The #1 Reason Power Attic Ventilators Don’t Help

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1. Allison, 
   
   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?

2. Great article Allison! I
   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?

3. It’s funny that you posted
   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.

4. Someone said “you can
   Someone said “you can fix ignorance but you can’t fix stupid.” 
    
   Allison, I admire your passion for attempting to fix both.

5. Some things just won’t go
   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.

6. Hey “Lady”, er,
   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! 
    

7. Steve – to your point, I
   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.

8. All of your info makes sense.
   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.

9. I once had a house with R/H
   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…

10. I have had the same positive
    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.

11. For David Eakin and others
    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… 🙂

12. @Steve & Bob: here we
    @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.

13. David, 
    
    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.

14. Not Allison’s #1 Reason, but
    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.

15. @Steve W, I fully concur with
    @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.

16. Had a long discussion with an
    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. 
     
    🙂 

17. I’ve been thinking about
    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!

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

19. Thank you Allison Bailes! I
    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!

20. here goes…while this
    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

21. boy…that installation of
    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

22. one more thing that is flawed
    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.

23. i’ve had earlier comments on
    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

24. @Mimi, in new construction,
    @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.

25. @ David Butler  &lt
    @ 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.

26. @Mark, you are correct, my
    @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.

27. @David Butler  &lt
    @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

28. @Mark, my previous comments
    @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

29. With a hot attic, the ceiling
    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?

30. So what are the solutions?
    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?

31. EW: That’s
    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.

32. @EW, regarding those light
    @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.

33. Allison writes: “Using a
    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.

34. My problem is noise from the
    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.

35. @Luke easily addressed…to
    @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

36. Luke: It’s
    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.

37. I live in Honduras. Our
    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?’

38. If you don’t have AC, a PAV
    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.

39. I don’t agree with the
    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, 

40. Wow, I was literally minutes
    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!! 
     

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

42. I disagree with the author’s
    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.

43. Our house is a two story that
    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

44. I live in a two story house
    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.

45. My house was built in 1945
    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.

46. Roof vents and power roof
    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.

47. I did dinner parties selling
    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).  

48. How about using an attic fan
    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.

49. I recently bought a 2 story
    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?

50. Thank you for the info! I’ve
    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.  

51. The only reason you have a
    The only reason you have a negative airflow in the attic is because there at least 2 in the roof shingles, you only need 1 but I often see houses with 4 sometimes 5, each one installed they fight each other to get the hot air out

52. Help me here then. My return
    Help me here then. My return is in the attack and it is an insulated flex duct. when it reaches 90 outside and above 115 in attic, the AC loses ground because the return air is warmed as it flows through the return. I put in the attic ventilator and have not had this problem since. Is there a better way to keep the return air from heating up other than control the attic temp

53. i did not have time to read
    i did not have time to read all of this but please be aware the air in the attic is much hotter than the air outside because it is heated by the same radiant effect you are referring to. so removing this hot air helps to decrease the air from also warming the other materials in the attic and more importantly replaces the very hot air with somewhat cooler air that can now act better as a buffer of that radiant heat transfer.

54. WOW, so much misinformation..
    WOW, so much misinformation…(not the original post, but the comments). 
     
    Karen with the giant west facing window: You need to have a BPI certified contractor inspect your house. They can help you solve your issues. 
     
    People with fans in your ceiling/or wanting to cool your house with exhaust fans: These fans are called “whole house fans”. If you get one that is insulated and sealed, and you don’t live in a humid climate, they can work really well. 
     
    People who think radiant barriers are magic: They’re not. Radiant barriers will definitely help to keep your attic cooler, but they’re not necessarily “better” than insulation somehow. They are much more valuable when you have ducts in the attic and they make a bigger difference in an attic with little ceiling insulation. It’s a cost/value equation. Sometimes your money will be better spent installing insulation, sometimes installing a radiant barrier. Again, a certified BPI contractor can look at your house and help you with real answers based on actual building science. 
     
