The #1 Reason Power Attic Ventilators Don't Help

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power attic ventilator fan gable vent

Three years ago I wrote an article titled, Don’t Let Your Attic Suck - Power Attic Ventilators Are a Bad Idea. Nearly a hundred thousand page views and 93 comments later, it's still generating lots of heat. I don't know why so many people are so defensive about power attic ventilators, but here are a few of the things they've said to me in the comments:

You really should do more research before you post blogs like this.

Common Sense!!!! is in the building world, you really ought to check it out sometimes.

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.

Oh, and let's not forget: "You just are not getting it lady that it's not just the cooling effect we are after here." Yesterday I got another one so clever and witty I was at a loss as to how to respond:  "Ok... I'm going to say it because no one else will... Allison Bailes, you're an ASS!! :-)"

It makes me feel so much better that he used that smiley face at the end. ;~)

One reason power attic ventilators don't help

In my original article, I focused on makeup air. That is, I said that when you run a power attic ventilator in a typical house, yes, it will cool the attic down. A significant amount of that cooling is likely to come from conditioned air being sucked up from the house below. Most ceilings aren't air-sealed well, so putting a negative pressure on the attic will do that. I quoted Peter Yost in the earlier article, and in the comments, David Butler made a similar remark:

"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."

That's as true today as it was three years ago when I wrote the first article. And it's part of the reason that my state, Georgia, has banned power attic ventilators (unless they're solar-powered, which was a concession needed to get the grid-powered fans banned). But there's really a more fundamental reason that power attic ventilators won't help a lot, and for some reason, I didn't mention that in the original article.

The #1 reason

How does heat get into the attic? Well, it starts at the Sun and radiates down to the rooftop. We make sure that most rooftops can soak up as much heat as possible by using asphalt shingles. They're dark-colored. They're granular. And many roof surfaces are tilted toward the Sun for enhanced absorption.

That heat then conducts down through the roofing materials. The underside of the roof deck can get very hot, so hot you can't keep your hand on it. At temperatures of 150° F or more, that's a lot of heat sitting there in that plywood or OSB. Some of it will go directly into the attic air by conduction, but that's a small amount because air isn't a good conductor.

Radiant heat transfer from the roof deck to the attic floor

The main way that heat gets into the attic is through radiation. That hot roof deck radiates heat down into the attic. But that radiant heat passes through the attic air and hits the solid materials. It gets absorbed by the framing, insulation, the stuff you're storing up there, and, unfortunately, any ductwork and HVAC systems that are up there.

Those materials heat up. They give up most of their heat by conducting it downwards into the house or into the ductwork and HVAC system and then into the house from there. Some of that heat gets into the air above the hot materials on the attic floor, but the attic air getting heated up is a secondary effect. See it now? Here it is:

Using a fan to blow hot air out of the attic doesn't address the radiant heat flow from the roof to the attic floor. Much of that heat then conducts downward and finds its way into the house.

Trying to solve the heat gain problem in your attic by using a fan is like lying out at the beach with a fan blowing over you and thinking you're not going to get a sunburn. A radiant barrier would be a better way to attack this problem, but their cost effectiveness is marginal.

Regarding this point, Danny Parker wrote in a literature review of attic ventilation, "Detailed simulations suggest that the heat transfer in an attic to a residential building interior in mid-summer is dominated by radiative gains from the hot roof decking directly to the insulation surface. This mode of heat transfer is more effectively limited by 1) increased attic insulation, 2) a truss-mounted radiant barrier or 3) a white reflective roof surface that limits solar gain to the attic structure."

Power attic ventilators cause problems

I've been in lots of attics. I've seen lots of power attic ventilators, including the one in the top photo. That one was hooked up at one of the gable vents. The other 7 were spread across the roof, and they were naturally fighting against each other. There's no way there was enough open vent area in that attic to supply all 8 of those fans.

In another home, I found three power attic ventilators in the roof. We had been called in to solve a mold problem in two of the bathrooms, and those three fans turned out to be the main problem. We turned them off and the negative pressure pulling humid outdoor air into the bathrooms has gone away.

Addendum: What to do instead of using a PAV

If you've got a vented attic that gets hot in the summer, your first option is leave the attic vented and make sure the ceiling is air-sealed and insulated as well as possible. If you've got an older home and it needs more insulation, you've got the perfect opportunity. By removing the old insulation, you'll expose the ceiling. That's your air barrier, and it's probably not doing a good job at it. Here are a few articles that can help:

The 3 Rules of Air Leakage (Plus a Bonus!)

Mind the Gap - Air Leakage at the Top Plates

Hidden Air Leakage Sites in Your Attic

Once you get the ceiling sealed up, then and only then are you ready for more insulation. You could get a blower door test to make sure you've got it airtight. When you're really ready for insulation, you've got three choices: batts, blown, or sprayed. Another option is to spray foam on the roofline. It'll be more expensive, and you'll need to think about a ventilation plan for the house as well as treating the attic air to prevent moisture from collecting in the foam.

If your building enclosure is in good shape, a power attic ventilator is unnecessary. If it's not in good shape, air sealing and insulation will save you more money and make your home more comfortable than power attic ventilators.

 

Related Articles

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

Power Attic Ventilators Banned by New Georgia Energy Code

Oooh, Shiny Stuff! - Radiant Barrier Fundamentals

 

Footnote

My friend Mike Barcik gets credit for this analogy, except that when he tells it, you're lying naked in the sun. Naturally, this being a family blog and all, I don't talk about naked people here. (Don't be scared; you can click the link without leaving the Energy Vanguard website and without seeing anything indecent. And no - that's not me in the photo.)

 

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Comments

Arlene Z. Stewart
May 29 2014 - 9:03am

Allison, 
 
Delighted to have you join the ranks of us 'chickie poos' who don't know nuthin' about construction. 
 
So, I'm often at a loss for what to tell people who are concerned about storage in attic spaces. Like it or not, George Carlin was right, we humans collect a lot of stuff. Often, especially in Florida, the only place to keep that stuff on site is attic or the garage. Obviously, the temps in the attic will make that location really undesirable. Why bother to pack away the toys for the grandkids when all they'll have is a plastic glob in 20 years? It's a lot easier to tackled in new buildings where it makes sense to radiant barrier even the garage space because it takes the heat down to a storable level.  
 
