned, go buy a manometer and buy a carbon monoxide meter, then do this test themselves? 
Yes, of course, but a manometer by itself won't tell you how much leakage you have. The amount of leakage depends on the pressure difference and the pathways, the size and shape of the holes. You'd need a blower door, too. 
And in general, if the carbon monxoide issue was really an issue, wouldn't homes all across the US experience their carbon monoxide detectors going off? 
Sadly, most CO detectors don't detect low levels of CO in a home. I wrote about that a while back: Don't Compromise — Get a Low-Level Carbon Monoxide Monitor
Posted @ Monday, June 23, 2014 6:29 AM by Allison Bailes
Allison, thank you. I also appreciate you taking each question separately so I can see what has been answered. I do have other questions.  
Ok, so the less pressure in the home makes sense from backdrafting, but then how does the carbon monoxide actually get into the home? So, I'm picturing, carbon monoxide is supposed to be released through the vent pipes, pressure is lowered then the gas can't travel upwards and out properly, therefore slowly seeps backwards or becomes stagnant. But, if the gas doesn't travel properly or reverses, where does the gas go. I believe you'd say into the home, but how? Does the gas go into the appliance then shoot out the side of it? Seems dangerous for the carbon monoxide to backup but wouldn't it just accumulate in the pipes or unit with no where to go since the appliance is wrapped in steel? 
The 1/8"-1/4" gap holes in the top plate, are you referring to the top plate of recessed lighting? In either case, if this was properly sealed, then how would air leave the living space otherwise? Is there any other possible means of travel for air from the living space to get into the attic? In either case, you mention that a blower door would be needed to know if air is leaving the living space, not sure what a blower door is, but regardless, wouldn't the electric bill determine that one's air conditioner is being used more then usual? Meaning, without doing any tests of if air is being lost from the living space, couldn't one just track how much their electricity bill is to determine is they are using more or less of their air conditioner? Of course the attic fan runs on electric to, but if the attic fan causes the air conditioner to run more, than there should be a significant increase in AC use and therefore seen on the electric bill. 
Your point on a low level carbon monoxide detector sounds interesting, didn't know a low level detector existed, I will have to look into that. Interesting point. Do you know of a company or specific model you recommend? In either case, couldn't the concerned home owner buy a low level carbon monoxide detector or carbon monoxide meter to see if their attic fan is causing C02 build-up? 
Another thought and concern here not mentioned, methane gas. If carbon monoxide does backdraft into a home, despite me still not understanding this, wouldn't methane gas be backdrafted into the home as well? And if this was the case, wouldn't we have more deaths, possibly having homes explode once they turn on their gas ovens, and wouldn't there be county issue of homes having bad odors? 
And speaking of deaths, very unfortunate to hear that people have dies due to C02 build-up, but if this was a major issue, wouldn't this be reported and a known fact or speculation to the public? Also, I had 10 different roofing companies come to my house, I also spoke with several different architects, I didn't know about this possible attic fan problem before speaking with them, however, none of them even hinted that this could be a problem after speaking with them about attic fan installation.  
Thank you for your time. 
Posted @ Monday, June 23, 2014 10:31 AM by John
John wrote: "how does the carbon monoxide actually get into the home" 
You obviously don't understand the difference between atmospheric and sealed combustion appliances. The majority of furnaces & gas water heaters are atmospheric (due to big difference in cost). Backdrafting can easily occur in atmospheric appliances. Not only is the combustion chamber open to the house, but there's necessarily a large gap between appliance flue and bottom of vent. See this previous article: 
"couldn't the concerned home owner buy a low level carbon monoxide detector or carbon monoxide meter to see if their attic fan is causing C02 build-up?" 
Most PAV's are installed without consideration of backdrafting or potential negative impacts on energy consumption. How many homeowners do you think are buying test instruments? Thus the importance of articles like this. 
Irrespective of PAV issue, anyone with a combustion appliance should invest in a low-level CO monitor, especially if they have tight home, PAV or whole house fan. The article linked at end of Allison's last comment includes sources (be sure to read the comments). They're not cheap. Or better, get rid of atmospheric combustion appliances altogether. As for methane, there's very little methane in flue gas as it's already burned. As for relatively low incidence of CO deaths... there's plenty of evidence that many, many people get sick from CO poisoning, most without knowing the cause. 