    People who really think attic fans work: I know it seems, intuitively, that these things would work great but they just don’t. You’re pitting your intuition against physics and data. If you really can’t believe it, set up some experiments of your own. Turn your fan off and monitor the temperature in your attic up unitl 2pm or something, then turn it on. Or look at the weather for 2 days with the same weather and monitor temps with and without the fan on each day. Prove it to yourself either way. 
     

55. I am seriously doubting the
    I am seriously doubting the efficacy of either the current practice of soffit/ridge vents or dual eave vents or any powered ventilation truly cooling the underside of the roof deck – which is where the true issue lies. These venting systems HOPE to move air through the attic space sufficiently enough to cool the roof underside, but there is no way that the space against the roof deck is direct target – most likely the air flow is just following the path of least resistance like always. I suspect that the reason some have seen cooler attics when a reflective barrier is placed under the roof is because it traps air between the roof deck and the barrier, causing the air space to heat up even higher than without the barrier and causing a greater thermosyphon action to occur against the roof deck underside. To see if this might be so, I have experimented in my attic (about 1931, no soffit vents, ridge vent (roofers…) and 6 windows now partially open to supply outside air) by adding conventional pink foam attic vent troughs along the entire under-deck area to accelerate the air flow against the roof deck and reduce radiation into the attic space. I have experienced a lowering of the attic air temps and lower radiant ceiling temps even though there is no direct path of outside air to the bottoms of the troughs. I may even try to improve on this by going to a wider trough system (the pink foam jobbie is only 11″ wide). Would really like to see BSC or some other research group do more definitive/controlled analysis as I really think this is the root issue.

56. David E, I have seen research
    David E, I have seen research presented which supports what you’re saying about where the air goes. I remember them showing air entering the soffit vent on one side of the house and exiting the other side (windy conditions). I can’t remember who was presenting it.  
     
    What you’re describing with your baffles is similar to above sheathing ventilation. There is really good information on this here: 
     
    https://www.google.com/search?q=ornl+%22above+sheathing+ventilation%22 
     
    We did this on a retrofit house with great results a number of years ago. We are working on a house right now where we used radiant barrier sheathing, included the normal soffit & ridge vents, installed the baffles throughout the entire roof, and then sprayed foam to the under side of the baffles. So it’s a sealed attic which is also vented above and includes a radiant barrier. To me, in combination with a reflective metal roof, this is the ultimate roofing assembly. I’m not saying everyone should do it. Every house is different and the benefits are not always enough to pay for an expensive approach like this, but as far as shear performance I don’t think it gets better. 

57. @peter…it is NOT pure
    @peter…it is NOT pure aluminum. and it is mylar not molar. heat can and DOES transfer through the RB…. however it greatly retards that process. dont get me wrong, i am all for radiant barriers. BUT making statements like you have gives the product a bad reputation because the people professing its attributes are overselling it and that looks bad

58. @tara…no, that is not an
    @tara…no, that is not an attic fan. you simply have a ceiling fan. in this regards it is probably acting like a mini-whole house fan tho its installation is most likely sub-par and is letting IN hot air from the atic when the fan is not in use

59. Cory: You
    Cory: You wrote: “You are incorrect. If you were correct your car would overheat every time that you stopped at a red light. Radiator FAN.”  
     
    Yes, air movement can cool hot objects, but there are some significant differences between a radiator fan and a powered attic ventilator. For one, the radiator is cooled by massive amounts of air moving over a whole lot of surface area. Take a look at all those cooling fins. I’ve never seen an attic built with cooling fins. Second, the air moving over the radiator is making direct contact with the surface of the radiator. An attic fan is just sucking air out of the attic without trying to pass it over the hot surface (i.e., the roof deck).  
     
     
    If we were to build roofs like radiators, I’d agree with you that we could cool them with fans. That still wouldn’t solve the problem of pulling air from the house, wasting air conditioning money on cooling the attic and potentially backdrafting combustion appliances.