I just haven't found good retrofit option yet. Any ideas? maybe a separate blog topic?

scott suddreth
May 29 2014 - 9:05am

Great article Allison! I teach this in building science all the time. "You cannot cool radiant surfaces with air. For example - we use soffits and ridge vents to move air in attics due to the difference in temperature. This creates a nice stack effect and moves air the hotter it gets. My question is How is that working out for you? Even with this air movement the attics still reach 150 degrees! 
 
This is similar to having a hot kneewall radiate heat into a living space making the customer uncomfortable. Until you address the hot surface created by the kneewall, blowing colder air into the room doesn't fix the problem.  
 
This is why passive solar homes can stay toasty on a sunny day when its 0 degrees outside. Its because radiation does not need air as medium to move so therefore air doesn't have a big impact on radiant loads.  
 
What is the ROI on powered attic ventilators? Anyone got any proof they are a good investment?

Parker Cannady
May 29 2014 - 9:05am

It's funny that you posted this article this week because I was listening to the "At Home with Gary Sullivan" radio show this past weekend and a Homeowner called in wanting advice on how to improve her attic. He recommended blown-in insulation and an attic fan if she could'nt afford spray foam. I caught myself arguing with the radio before I turned the station out of frustration.

ted kidd
May 29 2014 - 9:14am

Someone said "you can fix ignorance but you can't fix stupid." 
 
Allison, I admire your passion for attempting to fix both.

W. Blake
May 29 2014 - 9:18am

Some things just won't go away like unqualified installers and products. Criticism is human nature. Use profanity and belittling when one runs out of logic and proof. Glad I am not married to one.

Steve Waclo
May 29 2014 - 9:29am

Hey "Lady", er, Allison ??, 
 
Here in the high, dry desert of northern Nevada, I am quite fortunate in being able to use nighttime ventilation for nearly all of my cooling. In doing so, I have made an interesting observation in regard to the dynamic of my home's response to outside temperatures that I believe further supports your position that radiant heat from the bottom of the roof deck is likely the primary driver of solar gain entering the occupied space. Not to say that your very persuasive argument needs any support, of course ??. 
 
I'm sure I have described my situation before, very likely within the 93 comments you mention, but here goes: 
 
Diurnal temps here in NV, especially during spring and fall, can approach 35*, and by power ventilating during the night, I can often bring inside temp down into the high 60's by 6am, from a previous evening's high of perhaps 80*. What I found to be fascinating was that when the sun sets, and ambient begins to plummet (hey, it's a desert!), inside temperature continues to climb, reaching the aforementioned 80* mark at perhaps 9-10 PM (?!?!).  
 
This always baffled me, and although I must admit I am rather easily baffled, it was not until I read your discussion of the building science behind the fallacy of attic ventilation, I never understood what was going on. Now I know that heat radiating from the bottom of the roof deck and, to some extent all the trusses that have been baking in the attic all day, finally penetrates the R-40 attic insulation and turns my ceilings into radiant panels! 
 
Here's an additional point to ponder and I welcome your insight. Would not radiant cooling of the roof into a dark night sky (common here) cause at least some of the stored heat to leave by that mechanism, thus reducing, to some extent, radiation downward? I would not know how to begging running the numbers. 
 
Best wishes and don't forget, "Sticks and stones...". Well you know the rest! 
 

David Eakin
May 29 2014 - 10:28am

Steve - to your point, I would also like to hear the experiences of anyone who has replaced their asphalt roofing system with metal (either standing seam or aluminum shingles) as metal is a much better radiant medium.

Steve
May 29 2014 - 11:23am

All of your info makes sense. However when I lived in Central NY we had a whole house attic fan and we found that if we turned it on when we got home from work, it quickly lowered the temperature in the upstairs bedrooms and with the reduction in outside temps at evening time the attic did not heat up again until next day. Prior to the fan, it took hours for the upstairs to cool down. Our fan was installed in the upstairs hall ceiling blowing into the attic. When it would come on the slats under it in the ceiling would all flip open. It seemed to act like a late day thunder storm, reducing the heat till the cycle starts again next day.

Skye Dunning
May 29 2014 - 12:55pm

I once had a house with R/H running in the 70's from an attic fan pressurizing the house. I put my fan in the blower door and the canvas snapped in before I even turned the fan on. The house was under 12 Pascal negative pressure from the fan! 
 
In another house I hooked a duct blaster up to the ducts (ducts in the attic) and I was getting pressure before I turned the fan on. It took a a minute of head scratching to realize that my gauge wasn't faulty. The attic fan was pulling air from the house, into the ducts, then out of the ducts & in to the attic through leaks in the ducts. 
 
That same week, in the same area, I saw an attic fan someone had just purchased, right next to a dehumidifier - also new in the box. A marriage made in heaven...

Bob Ellenberg
May 29 2014 - 12:56pm

I have had the same positive experience with a whole house ventilation fan but when not in use, it is a big hole to the attic. I installed a Tamarack sealed insulated model for one client but don't know if there is any data on how well they seal.

Cameron Taylor
May 29 2014 - 12:58pm

For David Eakin and others reading the comments concerning the request he made: I have direct experience with changing my house roof from asphalt shingles to white reflective standing seam metal roofing. I measured attic temperatures prior to and following this change, and since the change I have also observed overnight behavior. 
 
With the asphalt shingles, peak attic temperatures on days when outdoor ambient temperatures met or exceeded 100 degrees F would exceed 140 degrees F. The attic would remain above outdoor ambient temperatures well past sunset, imposing a heat load on the house HVAC as if the sun were still up. I remember many nights where although the indoor air temperature and humidity levels were within standard comfort range (75°F at ~48% RH), the room was still uncomfortable due to an elevated mean radiant temperature of surrounding surfaces, particularly of the ceiling beneath the attic. I would require a fan to sit in my easy chair comfortably, or a running ceiling fan over the bed to sleep well overnight. 
 