Regarding attic leaks, there are lots of paths between house and attic... for example, cracks between drywall and top plate, and chimney chases. Maybe you should consider taking a building science course. This basic topic is covered in a beginner's course. 
Regarding the 10 roofers who didn't know about PAV issues... no surprise. Again, that's why blogs like this are so important. I moderate the largest home performance group on LinkedIn with 8,000 members including energy raters and other home performance professionals, building scientists, builders, HVAC installers, energy program managers, insulators, etc. We even have a few architects and roofers. But that's only a small fraction of the total population of people involved in home construction and remodeling industry, especially when you include everyone involved in sales & marketing various products for the home.
Posted @ Monday, June 23, 2014 1:32 PM by David Butler
David, "You obviously don't understand the difference between atmospheric and sealed combustion appliances," you said this to me. Is this necessary to even state towards me? If it obvious that I have no clue to sealed combustion appliances, is it necessary to tell that person that they are unknowledgeable rather than educating them? Not sure how professional that is. The weblink provided would have been enough. David, just trying to learn here, and I wouldn't be emailing you if I had knowledge, isn't that what you're here for anyway, to teach people with no obvious understanding? 
Not sure if you're trying to seem like you're talking down to me, maybe it's not intentional, but again, here we go... you state, "Maybe you should consider taking a building science course." Really? Do I really need to take a course? I'm just asking some average homeowner asking questions, and I'm sure the average uneducated person can just be as well educated in a forum like this without having to be told to take an education class on the topic. And, like in anything, not everyone has knowledge of particular subject matters, thus, basic questioning should suffice unless one has desires or need to full-fill formal education. I have asked many many questions to professionals in my life, and have never been told once that I should consider taking a course. Most people answer my questions because they have the knowledge and care to share, or don't, or just don't have the knowledge. I guess you're done or tired of answering my questions, so maybe I'll take my presence elsewhere. 
And a-las, another, well, talk down to the unknowledgeable person reply, from you..."This basic topic is covered in a beginner's course." Honestly, I'm starting to laugh now, is this an insult to my low level of education of this topic?  
David, can I ask you what a top plate is, or are you going to tell me that I should take a building science course or this is basic knowledge taught in a beginners class? Or would you like to inform me without making me feel stupid? 
I'm almost afraid to ask you anymore questions.  
Geeze.... despite all of the information you provided, yes, it is good, and I do appreciate the science, articles, ect, but honestly....yes, this info should be taken seriously, however, I think it should be noted that this may or may not pertain the to the average homeowner. Is there an average consensus that points to the percentage of people that should be concerned? Should all people be concerned? I am getting the impression that all attic fans are a problem to a persons home. Go to homedepot or and view ratings of what people think of attic fans. Most are rated at or above a 4 out of 5 stars. If you read any of them, notice how people state the lower electricity bills and if not, then cooler home environment. Of course, if their fan is causing CO2 build-up, then that is a huge concern. Most of the positive reviews outweigh by far the negative reviews of attic fans, so yes, these aren't perfect, but regardless, doesn't all the positive personal reviews/life experience reviews count for something?  
Posted @ Wednesday, June 25, 2014 5:38 PM by John
@John, your questions show that you're very interested in understanding, not just having someone give you an answer. Your obvious curiosity is what prompted my suggestion to consider taking a course. I did mean that quite literally. 
On the other hand, I must apologize for my comment regarding atmospheric appliances. That wasn't very tactful. But the rest was intended in the spirit of encouragement, not condescension. Who knows, you might get hooked!  
BTW, a top plate is the horizontal stud (typically 2x4) at the top of the wall. Google 'top plate' for images.
Posted @ Thursday, June 26, 2014 3:28 PM by David Butler
I appreciate the follow-up, I guess I took you the wrong way, sorry about that. Your follow-up tells me that you are professional and just trying to assist. 
Thanks again.
Posted @ Friday, June 27, 2014 3:02 PM by John
Cameron Taylor - Loved your story. I live in FL and have a cement tile roof no ridge vent and only soffit vents every three feet all the around the perimeter of the house. Attic temps are reaching 110 to 120 . My garage temp is reaching over 100 with an ambient outside air temp of 90 to 94. My question: What exhaust fan should I use to reduce the temp and would it be possible to exhaust it through a a 4" aluminum duct with a hole in the soffit? Rick
Posted @ Monday, June 30, 2014 2:01 PM by Rick
Interesting conservation. Re: top plate issue, 
I don't understand the gap in the top plate remark. Please explain. My dry wall system provides an air tight envelope that contains conditioned air in the living spaces. 