60. @skye dunning… i hope your
    @skye dunning… i hope your comment was pointed at my replies. there certainly was NOT any misinformation on my part AND i have done just what you suggested. the attic fan kept my house cooler by ~8F compared to pre-fan installation days. this was observed on numerous days following the installation. during workdays would leave air conditioner off when leaving in the morning: temp 92+F at 5pm. after attic fan installation (AC still off during day): temp 84F at 5pm. 
     
    i am not saying an AF is for everyone in all climates. but in my location…northern portion of california’s central valley with temps to near 120F at times, the installation was VERY beneficial. 
     
    also, prior to the installation the AC had a difficult time keeping up (it was actually a bit undersized). but after the installation the AC was easily able to keep up

61. i said it before but one
    i said it before but one reason AFs are not as effective as they could be is because of POOR installation. the fan in the pic at top of article has several problems. first, a portion of the fan overlaps solid surfaces. second…the whole surface the fan is mounted on should have been blocked off with plywood that had a hole the identical size of the fan shroud. as installed, a portion of the exhausting air bounces back off the louvers and re-enters the attic. 
     
    hopefully this not not a “professional” job because it is pure hack. if it was the owner should get their money back.

62. Allison…your comments are
    Allison…your comments are infuriating and show and general lack of understanding. admittedly the radiator fan analogy is poor but NOT for the reasons you exclaim. 
     
    the car radiator is using air to cool a engine block but is 2 steps removed (water cools block, fins cool water, air cools fins). whereas they are both cooling, the example is poor. the energy density in the engine is MUCH higher. that would make the two examples hard to compare 
     
    however, your rationale is near repulsive.  
     
    i am the first to say the installation of an attic fan is NOT always recommended. BUT there are times when the installation a unit can be beneficial but it depends on several things…1) proper installation 2) proper conditions (environmental factors) 3) on a lesser note….whether it is a DIY installation or a professional installation which is costly and hence, possibly not cost effective 
     
    i have posted several replies to the article and also to comments. you have never commented on any of my statements though it is possible you did and i simply missed it. you have taken a stance based on one example and the words of some “expert(s)” and give a blanket recommendation based on that. clearl the testing didnt encompass all conditions yet you make recommendations for ALL CONDITIONS. that stance is truly absurd and a disservice to your readers

63. @Mark, Allison, IS an expert,
    @Mark, Allison, IS an expert, and he’s far from alone on this. He’s not taking anyone’s word for this. Nor am I. 
     
    A PAV may reduce energy bills and/or improve comfort in a home with serious defects, but it’s unlikely to be the optimal solution in terms of long-term cost benefit.  
     

64. mark:
    mark: Perhaps you should read the literature review of attic ventilation done by Danny Parker at Florida Solar Energy Center. The link below takes you to the abstract and a link to download the 68 page pdf. 
     
    Literature Review of the Impact and Need for Attic Ventilation in Florida Homes 
     
    Yes, their focus is on Florida, but their literature review covers research in a number of locations. The main takeaways from the research are: 
     
    1. PAVs may help reduce cooling loads a little bit, but their energy use wipes out most or all of the savings. 
     
    2. PAVs can backdraft combustion appliances. 
     
    3. Regarding the point of my article here, Parker wrote, “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.” 
     
    What I find infuriating is people who come here to comment without understanding physics or what the literature says about this topic.

    1. So I follow and agree with
       So I follow and agree with all points, however, my point in installing an attic fan, and everyone I know who has, and does, and wants to, is exactly because we want #3 to happen. The physics are not in question. What our goal is and what the intended goal of attic fans has always actually been is, to draw cool air up through the living space from the cellar and deep crawl spaces and out of the attic. You do this during the part of the year where the outside temperature is not so high as to overwhelm the natural cooling in the subterranean areas. I have never heard of anyone 'ceptin' city folk who actually is trying to "cool" their attic to reduce the effect on the living area. The other benefit of doing this, is it keeps air moving through the cellar and crawl spaces helping to reduce certain build ups of naturally occurring gasses… I recently discovered that my use of my attic fan has been extended a bit in that I recently installed a GeoSpring water heater. This unit uses a heat pump as the primary heating source for the water in the tank. It outputs cool air into the cellar as a result and dehumidifies… So far my results are fairly exciting. G

65. How come my post wasn&amp
    How come my post wasn't accepted? I commented sometime last week. In either event, my post was in regards to me testing my attic fan with no insulation to see how much air leakage I had. I used a smoke puffer, and there was no air leakage. I'm trying to understand if this is the case, why would an attic fan be bad?