Since the metal roof was installed this has all changed. Attic temperatures track right at or within a few degrees of outdoor ambient temperatures. After sunset the attic temperatures continue this trend. Where my attic temperature sensor is located may be slightly affected by heat loss from the house whenever the attic temperature falls below indoor temperature, so the effect of night sky radiant cooling on the metal roof and attic coupling is to me not sufficiently measured. However the affect of this roofing material change is remarkable elsewhere. HVAC outlet temperatures in cooling considerably cooler than under similar conditions, mean radiant temps much more comfortable, HVAC run time reduced, energy bills way down. Win. 
 
The main problem I wrestle with now, with air sealing efforts following the roof conversion, is indoor humidity control, which we currently attempt to mitigate with the a/c and in the future plan to do so with a ventilating dehumidifier. In shoulder seasons the a/c run time is considerably less, so thanks to the recent addition of an Ecobee thermostat, it can now initiate a call to dehumidify using the a/c and overcooling up to my preset limit. But I digress... :)

David Butler
May 29 2014 - 3:27pm

@Steve & Bob: here we go again... this thread is about power attic ventilators (PAV's), not whole house fans! Different animal. Different topic. 
 
@Steve W., also being a high desert inhabitant, I can relate. But you'd get more benefit from a whole house fan or even a window fan. The best way to reduce ceiling loads is with air sealing and insulation. 
 
Detractors need to keep in mind that the temperature at top of attic is a lot higher than temp at top of insulation, which is what matters. 
 
@Cameron, great point about impact of ceiling temperature on MRT and comfort. We think of ceiling fans in terms of evaporative cooling of the skin. However, a ceiling fan in a room with attic exposure also acts to disrupt the air film, reducing it's R-value, which brings ceiling temperature closer to room ambient. 
 
I had a client whose wife didn't care for the sensation caused by ceiling fans. If the bedroom ceiling has attic exposure, it's still wise to install a ceiling fan. Just reverse the direction and use the lowest speed. This will lower the ceiling temperature and MRT, thus allowing a higher AC setpoint at night.

Steve Waclo
May 29 2014 - 4:45pm

David, 
 
Thanks for your comment! 
 
I went slightly off the reservation in discussing nighttime ventilation only to demonstrate an effect I observed in my home regarding Allison's thesis that attic temps are not a primary driver of occupied space conditions. It's the hot, hot underside of the roof and subsequent radiation. 
 
My apologies for using the rather vague term "power ventilation" in my post. Essentially, I do have a whole house fan but because of the Rube Goldberginess of my set-up I was hoping to avoid full disclosure. 
 
A neighbor gave me an elderly propeller fan with an approximately 300W (per Kill-A-Watt) motor. I adapted it to my attic access hatch, used a GFCI and a timer for (limited) control. Noise was an issue (I close the closet door to eliminate the "hole to the attic" effect during the day) and I discovered flex duct, in 18" diameter is hard to come by. My intention being to move the fan assembly further into the attic. 
 
As you may know, many dogs enjoy hours of harmless fun running through long, tubular "dog runs" and I scored one of this on the interweb for cheap. The fan now resides on rubber mounts, about 20' into the attic and noise is no longer an issue. Not surprisingly, the 90* bend at the attic hatch desperately needs work (turning vanes ???) and the entire length of the tube shrinks down against the coiled wire, but hey it works pretty well. Summer electric costs are substantially less that winter. 
 
Control remains an issue, and until I connect the cooling only stat I bought, over cooling remains an issue (awoke to 65* this morning ??). 
 
Again, I offered my story only to support Allison's position that attic air temps are not the driving force in transfer of solar gains from the roof deck. 
 
Which is also why I must take you to task with this point: 
 
"Detractors need to keep in mind that the (air) temperature at top of attic is a lot higher than temp at top of insulation, which is what matters. " 
 
Correct me if I'm wrong but I thought one of Allison's points is that attic air temperature, in well insulated, air sealed attics, is not the primary driver of occupied space heat gain. 
 
Thanks for your comment! 
 
Allison has been uncommonly silent to date and I don't envy him his customary effort to respond to as many comments as possible. Jump in anywhere, Allison.

John Proctor
May 29 2014 - 5:55pm

Not Allison's #1 Reason, but perhaps his #2 reason: Depressurizing the attic draws cooled air from the house (cooling the attic). The cool air removed from the house is replaced by hot outside air.  
If the conditioned space to attic were perfectly sealed, this would not happen, but even in new construction in California we find: "51 percent of the house leakage area was between the attic and occupied space..." http://www.proctoreng.com/dnld/ECO_Report_CEC.pdf 
 
I also have to mention that since there is a leakage connection between the occupied space and the attic that depressurizing the attic can depressurize the conditioned space and the combustion zone space. I have personally experienced immediate backdrafting and CO off scale when Jim Fitzgerald turned on an attic ventilator while I was testing a furnace in the house.

David Butler
May 29 2014 - 6:01pm

@Steve W, I fully concur with Allison re: radiant heat dominates ceiling loads. I was simply explaining why the conducted heat transfer component is smaller than what PAV defenders would have you believe when they cite large reductions at top of attic as evidence. The reduction at top of insulation (which is what determines any reduction in the conducted load) is typically less than 10F. 
 
Regarding your attic fan... I misunderstood, thinking you were cooling the attic in order to indirectly cool your home. But 300 watts is a huge power draw. You can get much more efficient whole-house fans for the job (check out quietcoolfan.com, ES series). And if you're mainly concerned with nighttime-bedroom temps, a good window fan uses even less power. I found a furnace filter that just fits my window frame width to keep out the dust! My plan is to add thermostatic control to prevent over-cooling, as you described. Fun stuff.

John C
May 29 2014 - 6:50pm

Had a long discussion with an attic fan believer once.... 
 
He said: 
 
Before I had the fan, my upper story would be baking hot whenever the attic was hot, and would cool down when the attic cooled down. 
 
After I had the fan, my upper story doesn't get hot any more. 
 
QED. Take that. Attic fans work!! 
 
I said: Did you check that your utility bill was lower? 
 