My water heater is in my basement, attic fan would have to lower pressure in my basement to cause back drafting? 
Isn't radiant heat heat that has to be dispelled in order to lower attic temperatures? 
My attic fan is end gable mounted with a gable vent on opposite gable end of equal size, plus soffit vents, plus ridge vent. Do you think I could still have air transfer from air conditioned spaces? 
Thank you
Posted @ Monday, June 30, 2014 3:01 PM by John Callahan
@Rick, I think you missed the whole point of Cameron's story. 
@John C: unless every drywall panel in your home is gasketed, caulked or otherwise sealed, leakage paths will open up between drywall and framing members. Just because drywall is screwed to a plate doesn't eliminate the gaps. That's just the nature of the beast. 
In any case, gaps along the top plates is only one of the many possible leakage paths commonly found in homes. Take a look at Section 5 of this excellent reference: 
PDF Version: Energy Star Thermal Enclosure Checklist 
WEB Version: Energy Star Thermal Enclosure Checklist 
If your attic plane is indeed tight (easy to verify with a blower door and manometer), then a PAV would not rob your house of conditioned air or cause your water heater to backdraft. But that doesn't necessarily mean it would have a positive payback, especially if your ceilings are relatively well insulated. It all depends.
Posted @ Monday, June 30, 2014 7:28 PM by David Butler
Frustrated by your comments. You say you believe science and want to read about the science yet obviously you have not read anything or learned anything from the building science gained in the last 20 years. I just think before you try to debate something you need to do your homework. 
I do not have time to debate as I am busy solving problems related to people who don't understand air leakage but I was bothered by this argument "Wouldn't people have co detector going off" They were going off so the standard was changed. A normal household Co detector only goes off if: 
100 ppm averaged for less than 90 minutes;  
200 ppm averaged for less than 35 minutes;  
400 ppm averaged for less than 15 minutes.  
We do all admit levels much lower than this are unacceptable IAQ?
Posted @ Monday, June 30, 2014 9:41 PM by Chad
I see most of what I said was already covered. To many comments to read. Lola. Heated subject I guess. 
I am with Allison & David on this one and I started out as a roofer. I have built several homes and roofed close to 100. I moved into Home Performance and have treated over 100 homes top to bottom. I am now a WAP Energy Auditor and have been studying Building Science explicitly for the last 5 years. 
My point is I have been on both sides of this battle. I HAVE mass amounts of REAL World experience. My father was an old time "the house has to breath type of guy". I still managed to see the truth. Come on over from the dark side guys its nice over here. Everything just starts to make sense. 
Posted @ Monday, June 30, 2014 9:54 PM by Chad
Chad, just because I have no knowledge of this subject doesn't mean I have to do homework before asking questions. I think science is important, but that doesn't mean I need to be so passionate about learning and go learn. Asking basic questions is always a good start.
Posted @ Tuesday, July 01, 2014 1:16 PM by John
Logically and scientifically its likely that air could leak from a conditioned airspace around electrical/light fixtures in a ceiling. 
unless every drywall panel in your home is gasketed, caulked or otherwise sealed, leakage paths will open up between drywall and framing members. Just because drywall is screwed to a plate doesn't eliminate the gaps. That's just the nature of the beast.  
Unless you are including the area behind your sheetrock in the the conditioned airspace, gaps in any framing is irrelevant. Sheetrock should be taped, mudded and painted on the inside and there should be no cracks. The painted area inside your rooms, not in your walls are the conditioned spaces. 
So, lets assume there is leakage from the fixtures since no one on this blog has visible holes or cracks in their sheetrock....I hope. If the fan is pulling air, as we all can agree it will, like most things in nature, the air will follow the path of least resistance. Is air being pulled thru insulation(1 inch or 20 inches) more or less resistant than pulling thru the unimpeded openings of the soffit vents? Of course the soffit vents will impede less. Now less add the fact that if air is pulled from the conditioned space a pressure differential or vacuum is created in the conditioned space and the air naturally will need to be replaced to eliminate the pressure differential. 
Now we're looking at the path of least resistance into the conditioned space for this replacement air. It could draw air from the carbon monoxide producing appliances or what about bathroom ventilation fans the same way? Although neither are terribly difficult pathways there will be resistance. This resistance has to be added to the resistance of the air leaving the conditioned space for the attic, in the real world. 