66. @Gary, you're
    @Gary, you're talking about a whole house attic fan. That's not what this article is about, as noted elsewhere in the comments. That's a whole 'nother topic.

67. @John, you didn't
    @John, you didn't provide enough information but I'll make the following general observations…

    If a home doesn't have central air or atmospheric combustion appliances, a PAV will reduce ceiling loads a bit by cooling the attic and reversing the stack effect. (In summer, stack effect tends to draw hot attic air into the house). However, a whole-house fan would be a lot more effective if the objective is to avoid the use of A/C. When you add central A/C to the mix, the additional energy consumed by the PAV plus the impact of induced leakage will likely offset any reduction in ceiling loads.

    Your puffer experiment doesn't prove no leakage. Smoke can be helpful at finding larger leaks but it can't be used to prove a lack of leakage. For that, you'd need to have a blower door test done along with some simple pressure diagnostics. In any case, I've never seen an air-tight ceiling. The only ones that come close are in purpose-built tight homes (e.g., Passive House level), where considerable effort is expended to seal all fixtures and top plates, including sealing the cracks between ceiling drywall and top plates.

    Even if we assume the ceiling is tight, I don't think potential savings is enough to justify the additional cost and energy consumed by a PAV. I went through the math here, in another forum. If a home has serious defects that cause unusually high ceiling loads, a PAV might be cost effective, but in most cases, it would cost less in the long run to fix the defects. YMMV.

    1. Thank you for your
       Thank you for your information. I would be happy to provide additional information, please let me know what information would be helpful. And I agree with the additional energy costs, but my attic fan is a 1500 CFM @ 40 Watts. And this is a gable fan. I understand a blower door test would show the air leaks, and I've seen this 2x, but I'm trying to understand with an attic fan on in an unsealed attic, the attic fan should pull the air in the home up into the attic? If this is so, I didn't really see any smoke being pulled whatsoever. With the attic fan on, I tested the outlets and light switches in two rooms below the attic fan, and the smoke puffer didn't get pulled. I also tested the ceiling fan and the recessed lights, didn't have covers and not sealed and there was a 1/4in gap around the lights, and still no pull of smoke. Please help me to understand, that although the attic fan is on, there is no insulation in the attic, the attic isn't sealed, why is the smoke puff test not reliable in this situation? Thank you for your time.

       1. @John, there are a couple of
          @John, there are a couple of things about your comment that aren't clear. When you say your attic is not sealed, that goes without saying… you can't have a PAV in an encapsulated attic, so it's obvious that you must have a vented attic. Did you mean to say that your ceiling is not sealed? Also, why is there no insulation? Presumably this is a temporary situation?

          As for the result you described… without being on site I can only speculate, but to reiterate, smoke pencils (puffers) are only useful for finding larger leaks. Leakage between your house and attic likely consists of many tiny leakage paths, too small to affect the smoke but collectively enough to impact the result. Or there may be larger leaks that you simply didn't find with the puffer. That's why pressure diagnostics is the only way to demonstrate the impact of the fan (or lack thereof) on house air changes.

          1. Thanks again. I removed my
             Thanks again. I removed my old insulation, had the attic air sealed, and had blown-in done. Your answer on the puffer not being able to detect small leaks makes sense, I just thought being 20 feet away from my recessed light, below in my room, to my gable fan, in the attic, would be able to show air leakage with the puffer, but I guess it is leaking just not strong enough to pull the smoke but collectively as a whole it could be a problem, I can understand this. Either way, regardless, I guess I don't have to worry since the attic is complete now, air sealed and re-insulated. I do have the gable fan running though, and now that the hot air isn't coming into my home the attic is super hot and the fan is running trying to remove it. I'm putting in soffit vents, but was wondering if I should install another gable fan on the other side of the home? I just feel that if there isn't a significant pull of air from the other side of the home then it will just take much longer for the one gable fan to pull all the hot air out, even with soffit vents. These gable fans, being only at 40 watts and pulling 1500 CFM, I don't think my energy bills will be high, but having another installed to pull air in should help a lot. I was thinking of just having them wired together, so when the one goes on to pull the air out, the other will be pulling the air in to help. I'd appreciate to hear what you're thoughts are on this?