No. I didn't bother. 
 
Did you measure how much cooler your attic was after the fan was added? 
 
He said about 10°F. 
 
I said: 
 
So, taking you attic from 130°F to 120°F, above R-50 insulation completely removed the heat leaking into your second floor? Sounds fishy to me. 
 
I said: 
 
You can also calculate the BTU load...50°F Delta-T, and 750 sq ft of R-50....BTU/h = 750*50/R-50 = 750 BTU/h expected heating...about the same as 2 100W bulbs, hardly going to make a whole upper story be super heated. 1/15 ton of AC would fix it. 
 
Dropping the temp by 10°F would reduce the heat load to the second floor by 20% (of 750 BTU/h), so the attic fan reduces the conduction cooling load by 150 BTU/h, or 50W!! 
 
The math doesn't add up. 
 
Instead, I argued that before the fan, reverse (summer) stack effect causes the heated air to flow through the FG and his not airsealed attic floor into his second story living space, heating it a lot more than 750 BTU/h (duh). 
 
After installation, the attic fan depressurized the attic, reversed the flow, so his second story was now being cooled by drawing the cool air from his first floor. The makeup air is prob leaking in in his basement, increasing his basement humidity problem (which he confirmed he had) and increasing his AC bill and dehumidifier energy consumption. 
 
:) 

Nate Adams
May 30 2014 - 12:37am

I've been thinking about writing about this as well! =)  
 
The big complaint that leads to an attic fan request is usually a hot second floor, which can largely be mitigated by air sealing the attic and installing a generous bit of insulation - drastically reducing conduction and convection. All of a sudden that attic fan isn't needed, anyway.  
 
Or the walls could be getting heated up by the sun during the day and conducting into the house during the evening (like my master bedroom or the brick house I audited yesterday). 
 
Of course, an oversized AC that cools the first floor quickly, satisfies the thermostat then shuts off the AC before the second floor gets cooled could also be a root cause.  
 
The only way to figure it out is to go look, house by house, room by room and design a solution to fit the homeowner and the home. Yay energy audits!

Bill DIY-er
May 31 2014 - 3:06am

What do folks think about installing a reflective surface to the underside of the rafters as a solution to the radiant problem?

Mimi
Jun 12 2014 - 10:33am

Thank you Allison Bailes! I am thankful I ran into your blog. I am a homeowner who was about to embark on paying an installer to repair/install new attic fans (electric) in a ceiling of the main house and a gable-mount in the guest house detached from the main house. Due to your blog, I am now going to pass on all that :) Your logic makes sense. Knowledge is power!

mark
Jun 12 2014 - 7:45pm

here goes...while this information MAY be accurate it is also misleading. certainly, some installation site should NOT be used. However, to say imply they are ALWAYS inappropriate is shortsighted and ignorant.  
 
i live in a area with 100+F temps common from May thru Sept. 20yrs ago i installed a gable mounted fan. on days prior to installation i took the temperature of truss members with a infrared temp sensitive yielding a 150F reading i completed the installation at 2pm on what turned out to be the hottest day of the year in Mid july. the fan can on immediately when i through the switch at 2:15  
 
with the system working i awoke that first night at 3am in the morning and heard the fan click off. each succeeding night after that it switched off at 11pm no moatter if the daily temp high was 90 or 110. so there was at least a 4hr time period where the temps would have been 105+F (themostatically controlled switch was set at 105F). the actual time period was most likely greater since that first night the attic temp surely would have been greater past 3am if it hadnt been for the fan exhausting heat. 
 
i took the temp of the same truss members on several days the fan would come on (typically around 10am) during midday highs. the result...118F. sure this is still HOT but it is considerably and substantially BELOW the 150F it otherwise would have been. 
 
still not good enough for you? on work days while gone we would NOT have the AC on. before installation the temperature in the house upon arriving home would be 92+. YES that is a hot house to come home to. after installation the inside temp would be 84. That would be an 8F temperature difference that now relieved the AC from having to cool down....not just the air in the house though...that would be the furnishings, the walls, etc. that is a considerable difference. 
 
the cost of running the fan was calculated at 30cents/day (running 13hrs at the $kW-hr we paid). the AC ran considerably less after the fan was installed PLUS we had a better quality of comfort which is hard to put a dollar value on. this has further saving due to the AC running less the years it will run before something wears out now is extended. AC work is EXPENSIVE. 
 
ADDITIONALLY, before the install the air coming out of the ducts (rigid metal flex ducts with little insulation installed in the late 70s) when initially turning on air would be HOT for approximately 30s. AFTER install the ducts would blow hot air (but not as hot) for approx 5-7s.  
 
and you tell me it wasnt worth it. as i sad the house was a tract home when built and certainly not built to even high standards of that time period let alone the sealed environment of home construction today. 
 
your article is irresponsible in that it gives a blanket recommendation for a product. maybe they arent recommended for MOST of the US, including Georgia, but here in the central valley of CA, and the arid southwest they will certainly have a payback.  
 
installation for my including fan and a switch (there are some days you dont want to remove heat from your attic when the temp is above 105F such as the early spring and late fall) and associated wire was less than $75. if it didnt pay for itself in the first year it certainly did by the second AND the house was MORE comfortable

mark
Jun 12 2014 - 11:46pm

boy...that installation of the gable vent is sloppy. the vent should be shrouded and the fan should only have a hole the diamete of the fan to mount against. without that set up much of the air exhausted by the fan simply bounces back off the louvres and into the attic. 
 
that said...i know many people will say that would only make the situation worse with MORE air being drawn from the house into the attic (if more is being sent out of the attic). but there ARE times when a PAV can do the trick. but NOT always i understand. this installation in the above photo could very well be one of the BAD sites. looks like there is little insulation to begin with which would allow less resistance to airflow coming from the interior of the home. 
 
before i installed mine, as mentioned in an earlier reply, the AC (roof mounted) could barely keep up with the demand. on the hotter days, above 97 or so, once it came on it wouldnt go off until well into the morning. after the installation the AC would cycle on and off. with this condition the attic ventilator could NOT have been pulling an appreciable or substantial amount of air out of the house or the AC would have continued to run continuously. 
 