So, when we look solely at the forces required to move the air, it is much easier for the fan to pull from the soffit vents than any insulation and holes you would obviously see thru your painted sheetrock coupled with the force needed to draw in replacement air. 
Now lets tackle the heat issue. Lets assume we have the recommended insulation in the attic. Whether the heat is UV, IR or squirrels having sex..... the more dramatic the temperature differential between the attic space and the conditioned space, the more heat will transfer to the conditioned space or anything adjoining it. Your ceiling and upper portion of your room WILL be hotter if the attic temperature is hotter. No study or manufacturer claim. The Second LAW of Thermodynamics- Thermodynamic equilibrium 
More work for AC unit! 
What if, like some of us, your AC unit for the upstairs is in the attic? Logic dictates that even your insulated ductwork and uninsulated air handler will have more heat transfered to it if there is a large temperature differential. The Second LAW of Thermodynamics- Thermodynamic equilibrium 
Even more work for the AC unit! 
So, cooler attic means less heat transfer to conditioned space and AC equipment. 
Cooler Attic........ 
We agree that heat rises. If not, take a look at the Ideal Gas LAW. We can also agree that if the outside temp is less than the temp in the attic & there are openings near the top and bottom of the roof, the cooler air will naturally come in from the bottom and replace the warmer air rising out of the top. The rate at which this air moves depends on a ton of factors. Amount of direct sunlight, ambient temp, roof pitch(volume of air), roofing materials, roof decking...... 
But what if the rate at which this air moves is slower than the rate more heat is being generated by the sun and the squirrels? You'll have a buildup in heat in the attic space. 
Work for the AC unit. 
If we were able to naturally exchange the heated air in the attic at a rate equal to or faster than its being heated up there would be no need for any mechanical assistance. Try jumping into your black car at 3PM in July turning your AC fan on 1 instead of 4. Of course you'd blast it until the temp was comfortable. Some of our attics need mechanical assistance removing the heated air at certain times of the day inorder not to overload our AC units. I think only one person mentioned that the fans have thermostats. Set it to turn on at a temp above which your AC isn't able to keep pace. So naturally, it wouldn't be an issue with it turning on in the winter, spring or fall.....for most of us. 
A combination of a house's characteristics, its climate and sun exposures are as diverse as we are. There is no panacea. 
Common sense isn't as common anymore. Steve 
Posted @ Thursday, July 03, 2014 1:12 AM by Steve
@Steve, no one is saying PAV's don't reduce attic temperature, and thus reduce the cooling loads. The question is whether they provide a positive benefit after accounting for motor watts and induced exfiltraton.  
When you do enough zonal pressure diagnostics and energy monitoring and modeling, it's pretty obvious the answer is... usually not.  
As for air not leaking into interior walls... again, when you do enough testing, and read studies by others who have done even more testing, it's obvious that interior walls are big contributors to attic leakage if those seams aren't sealed at the top.  
Air enters interior walls in multiple ways -- cracks around door trim and strike plates, light switches, receptacles, etc. It's easily to see this with a smoke pencil and blower door.  
Multistory, basement, and crawl-space homes also have a bunch of holes in the bottom plates of interior walls that provide a direct path from the floors system, which typically has lots of leakage paths to the outside. For this reason, single floor homes on slabs are typically a bit tighter than average, all else being equal.
Posted @ Thursday, July 03, 2014 4:37 PM by David Butler
Steve: To add to what David Butler said, I can confirm that air moves through the walls even when you don't see visible holes. Check out this article, with a video, about  
air leakage at electrical switches and receptacles. The video shows a lot of air moving through a switch during a blower door test. And the homeowner had even put a gasket in that switch to reduce air leakage!
Posted @ Thursday, July 03, 2014 5:15 PM by Allison Bailes
@Allison Bailes @David Butler 
My issue is with the topic title, "Don’t Let Your Attic Suck - Power Attic Ventilators Are a Bad Idea". A little extreme since PAVs do serve a purpose and everyone's home and environment is unique. 
no one is saying PAV's don't reduce attic temperature, and thus reduce the cooling loads David 
That's a good thing! 