68. For some reason, the links
    For some reason, the links were stripped from my last comment:
    * example of air sealing top plate to ceiling drywall: http://bit.ly/1JaBWmR
    * link to another forum with thumbnail math on PAV's: http://bit.ly/mdIcXP

69. I installed 2 attic fans last
    I installed 2 attic fans last fall. Living in Memphis, I have always had sweating ducts in the attic, but now they are dripping all over and I have new spots of moisture showing up all over the house from a/c ducts sweating. Could it be that although the attic is a little cooler, it is, in fact, drawing in more humid air from outside and, because the temp in attic is lower, the dewpoint is also lower?

70. Larry, more likely your AC is
    Larry, more likely your AC is running more because the fans are pulling air through your house. Longer AC run times mean colder ducts. You could also have duct leaks exacerbating the issue (well you do have duct leaks because we all do, it's just a matter of how much).

    My recommendation is to turn the attic fans off. I would also recommend getting a BPI certified contractor to diagnose your house for you. They can tell you exactly what's going on and lead you to the correct strategies for improving your situation without wasting money (like on attic fans).

71. Everything you say makes
    Everything you say makes great sense. For Georgia! But work with me here please? I'd love to hear your thoughts.

    I really respect where you are coming from but I am in Oregon, where it is very dry in the summer; and we run AC very little (maybe 4 days last year). Setting those days aside for the moment…I don't care at all if air is being drawn through my ceiling(see note below)….I have the windows open in any case most of the time. An attic ventilation fan seems quite reasonable. I have pretty good soffit venting and turtles on the roof, but the attic heats up a great deal. Even if the heating is radiant and conductive, better airflow will still cool the surfaces somewhat—and I have no moisture problem or AC efficiency problem to worry about. So why not?

    When it gets HOT I close the windows in the morning (maybe 15 days a year). On those days, there will be more resistance to drawing air from the house and if some does, I'll get some outside air in. But I don't see this as being much of a problem—I should just experiment and see what keeps the house cooler.

    On the few days when it gets REALLY HOT and I turn on AC, perhaps I'll lose some AC energy into the attic. But I could also just turn off the exhaust fan on those days (after doing the experiment—because it's so rare that cost isn't an issue, and there's zero likelihood of damage due to humidity, so really it's cooling that I care about).

    note: I actually have a whole-house fan in the ceiling that pulls air from the house into the attic. So there's a monstrous "leak" that is intentional. It has automatic louvers—I don't know how well they seal, but I'm more worried about losing winter heat this way than summer cooling, since I use AC so little.

    1. Ok, I'm sold! I&amp
       Ok, I'm sold! I'm moving to Oregan!!!

72. @Ethan, I realize there must
    @Ethan, I realize there must be at least 100 comments posted here, but your situation has already been addressed more than once. If an attic fan allows you to avoid using the A/C (and you don't have an atmospheric combustion appliance that would be affected), then why not indeed. However, you'll get much better results by using the whole house fan. But it makes no sense to run the PAV at the same time as a big WHF.

    @John, holding a smoke pencil even a foot from a leaky fixture is unlikely be detect anything. You mentioned you plan to install soffit vents. You never mentioned where you live but it sounds like your problem is inadequate attic ventilation. And no, I do not recommend adding a second fan. You should turn off the fan you have, add some soffit vents and see what difference your new insulation and air sealing efforts make.

    1. Thank you David. I added the
       Thank you David. I added the soffit vents (16 x 4 vents), 10 on each side of the home, evenly spaced apart. It does make the attic cooler, but the temp on an 85 degree day still reaches 110 for most of the day. The insulation and air sealing keeps out the radiant heat, my interior home temp is fine. I am just worried if my roof and shingles will warp since the attic temp is high and the heat remains trapped in the attic. Should I install a ridge vent or leave the attic the way it is? Thanks Again!

       1. @John, the attic temperature
          @John, the attic temperature will vary from top to bottom, highest at ridge and lowest at ceiling level. Regardless of where you took the measurement, 110F is not very hot for a vented attic in summer. Shingles are designed to withstand much hotter temps. It's not unusual for a roof deck to be 50+ degrees above outside ambient.