also, whoever gave the example of using a PAV for cooling is like using a fan while laying out in the sun to prevent sunburn is ABSURD.  
 
first...what gives a sunburn is UV. the majority of radiant heat received is more greatly IR. but point well taken (this isnt the main argument against what was said) 
 
second, while radiant energy does continue to be emitted from the underside of the roof deck, that doesnt mean the roofdeck cant be cooled by a little flowing air and that is EXACTLY what happens. the flowing air cools the underside of the roofdeck and also cools anything else in the attic which is radiating energy to the top of the insulation.  
 
when considering this, one must be knowledgeable of the fact that radiant energy is dependent on T^4, with the temperature being K, or Absolute otherwise known as Kelvin. it is this that is SO IMPORTANT: temp in K AND to the 4th power. so you see...if you can simply reduce the temp a little less than 2F (1.8F actually) from even 140 to a bit more than 138F, then the savings are ~140W/m^2 or about 13W/sq.ft. but in some area, mine for instance the attic temp can get over 155F. at this temp, because of T^4, the radiant energy emitted is about 13W/sq.ft. of course building wood materials arent perfect emitters so lets take 80% of these numbers: 10W and 12W/sq.ft.  
 
lets take a modest home with a 1500sq.ft attic floor and calculate BTUs for the entire attic floor: this comes out to be ~60000BTU. that is ALOT OF ENERGY falling on the top of the insulation. and REMEMBER that is with only a 2F degree differential. if the attic surfaces cool down 10F then the energy is 5X as great. 
 
though you state the moving air does NOT cool the attic surfaces NOWHERE do you state any measurements of such. this makes me believe you are utilizing only flawed logic to determine your opinion 
 
possibly something else that isnt taken into account with air being pulled from the house with a leaky ceiling, is how much heat from air infiltration would find its way into the living environment from the attic with NO fan exhasting the attic. if its a leaky attic then it can leak both ways. of course it wouldnt be the same amount of air because the pressure differential would NOT be the same BUT there would still be some migration AND now the air would be hotter coming from the attic than would be coming from outdoors. 
 
one last thing is...if the AC is NOT on, the house is heating up. the ceiling radiates heat according to T^4. if it is possible to cool that ceiling down just 2 degrees then alot of radiant energy is NOT falling on the objects in the house INCLUDING people and making them feel hot and also heating up all the objects in the house

mark
Jun 12 2014 - 11:53pm

one more thing that is flawed in your article. you say some of the heat will go directly into the attic but not much because air isnt a good conductor. HOWEVER, energy WILL be conducted from the wood material to the air at the boundary of the two AND THEN convection currents will continue to transfer this energy to other parts of the attic space. granted, the majority of the energy transferred is radiant but as for heating up the air in the time period it does, convection and conduction increase the rapidity that it occurs.

mark
Jun 13 2014 - 1:28pm

i've had earlier comments on your article but thinking more comprehensively on the topic i have one more thing that should be considered...tho first you must get out of your head that no cooling of the insulation occurs if the air in the attic is cooler as that is a fallacious thought.  
 
R-value ratings are calculated at a temperature of 70F (possibly 75F, i forget). as temps rise R-value goes down. R30 at 130F is around R19 if i remember right. at 150F it is even lower.  
 
so as temps rise in the attic, and hence less energy is convected off the insulation into the attic, the insulation on the ceiling is less and less effective, potentially by a LARGE amount. by cooling that air, and hence the insulation even a small amount, it is more able to fulfill its purpose and reduce the energy transfer into the house from the attic. 
 
as for your erroneous thought on cooling the attic. the air in the attic gets hot. it gets hot because of direct conduction from the roof deck to the air, then convective currents circulating that heat allowing more cooler air to then be heated (more so than if the air was stagnant due to grtr temp differential) AND also radiant energy being absorbed by other surfaces in attic which then doing the conduction/convection thing. 
 
the fact that the air in the atic gets hot in the first place, unbearably hot in fact, shows that energy transfer occurs from the solid surfaces to the air (and some of it actually is radiant energy from the surfaces to the air directly). doesnt it seem prudent then that reducing the air temp would then reduce the surface temps? 
 
as heated air is evacuated from the attic, cooler air is brought into the attic maintaining a higher energy transfer from the roof deck, and other surfaces, to the air thereby reducing those surface temps...INCLUDING that of the insulation.  
 
ALSO, as the roof deck is cooled, even slightly, due to radiant energy being proportional to T^4, a LARGE effect can be had with a small difference in temperature in REDUCING the amount of energy that is radiated to the top surface of the insulation and keeping it at a lower temp so it can do its job better. 
 
Additionally, through time conduction/convection from the insulation surface to the air above is increased the GREATER the temperature difference between the insulation and the air. and, as the members of the roof trusses are cooled, they will also absorb MORE radiant energy from the insulation surface and the roof deck further reducing the temperature of both, tho i admit, this is a smaller consequence. 
 
GRANTED, it is difficult to cool the radiant surfaces of the roof deck and the insulation with air movement. in fact, it is a small amount relative to the energy content in those surfaces. that is why the attic can still get to UNBEARABLE temps even with a PAV. 
 
Your belief has some grounds in fact but the all inclusive application of those beliefs is misguided 
 
i once read quotes of a university professor stating that the use of a mulch in a garden did NOT save water because the mulch interfered with the water absorption of the soil...and absorbing water itself. of course we know this to not be true. thats like saying insulation doesnt work. your arguments appear to be along the same lines

David Butler
Jun 19 2014 - 3:22pm

@Mimi, in new construction, roof sheathing with factory applied radiant barrier can be cost effective (RB adds about 10 cents per ft2), but retrofit radiant barriers cost many times that amount, and will never save enough to justify the the cost. Better to spend your money on air sealing and insulation.

mark
Jun 19 2014 - 4:24pm

@ David Butler 
i am concerned about the information you provide. you replied to Mimi, though it actually should have been David DIY-er, regarding radiant barriers in the attic. while it is MUCH easier to have a RB installed at time of construction, ie on OSB, in CAN still be cost effective as a retrofit. 
 