The PAV's reduction in attic temps which reduce cooling loads should be what PAV manufacturer's and installers promote. If the units have a thermostat then it will only come on when the temp is at a level that thermal transfer, thru the insulation, between the unconditioned attic space and the conditioned interior envelope is taxing the HVAC system. Homes with HVAC systems in the attic are more susceptible because their units are in the unconditioned, high temp spaces. 
The purpose of the blower door test is to find leaks in a home by applying a force approximately equivalent to a 20 m.p.h. wind blowing against all surfaces of a building. A 20 m.p.h. wind blowing against all surfaces would never happen in nature even in a hurricane because its winds come from one direction. So the test only approximates the force, 20 m.p.h, not a scenario that might happen in a home. 
The video shows a lot of air moving through a switch during a blower door test. And the homeowner had even put a gasket in that switch to reduce air leakage! Allison 
My point exactly. Enough directed water pressure against a basement wall with no french drain to relieve will force the water to find a way thru. 
By sealing the fan at the door the test applies direct force specifically to the building's interior envelope. An attic fan does not. It's force is applied to the attic not the interior envelope. The blower door fan is either pulling or pushing air directly into or out of the interior envelope. The attic fan only pulls from the attic space which should have unimpeded soffit vents to supply enough abundant outside air. There would have to be no soffit vents or any other openings in the attic for the attic fan to need to draw from the cracks and holes in the building's envelope. 
The blower door test is for finding leaks. It's affects on a structure can't be transposed onto the attic fan just because they both have fans and move air. If the same preparation were taken in the attic space that is taken before a blower door test, then you might be able to correlate the two. Known openings would have to be closed. With the blower door, exterior doors & windows, flues.... With the attic fan the soffit, ridge or any other venting should be closed. Because there are cracks or leaks in the envelope does not mean the attic fan will exploit them. If there is no venting, the attic fan will emulate the blower door test. Then and only then. As long as the attic fan has the option to pull air from the abundant atmosphere thru attic venting there would be no need for it to pull from the interior envelope.
Posted @ Saturday, July 05, 2014 10:14 AM by Steve
I just read an article about attic ventilation linked above ( I have two powered attic fan ventilators in my house. Both of them became very squealy when in use (a bearing within the motor, I was told). To prevent overheating I disconnected the fans. I have been looking at buying replacement units, but upon doing some research, it appears powered attic fans are often discouraged. My question is, is it better to just leave the fans in the attic as is (disconnected) which would allow for hot air escape from their vents, or would something like a Whirlybird or other be a better idea? Thanks! 
Posted @ Thursday, July 10, 2014 1:29 PM by Richard
Interesting discussion. However, nearly all of the debate here is about cost/benefit analysis of attic fans, with the chief energy-cost culprit being leakage of conditioned air into the attic. I live in the NW, where very few homes, including mine, built in 2001, have AC. For a couple months every summer, it's uncomfortably hot, but not enough to justify an expensive AC system that sits idle 90 percent of the time. (Common conundrum here.)  
The house is not shaded and is blasted by the sun for extended periods in the summer, especially during our long days when the sun doesn't set until 9 p.m. 
I have not measured the temp, but have been up there enough to know that during long warm spells, the temp in the attic is alarmingly hot. And, like another poster here mentioned, the second floor of the home gets uncomfortably warm, with a huge temperature variation between the lower floors and upper, late in the day and through the night. The effect increases noticeably with consecutive hot days. It seems very clear to me that the existing ridge vents and soffit vents are not allowing the attic to cool to a reasonable degree before it reheats the next day. 
Attic vents seem like a natural solution, because seepage from AC to the attic is NOT an issue here. In fact, I welcome the notion of air from the stuffy house moving upward, anywhere, for any reason, to pull in cooler air from the lower-floor windows. We typically get comfortably cool temperatures at night, but that air simply is not at present being drawn into the house sufficiently to cool the upper levels.  
So I guess my question is simple: In a house with no central AC, when increased cooling of the upper level is the primary goal (I don't care if I spend money to run the fans; I just want to be able to sleep!), aren't most of the arguments against attic fans less relevant? 
It just seems obvious to me that, like another poster indicated, there's a giant heater sitting on top of my non-air-conditioned house, for weeks at a time. BTW, yes, my house has sufficient ceiling insulation, at least in the opinion of several builders and HVAC people. Thanks for any opinions.
Posted @ Saturday, July 12, 2014 12:20 PM by Ron
@Ron, since your home doesn't have A/C, you should consider a whole house fan. It will do a much better job cooling your attic and house than a PAV. 
