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

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power attic ventilator building science infiltration air leakage

Trade 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.


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M. Johnson

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?

Allison Bailes

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.

M. Johnson

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.

Allison Bailes

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.

Ira Eisenstein

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"? 

Allison Bailes

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.

Tim Anderson

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.

David Butler

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...

Allison Bailes

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.)

Melanie Grove

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.

Allison Bailes

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

David Butler

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.

Melanie Grove

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.

Richard Taylor, AIA

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.

Dennis Brachfeld

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!

Steve Meyer

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!

M. Johnson

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.

Allison Bailes

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.

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.

Steve Byers

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.

M. Johnson

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.

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.

Gene Wilhoit

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. 

Tim Conway

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. 


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...


Sorry for the awkward post.. 
Note: this is a vaulted ceiling that seems to trap all the air


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


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.

Allison Bailes

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.


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.  

Allison Bailes

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.


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.  

Allison Bailes

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.


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.


Sorry meant to say Mister at the lady part.

Allison Bailes

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.


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.

Allison Bailes

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.


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. 

Robin Boyd

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.

L. Albertson

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. 

Luke Albertson


Allison Bailes

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.


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?

Allison Bailes

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.


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!


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?


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.

Bryan Hersman

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!


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!