500sq.ft. (4 x 125') rolls of perforated RB (foil with scrim) are available at Lowes for $75 or ~$0.15/sq.ft. this is NOT a huge cost.  
 
If one was to rely on a contractor to install it, the cost I find is exhorbitant and really not worth it. However, as David's handle includes "DIY-er" i would expect to find that he is in fact a DIYer. that said he could do it for the cost the material, a mere 5cents above other methods 
 
it can be a pain to install in trussed attic but it is possible. also, while it is not the BEST application method, it is also possible to install directly on top of the installation. critics will say this method is unacceptable due to dust accumulation reducing the reflectivity of the material. However, studies have shown that typically this reduction is in the 5-10% range. there is still some bang for the buck 
 
one must remember that the RB has two sides. so, although the top side gathers dust the bottom side is protected.  
 
again the critics will say "but then there is NO airgap" which is necessary for a RB to be effective. i think critics just like to be negative and hear themselves talk. what they dont think about is that the fact that the RB wont lay perfectly flat and will NOT be in perfect contact with the insulation thereby maintaining an airgap. the areas of non-contact can be enhanced during installation improving on this area of non-contact, ie intentionally put peaks in the sheets when installed. 
 
also, the insulation isnt a perfect surface anyway. so even if the RB was perfectly flat, there is alot of space between many insulation fibers and the RB to give some effect anyway.

David Butler
Jun 19 2014 - 6:36pm

@Mark, you are correct, my comment was intended for David-DIYer.  
 
We agree that professional installation is a deal killer for RB. I've seen prices as high as $1 per ft2. For a DIY, I guess it depends on how much one values one's time. But man, that's hard work, and hard to do a good job in an existing home, even with a stick-framed roof. Forget about trusses. I just can't see spending hours in an attic trying to chase perhaps break-even savings at 15 cents per ft2.  
 
A large study showed that factory applied RB (~10 cents/ft2 extra) is only marginally cost effective in homes built to 2006 energy code. Over a large number of homes in Houston, as I recall, RB saved an average of 3% of cooling energy. Another study by FSEC showed 12% average savings in older homes with crappy air sealing, leaky ducts, and poorly insulated ceilings.  
 
I recall a study re: RB laid on top of insulation but can't put my hands on it now. It *might* reduce ceiling loads by 10%, but not 10% of total cooling energy. Maybe you can post a link to the study? 
 
Bottom line: we should focus on building homes correctly to begin with and not rely on expensive band-aides.

mark
Jun 20 2014 - 12:25am

@David Butler 
without knowing all the information regarding the large number of homes it is difficult to tell. was the use normalized based on degree days from previous years? wind? could roof pitch make a difference? set point of t'stat could also make a difference. etc 
 
i know in my current house, 1-1/2 story home, the knee walls in the second story were insulated but the interior temp of the wall would still be hot enough in the summer months that leaving your hand on it for more than 5s was uncomfortable. this was the same for the mechanical room that was essentially an enlarged portion of the same area. i installed RB on those walls and the temp dropped 10F as taken with an IR thermometer on the interior. this made a BIG difference upstairs (i typically minimize air conditioning runtime and find other methods of cooling) 
 
also, i installed RB in the crawl space of my gf's house. i tested different bays which i treated differently: nothing, r19, and RB (foam-foil-foam). the RB bays produced the same floor interior temp as the R19. both were several degrees warmer than untreated tho i dont recall the numbers.  
 
it was considerably easier to install the RB tha the fg insulation...cheaper too

David Butler
Jun 20 2014 - 4:45pm

@Mark, my previous comments were only in reference to RB at roof, or top of insulation. Knee-walls are a different story.  
 
Knee-walls are exposed to same temps as ceiling, yet typically are insulated to a much lower R-value. Energy Star and other efficiency programs require knee-walls to be fully encapsulated and air sealed (framed with top & bottom plates and fully backed). You gotta get everything right. Incorporating a radiant barrier on the backs of knee-walls (typically in the form of foil-faced polyiso for durability) is not a bad way to go. 
 
We're getting off topic, but here's a link to the Houston study: http://bit.ly/1m4W792

wd
Jul 2 2014 - 12:21am

With a hot attic, the ceiling heats the building. So, it's natural to think of cooling the attic. A fan cools things. Voila. 
 
I've had better success with insulation between the rafters and the hot roof radiates more to the surroundings. The attic cools and the attic fan goes off. The a/c usage goes down. 
Similarly, painting the roof white causes the roof to cool. Sunshine is reflected instead of heating the roof. The cooler roof radiates and conducts less to the attic. The cooler attic eases the a/c load. 
 
In these cases, the insulation and the paint get the job done with no batteries required. And, who wants to listen to an attic fan run? The idea is to reduce electricity usage without sacrificing comfort, isn't it?

EW
Jul 10 2014 - 1:27pm

So what are the solutions? Your website has lots of great insight and information, but I've read both your Canned Light page and this page, and I've read all the reasons we should not use canned lights or attic ventilators, but I haven't seen any solutions or alternatives. Could you help by providing some additional information for alternatives?

Allison Bailes
Jul 11 2014 - 12:48pm

EW: That's a fair question, and I should have had that in this article from the beginning. We've definitely covered it in other articles and in comments, but if you'll look back at this article, I've just written an addendum with some advice on what to do instead of using PAVs.

David Butler
Jul 11 2014 - 1:52pm

@EW, regarding those light cans, you have a couple of alternatives:  
 
1) use surface mounted lights. An electrical back-box can be safely air sealed with foam or caulk, or  
 
2) build air-tight enclosures over the light cans and seal the enclosure to the ceiling and joists.  
 
An enclosure can be constructed of drywall, foam board or duct board. Manufactured versions are available under the names Insullite, Tenmat and Seal-A-Light. 
 
Note that "AT" rated light cans are not really air tight (they're marginally better than non-AT fixtures), and the IC rating (insulation contact) doesn't mean you can cover them in urethane foam.