          I believe you said you have gable vents. As long as they have at least as much net free area as the soffit vents, you don't need to install ridge vents.

       2. David,
          David,
          My soffit vent square foot on each side of my home = 640sq ft, 1280sq ft when totaled together. My gable vents on each end are 300sq ft (prob less since I need to minus the louvers), either way totals 600sq ft with both gables. Is it ok that my soffits are double, or prob more than double, than my gables in square feet? Should I close some soffits to create more of a suction?

    2. @David, thanks. I did read
       @David, thanks. I did read all the comments (took a while) and came to the same conclusion. I also had two other thoughts as a I reflected on this, that may be worth considering and may actually drive the conversation forward a bit.

       First, the concerns are real—humidity, backdrafts, etc. are all worth consideration. I am grateful for the article and discussion which have been helpful and informative. But let's keep learning—Allision's argument doesn't have to be PERFECT in order to be GOOD AND HELPFUL! Let's make it better….

       Allison's main point that the ceiling has to be airtight BEFORE doing any ventilation is wrong for three reasons.

       First, airflow isn't on or off—it's relative. With soffit vents in place, there will be some balance of house air versus outside air drawn in once the pressure in the attic is lowered by the fan. The precise balance will depend on the balance of resistance of the soffit-vent versus through-ceiling paths (and the through-ceiling path also has resistance on entry to the house—from crawlspace etc). Not easy to measure but a good number of unblocked soffit vents would favor airflow from outside.

       Second, nobody on the entire thread has suggested that an attic fan **will not cool the house**. This is at odds with Allison's thesis that they are useless (the fan/sunburn argument). In fact there is good data in the comments that it **does** cool the house. The point is that in some cases the cooling could be inefficient or even dangerous (if drawing conditioned air or creating backdraft or humidity problems). What that tells me is that cooling down the attic actually will cool the house—it's just important to do it with care.

       Third, Allison's thesis that the ceiling must be airtight before venting the attic ignores relative costs of the two approaches. In an area like mine where it could be useful and safe, the question then becomes one of cost tradeoff. I'm not going to seal my ceiling **because I have a whole-house fan** and it would be difficult and expensive to do—refit everything, reinsulate everything, change the fan louvers, etc. For $150 I can install a gable fan with an accessible switch so that I can turn it off when the A/C is on. The louvers should seal well enough to not mess with the soffit-to-turtle passive airflow when the fan is off. I'll install it well 🙂

       Many thanks,

       Ethan

73. John,
    John,
    If you are concerned with your attic temps (which are really a symptom of the true problem – radiant energy from the roof deck), then you could run baffle panels the entire length of your roof deck underside. This would force the air coming in your soffit vents to travel under the roof deck (not just into the general attic area) and out the top before being drawn through the gable fan/vents. I have done this with most of my under roof area and have noticed a significant reduction in attic temps even though I don't have soffit vents (I left the bottoms of the baffles open to get air from the attic which is supplied by 6 partially-open windows; top of baffles ends in a ridge vent; no mechanical venting/fans).

74. Ethan wrote: "The
    Ethan wrote: "The precise balance will depend on the balance of resistance of the soffit-vent versus through-ceiling paths."

    True, but typical attic vent ratios don't provide NEARLY enough free area for a PAV to operate without creating a negative pressure zone in the attic, which is what sucks air out of the house. Prevailing winds will also impact ratio between attic vent path vs. ceiling leakage path.

    Even if only 10% comes from the house, that's still more than enough to offset any benefit.

    But again, this issue of folks in climates where attic fan might eliminate the need for A/C, as you described, is really beside the point of Allison's article. It doesn't take a lot of discussion to figure that out.