Air sealing and perhaps some more insulation should not be discounted just because you don't have AC. A tight well insulated ceiling not only helps keep u/s rooms more comfortable in summer, but more importantly, it's one of the most effective strategies for reducing heating bills.  
I typically specify R49 for homes in northern climate zones with a verified tight envelope (especially ceiling).
Posted @ Saturday, July 12, 2014 1:42 PM by David Butler
@Richard, passive turbine vents are relatively inexpensive and do a reasonable job of enhancing attic ventilation if ridge venting is inadequate. But it's ok to just PAV's in place.
Posted @ Saturday, July 12, 2014 1:46 PM by David Butler
To Ron: You seem to me, to have described the ideal situation to use a WHOLE HOUSE fan which pulls air through the attic. Distinct from a fan whose purpose is focused on the attic. Whole house fans used to be common where I live in Texas, before air conditioning got to be the norm. They do make a major difference in comfort sometimes (when outdoor humidity is not your main enemy). 
You are right in seeing infiltration as a non-issue when you have windows open for hours and perhaps days at a time. I have a little bit of a sincere question whether your bigger benefit will be changing the attic temperature, or pulling in cool night air from outside. 
Posted @ Saturday, July 12, 2014 1:51 PM by M. Johnson
P.S. to Ron -- You still want to check whether you have combustion appliances which might backdraft toxic gases into your house with the fan running. Worst case scenario, the fan runs and nobody opened up windows, you get maximum depressurization of the house. This is not an energy cost issue, it is an issue for safety of your loved ones. Hope this helps.
Posted @ Saturday, July 12, 2014 1:56 PM by M. Johnson
Ron: I agree with everything David Butler and M. Johnson said and want to emphasize the latter's post script. It's not just about cost effectiveness. Backdrafting combustion appliances is a serious health and safety risk, and something that PAVs have done in homes. Be especially carefully if you have any natural draft combustion appliances. Most gas water heaters are of that type, but if you have an old furnace, it could be a natural-draft type as well. Read this article on combustion safety to learn more: 
3 Problems with Atmospheric Combustion Inside the Building Envelope
Posted @ Saturday, July 12, 2014 2:38 PM by Allison Bailes
Thanks very much for those suggestions. I was mulling the whole-house-fan approach myself. It seems a reasonable solution for these short-term heat episodes, especially when we have perfectly cool air outside at night that isn't being drawn into the house. The house came with a "whole house" fan in a second-story laundry room, but it's no more robust than a typical bathroom fan. Thanks also for the warnings about combustion appliances. My gas water heater fits the description.  
Posted @ Saturday, July 12, 2014 11:25 PM by Ron
I actually read the entire thread. Wow, there are some difference of opinions here. It is my understanding that the author is against the use of power vents but recognizes the need for overall ventilation. If that's true what would be the author's recommendation for my situation? I understand every single situation is different but this is pretty straight forward: My 2500 sq. ft. house has a 4/12 pitch, hip style roof with plenty of soffit area but a total of 18 inches of ridge. I just don't see any passive ventilation solutions for my situation. I need about 17 sq. ft of exhaust venting for a passive system (if I figured it correctly) and yet very little area to work with (the house basically comes to one central peak) Thoughts?
Posted @ Tuesday, July 22, 2014 5:01 PM by Kevin
Hi Allison and David, it's been a while since I questioned your premise of attic fans sucking air out of the home. Well, I had a blower door test and I am sold on your premise. There were air leaks everywhere, he was walking around using this smoke puffer, every outlet and ceiling light had air leaks, and he was showing me my unsealed top plates as well. So, sorry for the doubts, but I guess I needed to experience the real thing. However, I and did appreciate you answering all of my concerns/questions and have learned a lot since. Now wondering, if I air sealed my attic would I still have air leaks, with and/or without an attic fan? My only other concern would be, I do have an attic fan for the gable vent, not currently installed, I'd be interested in buying a smoke puffer, don't air seal just yet, install that gable fan, and see if air is being pulled in from my home. I have gable vents on each end, with only one soffit, so this could be an interesting test.
Posted @ Tuesday, July 22, 2014 9:13 PM by John
@Kevin, I am not an expert but studied the question a lot before having venting added to my own house. Google "Ross 150 roof vent" to see the product used on my roof, the roofer convinced me this was better than the alternatives I suggested. This model is supposed to be good in hurricane country.  