Kevin Dickson
Jul 15 2014 - 3:37pm

Allison writes: "Using a fan to blow hot air out of the attic doesn't address the radiant heat flow from the roof to the attic floor. Much of that heat then conducts downward and finds its way into the house." 
 
mark has written a many reasons above why he disagrees with this. I'm with Mark on this one: If you move enough air through the attic, you CAN prevent the attic heat from reaching the conditioned space. 
 
The roof doesn't radiate directly to the attic floor. The radiation gets intercepted by the top layer of attic insulation, which is NOT transparent to radiant heat. Forced convection CAN draw heat from this top layer and get it out of the attic. 
 
One would think that a small amount of heat can reach the conditioned space through conduction in the wood truss members. That's only true when the ambient outside air temperature is higher than the temperature of the conditioned space.

Luke
Aug 17 2014 - 9:52pm

My problem is noise from the fan. 
It runs to one or two AM 
even on moderate (75 degree) days. 
Thinking of disconnecting the fan 
by the electrician who installed it.

mark
Aug 19 2014 - 1:18pm

@Luke easily addressed...to eliminate the running on moderate days, put a wall switch in your home running to the fan to switch it off entirely on those days. 
 
the thermostatic switch may also be set a bit too low and comes on/off at temperatures not appropriate. i think the recommended temp setting is 110F. 
 
also, to reduce the vibration you can put some foam between mounting brackets and the framing it is bolted to lower the vibration transmitted from fan to house. 
 
while doing the last step you might as well take the time to block off the area outside of the fan shroud so exhausted air doesnt bounce off the vent louvers and return to the attic. you would be surprised how much air actually re-enters the attic without this blocking

Allison Bailes
Aug 19 2014 - 1:28pm

Luke: It's even more easily addressed than Mark says. Just do what you were thinking of and disconnect the fan completely. If you want to spend money on something, get your attic air-sealed and re-insulated. That'll actually do some good, unlike the power attic ventilator.

John
Apr 22 2015 - 1:45pm

I live in Honduras. Our house does not have attic space insulation and is not sealed. Other than the air space along the eves and the roof material and an air space at the roof peak, there is no venting. We have no AC. I can feel hot air seeping into the house from the attic. My question would be 'Would a PAV help keep my house cooler by venting the hot attic air and sucking in cooler air from all of the open air spaces in the ceiling and around the outside roof?'

David Butler
Apr 22 2015 - 2:50pm

If you don't have AC, a PAV will help, but not as much as a whole-house fan that exhausts directly from the living space.

GG
Apr 29 2015 - 11:26am

I don't agree with the criticism nor perspective of this guy -totally. 
Depends on your local climate, and fyi, Georgia is not the center of  
the universe. It is not always a matter of energy savings. Only in  
those places where people live like rats is population a problem, thus  
the government makes laws that using electricity in regards to -is "not allowed".  
 
Air displacement is a natural way to dissipate heat. When we are hot, we want the  
feeling off of us, forget the damn savings or etc. In dryer climates especially, ventilation  
can be a positive effort to aid in dissipating heat from an attic, room or space. It  
may need to be balanced,but the mechanical displacement of air will also try to balance itself, 
that is what a tornado, funnel cloud, wind storms or air moving through a space is;  
forcing of balance. A ventilation system of any measure will help rid of the radiance... 
the question is at what rate? In dry western desert states, the extra movement may catch up with the radiant heat,  
or even stay ahead of it depending on the ability to 'balance' temperature equilibrium. 
In a humid climate, temperatures are not only contained in the air molecules, but in  
water molecules as well....where air movement may burden an extra factor. If a home can  
have an insulation barrier, then move the air on either side, one is gaining. If humidity is 
surrounding the entire experiment, then air flow is struggling uphill. Not to say it will not 
work, just may not gain much. In dry climates, 
yes have a some ventilation in the attic, 

Ker
May 8 2015 - 1:50pm

Wow, I was literally minutes away from buying a fan for my attic until I read this post. However I’m wondering if the message in this post applies to me. Can you check out my scenario and let me know what you think? 
 
I live in New York in a 40 year old house. My house is 2 floors but the second floor isn't a full second floor. My house is like a cape without the dormers. I’ll try and link to a floor plan but the house is rectangular measuring 64’ x 32’. The second floor measures roughly 64’ X 18’. The second floor consists of 2 large bedrooms, 1 bath, and a couple cider closets. There are windows on either side of the house for the bedrooms. The 14’ difference between the 1st and 2nd floor is divided evenly between the front and back of the house and is storage space. The roof travels from the peak to the top edge of the 2nd floor then down to the top edge of the first floor (visualize soffits) (the roof line is not a continuous run like a cape without dormers). This essentially puts the 2nd floor in the attic. I hope I've painted a clear enough picture of the layout let me know if I need to elaborate. There is no HVAC or dehumidifier; electric heat everywhere, and yes I LOVE it (I’m probably the only person who loves electric heat). There are soffits as I've mentioned and gable vents on either side of the 64’, no ridge vents.  
 
As far as sealing and insulation, the 2nd floor ceiling is completely insulated with plastic vapor barriers. The 2nd floor and 1st floor walls are insulated with vapor barriers (R-20 I think… pretty sure but don’t quote me). Actually, every wall interior and exterior has the same insulation and vapor barrier. The first floor ceiling is also insulated with vapor barrier. A whole house remodel last year required opening up enough walls for electrical confirm this construction. The 2nd floor windows are about 10 years old but all the windows on the 1st floor was replaced last year during the remodel with low-e foam insulated frame units. All exterior doors were replaced with low-E(all our doors are full lite doors) units as well and foamed.  
 
Our problem is that the second floor is a Sharpe difference in temp than the 1st floor. For example yesterday was about 88F outside, the 1st floor was a very comfortable 66F, but the 2nd floor was 79F and climbing when I got home at 6PM. Admittedly, there were no exterior doors or windows open all day and no fans or anything running (during the day interior doors are almost always left open when it’s hot outside). This morning it was about 54F outside, 63F on the 1st floor and 67F on the 2nd floor. All heat was off last night.  
 