    One of the main points of the article is that PAV's have a lot less impact than most folks seem to think. Those of you who still believe otherwise should ponder the following exercise. Make your own guesstimates and see what you end up with…

    a) what % of ceiling load is from conducted heat (vs. radiant heat)?
    b) by what % does PAV reduce delta-T between house and attic?
    c) what % does ceiling contribute to total cooling load?
    d) multiply %'s from (a), (b) & (c) to get % reduction in total cooling load

75. Wow. This is just STUPIDITY.
    Wow. This is just STUPIDITY. People will write anything these days. This is utter crap, and can be disproven by a 5 year old.
    EXAMPLE:
    Let's say your car is in the sun all day. You open the door, and the seats are hot. VERY hot. Well, according to Professor Bailes here, you're plumb out of luck. And you'll just have to burn your buttocks and sit on a hot seat. Unfortunately for her, anyone that's ever been INSIDE OF A CAR knows that if you open the windows and "air it out," the temperature drops drastically and within a few minutes, your seats are no longer too hot to sit on. Not to mention when you enhance this effect with A/C.

    It's called life lessons 101. Stop deceiving people. Basic physics here, really.

    1. Jeff Moore,
       Jeff Moore,
       The point of this site is to explain how attic ventilators take the air inside your home out of your home and into your attic. What don't you agree about this?

    2. Well, you seem to know as
       Well, you seem to know as much about physics as you know about my gender, Jeff. I've lived most of my life in Texas, Louisiana, Florida, and Georgia. I know a thing or two about hot cars that have been in the sun all day. I also know a thing or two about heat transfer in attics. You might want to learn that yourself before you come here with your drive-by attempt to flame me. Now, let's see if you can handle the heat when it's reflected back at you or if you fade back into the anonymity of the Internet.

76. @John, no, you don't
    @John, no, you don't want to close off any soffit vents. Your attic will get more ventilation with 2x soffit vent area than if you had fewer soffit vents. That said, you'd get even more ventilation if the vents are balanced. You could add one or two of those passive turbines.

    BTW, just as the louvers determine the net free area of a gable vent, soffit vents also have a net free area that's less than the total soffit vent size, depending on the type of vent. The manufacturer should be able to provide you with the net free area spec.

    1. David,
       David,
       Thanks again. Let me make sure I have my math right. In order to figure out net free area, I would do Width of inside frame x height of one air flow opening x the amount of lovers or openings?

77. WOW this really is the post
    WOW this really is the post that just keeps giving!

    John, that's correct, though it's usually easier to get the net free area from the manufacturer. I know it's not like you're at the store with one in your hand but you might be able to find a picture online and get the net free area there.

    Jeff, to your credit I will say that if we could roll the windows down in our attics it would surely make a significant difference. For me the equivalent area would be 4 hatches about 15 ft. long by 20 ft. high. If I could open those, Bahama shutter style I would no longer have an attic, but basically shade over my house with a large open area, and things would certainly cool down.

    Allison, it's so good to see you stand up to these men. Don't let them bully you! Will you marry me?

    1. Thanks Skye. I'm
       Thanks Skye. I'm just trying to be perfect now. Just not sure how to measure the opening of air flow on the gable. Is the measurement taken from the outside, the bottom edge of one louver to the top of the one below it, there is approx 1in between each louver from the outside this way. Or, is the measurement taken from the inside, the louvers are slanted, so from the top of one lover (can't start with the bottom of the louver since I am now behind the it facing the screen) to the one below it is 3 inches. Big difference, just didn't know which one was the appropriate way to measure this. Thanks again!

       1. @John, your question is valid
          @John, your question is valid. I suggest doing a Google search on "net free area" for gable vents with louvers: http://bit.ly/1RUyHPD

    2. skye, i may have the
       skye, i may have the misconception i am attributing to you but i believe you are a guy… from what i can tell Allison is also a guy so maybe your marriage proposal is misdirected

78. In simplest terms, you&amp
    In simplest terms, you're measuring the holes. So if you measure from the outside and it's 1" from the bottom of one louver to the top of the one below it, and the space is 10" wide, that's 10 square inches. Continue that way with the rest of the openings, add them up, and that's your net free area. If the openings on the outside are smaller than the openings on the inside, that's where you measure.

    1. Thanks Skye,
       Thanks Skye,
       So, my width is 18in and each louver has a 1 inch opening, but there are 7 of openings. So, my net free area for one gable vent is 18in x 7in =126 sq in?