Try this pro-Ross brochure: ross150roofvent.pdf 
There are plenty of competitors, some square and some round, and the consensus I hear from experts is these all can provide good ventilation. Hope this helps.
Posted @ Wednesday, July 23, 2014 8:48 AM by M. Johnson
@John: Three cheers for your way of being straightforward with your curiosity, and for writing back to confirm what you learned!
Posted @ Wednesday, July 23, 2014 10:18 AM by M. Johnson
Way to go John!!! 
And really nice work Allison & David! Selling "Comprehensive Home Performance" one convert at a time!! 
Now we need to sell the "Measured" part in the front.  
(P.S.: Folks - "I see savings" isn't even anecdotal - what savings? How do you "see" it? Show us the analysis, or please don't waste space and look silly.) 
Posted @ Thursday, July 24, 2014 12:59 PM by Ted Kidd
John: Thanks for coming back and posting about your experience with the blower door test. I really appreciate it and am glad you came back to let us know.  
As for your question, it's pretty much impossible to eliminate all air leaks when you seal an existing home, especially if you have a low-slope roof, complex framing, or obstacles that limit access.  
If you do the experiment with the fan, please come back and let us know what you find out there. Be aware, though, that it may be difficult to see the effect just with a smoke puffer. 
Posted @ Thursday, July 24, 2014 1:18 PM by Allison Bailes
@John, glad you proceeded with a blower door test. Those of use who've been involved in this type of testing for years have no problem accepting the research that says attic fans are not a wise investment. 
Not sure if this has been explicitly stated, but here's why leaky ceilings have such a big impact on cooling cost and comfort: Under typical summer conditions, stack effect causes hot air to move from attic to the house. A powerful fan in the attic will reverse the direction, which is a large part of why they can improve comfort. BUT, for every cubic foot of conditioned air that the fan sucks from the house, a cubic foot of hot (often humid) outdoor air is drawn in through other leaks lower in the structure. If the house is not air conditioned, the better solution would be a whole house fan rather than a PAV. 
Allison rightfully pointed out that it's virtually impossible to get a really tight ceiling, especially in an existing home. But a big improvement in ceiling leakage (perhaps supplemented by additional ceiling insulation) will dramatically reduce heat gains through the ceiling, to the point that any further reduction a PAV might contribute wouldn't justify the electricity it consumes. A well insulated tight ceiling typically accounts for less than 20% of the total cooling load. So you might as well toss or sell your gable fan, but only after you report back on pre-retrofit smoke pencil testing!
Posted @ Thursday, July 24, 2014 2:03 PM by David Butler
Thanks everyone, I guess I am a convert, well for now I am, I was impressed to see this with my own eyes. You see, I'm really not that bad of a guy. Just someone who's trying to use reasoning and common sense, and as I just experienced, that only gets you so far. I do plan on doing some tests, it would be interesting to see what happens and I will definitely get back to you on it, but it will not be for some time since I am involved in a lot of other projects. The blower door gave me a real life experience that I can't deny, but I am curious to know how much air I am actually loosing with no attic fan or attic sealing. If I use a smoke puffer, I'm sure air won't be pulled into the attic from my wall/ceiling openings without an attic fan, so is sealing the attic needed if this is the case? I am sure I am loosing air, since there are openings up through the top plates, but I would think that amount of air loss would be so negligible that it barely makes a difference in the air lost from the home. I could be wrong, as we all know I already have been, but my logic to understand all of this is still churning. Considering what I already learned, I'd speculate that if I install the gable fan, I should see some air leakage, but not installing the fan has me to think how much air could I possibly be loosing. I did buy a smoke puffer, did my research and choose one to buy, so I will be doing some tests with no attic fan running, with the attic fan running with the 5 inches of insulation and no air sealing which I currently have, and will remove that insulation to see the difference with the fan on as well. It will be some time until I do this project, but once I do it I will be sure to share my findings.  
David, good point about the additional ceiling insulation and air sealing not contributing to heat gains through the ceiling, but will getting that hot air out help to put less stress and deterioration on the roof and shingles? 
Thanks again!
Posted @ Thursday, July 24, 2014 3:03 PM by John
John wrote: " If I use a smoke puffer, I'm sure air won't be pulled into the attic from my wall/ceiling openings without an attic fan, so is sealing the attic needed if this is the case?" 