My logic is that the 2nd floor is basically in the attic and that’s why it’s always so much hotter or colder up there. One of my ideas was to replace the insulation on the 2nd floor walls with a higher R-value maybe even spray foam. Those walls aren't like the normal exterior walls because the adjacent space is way hotter than outside. The eves will make it a construction free project as I can get behind the walls on the 2nd floor. However, it’s not a cheap project rolling out or spraying (2)64’X8’ of insulation. I figured if I can cool the attic space by pulling the hot air out that maybe a viable option and maybe also fix the ice damming issue in the winter. Reading this post has me second guessing taking the time to install the attic fans but I’m thinking the advice given to seal and insulate the attic has been done in my case and I still have a problem so now I’m just confused.  
 
What do you suggest? 
 
Thanks and sorry for the super long post!! 
 

Cory
May 9 2015 - 10:53pm

You are incorrect. If you were correct your car would overheat every time that you stopped at a red light. Radiator FAN.

mitch
May 16 2015 - 3:07pm

I disagree with the author's premise that the gable vent won't cool the attic/home. Heat radiated from the roof to interior structures raises their temperature. As long as the outside air supplied by the fan is of lower temperature (and there is an exit for the air) heat will be removed and these objects will experience a temperature drop.

karen
May 16 2015 - 3:42pm

Our house is a two story that faces West and is in direct sun most days....NO mature trees to help shade the house. To top off the extensive sun exposure, the master bedroom has a HUGE arched window that faces West and is on the second floor. The room also has a cathedral ceiling. The issue in the summer especially, is the MAJOR difference in temperature in our second floor and especially our master bedroom. I feel like I'm in a greenhouse cooking! Our bathroom is in the room opposite side of the huge window and there isn't a window in there to provide any cross ventilation. So getting ready for is next to ...impossible with the level of he

karen
May 16 2015 - 4:23pm

I live in a two story house that faces West. We are in TOTAL sun. Our master bedroom in on the second floor and has a HUGE arched facing Wesand a cathedral ceiling. We have CA and ceiling fans in all rooms upstairs, but our second floor remains VERY hot throughout the summer. Our master bedroom has a bathroom and walk in closet on the opposite side of the room as the huge window. Getting ready in that bathroom is unbearable! We have an exhaust fan in the bathroom, but it doesn't do much of anything. I feel like I'm in a greenhouse. I always thought a solar attic roof fan would help remove some of the heat in our second floor. Even if it only ran late afternoon at the peak of heat until the sun set. After reading your article, you seem to advise against it. That being said, what do you recommend that would help? I've lived with this for 12 years and refuse to suffer through another summer. This house was a Spec and apparently the builder didn't consider positioning when deciding on the house design and types and size of windows...... Another thought I had was to put some kind of film or coating on our West windows to help limit the amount of sun that penetrates. Anyone have a good product suggestion for accomplishing that?  
Thanks in advance.

Tara
May 17 2015 - 5:07pm

My house was built in 1945 and our kitchen ceiling has what I assumed is an attic fan built into it. It is definitely sucking air upstairs, but after reading this article and viewing other pictures, I'm not sure if this is truly an attic fan. Without the AC on, and with this fan going, my house does seem cooler, as if the cold basement air is getting turned up onto the main floor. Am I dealing with something different? If so, what is it? I don't even know what to search for on Google. haha. New home owner probz.

Charles Mcfee
May 20 2015 - 2:41am

Roof vents and power roof vents are made to work in conjunction with vented soffits so that outside air will replace superheated attic air during hot summer days. 
Of course you try to make the attic scuttle hole, use air tight trim on your ceiling cans, but it would be foolish to ignore the benefits of a reasonably powered attic fan working in connection with adequate soffit venting.

Peter
May 23 2015 - 11:03am

I did dinner parties selling retrofitting barrier shield to homeowners on florida's west coast I spent considerable time with the inventor so designed it for the astronauts. We installed it on 26 inch width rolls on top of the existing attic insulation. It is made up of 2 very thin layers of most pure aluminum with a thin film of molar in between which makes it non conductive. So the attic heat no matter how intense cannot penetrate it, no matter how hot the attic gets. Most people think adding more insulation will solve the heat transfer from attic to living area but this is not necessarily correct. The insulation is like s storage battery. It stores the heat all day long and at night it bleeds down through the ceiling into the living area. Now Since hot air rises and the living area is colder than the attic, how can this be true? Are you ready for this? Because HEAT ALWAYS TRAVELS TO COLD. In the cold weather the heat in the living area rising through the colder attic area(since hot air rises) cannot penetrate into the insulation but cannot penetrate the barrier shield.CONCLUSION: if you use barrier shield in the method stated it doesn't matter how hot the attic gets, which makes the fan issue a mute point. However if you are living in the attic this would note be the best solution (just kidding).  

Mitch
May 23 2015 - 3:49pm

How about using an attic fan in a home during the Spring and Fall months when no air conditioning is on? It seems they would be useful during cooler evenings after the sun has made your house warm all day.

Herb Sevush
May 28 2015 - 12:15pm

I recently bought a 2 story plus attic home in the Northeast. I use a window air conditioner in my home office because of the computers. For the rest of the house we use individual ceiling fans and keep many windows open. I'm looking to keep the attic temperatures down to in order to be able to store some valuables up there. I'm not looking for it to cool the rest of the house so in my circumstances, is an attic ventilator still a bad idea?

Kevin
May 31 2015 - 11:46am

Thank you for the info! I've been researching for some time now on how to insulate the front part of my house. This part of the house is nearly 44 years old and has very little insulation in the attic. The a/c runs constantly and the attic is unbearably hot during the summer months here in sunny Florida. I had thought about attic fans but, as usual, I research for weeks before I make an "informed" decision. Funny thing is, I was lying in bed last night and it hit me. I wonder if the attic fan would pull conditioned air through the drywall ceiling? I know they create a ton of neg pressure. Now I'm thinking radiant barrier and more insulation in my attic. This part of the house faces east so the eastern and southern slopes of the roof get pounded for hours by this hot Florida sun. I wouldn't think too much about the neg things people say. It baffles me that there are so many individuals that cannot comprehend basic information like the kind you provide. Thanks again.