       1. typically, tho not always,
          typically, tho not always, there is a screen on the inside ite the gabe vent. this keeps out birds, bats,some insects, etc. the atea of the screen must also be subtracted

79. I have read many of the
    I have read many of the comments about how taking hot air out of an attic will not lower the temperature. I can't be convinced. Sucking out really hot air out of an attic has to have a positive effect on the home temperature in the area below the attic. Otherwise it would defy physics.

    1. Gary,
       Gary,
       You should really quote what information you are referring to. A general statement that you are making, without a reference, is vague, or at least state the persons name you are referring to their comment.
       I could do the same thing you're doing without a reference point: As far as I've read and learned, the problem isn't that you can't cool your attic, the problem is you are cooling the attic with the air from inside your home. I can prob provide you with a minimum of a dozen quotes from within this page that state this.

       1. john, your statement is
          john, your statement is incomplete and this all to common of a practice causes confusion amongst some readers. in some cases (maybe most) an attic fan can pull (some) air from the house to cool the attic . an attic fan can by no means pulls all the cooling air from the house as your statement could be read to mean tho i am sure that is not what you meant.

          in my limited experience of a gable mounted fan, its installation resulted in the home being 8F cooler upon returing from work (~5pm) WITHOUT the A C on

80. Results in: my gable PAV
    Results in: my gable PAV definitely works.

    Reminder: I am in Oregon, very dry so no humidity problems. We run the AC rarely, but recent tests have been with it on (forecast highs > 95 degrees). The attic has fairly good soffit venting and limited turtle vents just below the ridge. We have a whole-house fan as well. The attic is reasonably well insulated the ceiling is not well sealed.

    Our overall strategy is the same before and after: open the house at night and run the whole-house fan for a while evening and early morning, then close everything down and leave the circ fan "always on" with the AC set somewhere in the 76-78 range.

    Before gable fan: on a 95-degree day, starting from whole house under 70 degrees thanks to the cool nights and whole-house fan, by mid-afternoon the AC was running constantly and was unable to keep to the set point. Upstairs would get uncomfortably hot with radiant heat (ie stayed uncomfortable for some time even after the WHF ran for a while) and the thermostat (downstairs) would peak in the low to mid-80's.

    After gable fan: on a similar day/start point, the upstairs stays comfortable and the AC can maintain the setpoint. There appears to be far less radiant heat upstairs—it is always comfortable and just a few degrees warmer than downstairs, but no noticeable radiant heat.

    Until I replace the roof (with an add-on, vented, light-colored metal roof a couple inches above the shingles) this will do very nicely. The real solution IMHO is not more insulation (which is good, but which is solving a problem that would better be entirely avoided!) It would be better to reduce heat loading into the attic at all, which the new roof will do very effectively.

    Is it drawing some conditioned air into the attic? Undoubtedly. Is it increasing the cooling load by doing so? NO, because I know the AC was running full-tilt before and failing, and now it works (and even cycles). This was a good tradeoff.

    YMMV. Not a solution for Georgia 🙂

    1. btw ethan…i hope you
       btw ethan…i hope you blocked off the area of the gable vent outside the fan shroud. this really adds to the efficiency of the fan. otherwise a considerable amount of exhaust air bounces off the louvres and comes back into the attic short cycling the exhaist air

81. Ethan – when you replace your
    Ethan – when you replace your roof, DO NOT allow the old shingles to remain. Have them stripped off, have the decking repaired/replaced as necessary, have new peel-and-stick weatherproofing installed, have furring strips installed over the roof joist lines, THEN have the standing seam light-colored metal roof installed.

    Also – construct/purchase an air-tight box cover for your whole-house fan to seal it off from the living area when not being used (did this at my dad’s). Alternatively, you could get one of those thin bubble wrap/vinyl panels with hook-and-loop fasteners to cover the attic-side louvers (did this at my brother’s). You will be amazed at how much this will add to your comfort.

82. Interesting comment. Have you
    Interesting comment. Have you felt heat from UV?
    “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”

    1. @H…can’t tell if you are
       @H…can’t tell if you are being sarcastic or not but while UV does more damage to things (including skin), IR is where more heat is transfered

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