Did you read my previous reply (2nd graph)? Without a PAV, the direction of leakage (in summer) is upward, from attic to house, not the other way around. And of course the ceilnig needs to be sealed! 
"...there are openings up through the top plates, but I would think that amount of air loss would be so negligible that it barely makes a difference in the air lost from the home. I could be wrong." 
You seem to be missing the big picture here. Although those leakage paths may seem insignificant to you, they're not. So yes, you're wrong. This is basic building science and if you're not willing to accept, then you may enjoy taking a class. A previous commenter took offense when I suggested that! Oh, geez, that was you!  
Regarding shingle life, this topic has been previously well covered above. In particular, see the exchange between Allison and MS on June 21, 2012 (whew, this is a long thread!)
Posted @ Friday, July 25, 2014 12:36 AM by David Butler
Thanks again. I am sold on air leakage into the attic, but this was done with a blower door test. I'm assuming if an attic fan is on then the results should be that home air now makes its way into the attic. I'm just wondering if there is no attic fan, how much air is seeping through, in either direction. I understand the stack effect, but isn't this radiant heat building its way through the insulation and eventually into the home. Isn't this heat not air being pulled through? Seems like heat and air are the same thing, but if no air is being sucked from within the home, such as a blower door test, then wouldn't the air from the attic take very long to get through the wall plates and into the home, therefore it would mainly radiate into the home through insulation? Without an attic fan, how is air loss identified if I can't see it or feel it, such as I did when the blower door test was performed? 
Keep up the good work, I appreciate the education.
Posted @ Saturday, August 02, 2014 10:48 PM by John
"I'm assuming if an attic fan is on then the results should be that home air now makes its way into the attic." Not necessarily. The attic fan has other sources of air to pull from. The soffit vents. If the soffit vents are restricted or if collectively they don't opening area for the air to pass then air may be drawn from the interior envelope. 
As you suggested, not only would it take time for the air to move through the plates and walls into the attic, the force required for that air movement is more than the force needed to pull through the unimpeded soffit vents. So which source makes more sense scientifically? The one requiring less force. The soffit vents. 
There isn't an commonplace scenario where you will be able to see or feel the movement of air like with a blower door test. You certainly can feel drafts but they don't even come close to the force exerted by the blower door fan. It is exaggerated for the technicians to be able to spot the leaks.
Posted @ Monday, August 04, 2014 6:13 PM by Steve
Steve wrote: "So which source makes more sense scientifically? The one requiring less force." 
This is a misnomer. All possible paths will be active, depending on the pressure differentials. The volumes through the paths will be relative to the aggregate size of the leakage paths and the pressure differentials. See my June 19, 2014 comment above.  
One thing that hasn't been mentioned is the impact of duct leakage. If there's dominant return-side leakage outside of conditioned space (a very common problem), this will pressurize the house relative to outside. It doesn't take but a few Pascals of difference to substantially change the balance between leakage from the house versus the soffit vents.  
Posted @ Tuesday, August 05, 2014 12:56 PM by David Butler
A lot of helpful ideas here. Thanks to all. I guess I agree that the PAV is generally a loser and could sometimes be a hazard, not to mention noisy. Seems to me a big ridge vent on top and soffit vents takes good advantage of natural convection and is probably the best approach for most. But even that can suck on your living space if you wanna get nit-picky (which, apparently you do). Everybody's situation is different and your ideal "building science" posturing is of limited use to many of us, but I learn from the "dialogue", so thank you anyway. Attic air needs to exchange and if you can't do it passively for whatever reason consider using a fan. Me? I have a gambrel roof with just a little triangle on top, insulated on the top ceiling, but the angled part of the ceiling is 2x4 rafters with black composition roofing on one side and ceiling on the other, with some thin insulation. Terrible, and not much can be done to make it something it isn't. I like to stick a sprinkler on the roof and put it on a timer. It doesn't take much to make a big difference. The water (ours is ~50F) quickly chills the materials, greatly reducing heat conduction into the structure, plus you get evaporative cooling as it dries off. Then the arguments about fans become moot ;-) Practical here in the Northwest where water is cheap and the heat is only an issue for a few weeks and mostly just during the hot part of the day. Works for me and I can sleep with my windows open. Hope that's of some use to somebody out there.
Posted @ Thursday, August 07, 2014 8:37 PM by Ben
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