Is It Ever Helpful to Use a Powered Attic Ventilator?

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1. Any thoughts on wind turbines as “powered” attic vents? Are they really any better than just passive roof vents?

   1. Roy, I haven’t seen evidence that passive ventilation of any type provides any significant benefits. Yes, it can keep the attic cooler in summer, but it has little impact on keeping the house cool when the attic floor is properly air-sealed and insulated.

2. As a former northerner, I believe that our intent at venting the attic was to control humidity in the winter, especially since we didn’t use ceiling vapor barriers.

   But as a recent southerner, I know a lot of people who swear by the wind turbines as an attic ventilation device. I was just wondering if they actual result in higher airflow regardless of whether that airflow helps anything.

   1. Ah, that I don’t know. One thing I’ve read is that vertical venting (soffit + ridge) works better than horizontal (gables) because of the stack effect. I’ve seen the whirlybirds spinning pretty fast on the condo buildings where I used to live, but I think that was more often in winter, when it would have been better to have them closed off since we don’t have to worry about ice dams in Atlanta.

3. We seemed to have gotten away with just having ridge or gable vents without soffit vents in the north, probably because that was enough opening to let the water vapor diffuse to the outdoors before it condensed or froze on the underside of the roof sheathing. I hadn’t thought about this helping with ice dams. The only problem I ever had with ice dams was when the snow would melt on the skylight and then refreeze below it in upstate NY. I had to shovel the roof as well as the driveway after every snow storm.

4. What about the situation of when the attic area is used for storage?
   I have converted a portion of attic into storage for holiday decorations, luggage, and extra stuff that rarely if ever gets used. I’m assuming some of these items will not withstand temps going into the 140+ range. I live in the PNW, and this last weekend, the outside temps were 115 during the day. I can’t imagine what I’m going to see when I go out into the attic.

   I have installed a fan out there that operates on a thermometer. I’m sure it is probably pulling conditioned air into the attic, but other than insulating the roof deck and making the entire area conditioned space, do I have a better option if I’m going to continue to use this area to store items?

   1. Garen, all of the problems I listed above apply no matter how you use the attic. If you really think it’s necessary to keep the attic cooler and the other methods (reflective roof, passive venting, radiant barrier) won’t work for you, just be careful that you don’t have any natural draft combustion appliance that could get backdrafted and send carbon monoxide into the air you breathe.

   2. Garen – as Allison stated above “The heat gets into the attic by radiating down from the hot roof deck.” All the roofing solutions proposed (including the more common ridge/soffit vents using thermosyphon air movement) have been meager attempts to solve this. I also live in a 1931 home with walk-up attic storage (that we want to keep). Something I have been trying for several years is to truly mitigate roof deck radiant heat by extending the plastic roof joist ventilation panels to as close to the peak as possible (I have found that just about any long-lasting, solid material fastened against the bottom of the roof joists can be used to create an air chase). This creates a chase area for the roof deck to radiate into, significantly raising the air temperatures within this space and subsequently increasing the thermosyphon air flow under the deck – which helps to cool it without letting it radiate into the general attic space and “hoping for the best”. This greatly helps keep the attic space closer to ambient outside air temps at a fairly low one-time cost (just supplies and labor time). This also reduces the Delta T (difference of air temperature between the conditioned and unconditioned space) between our second floor ceiling and attic, reducing the AC load of the second floor rooms. I have also closed off any air passages through the second floor ceiling into the attic as well as constructed an insulated, movable attic floor hatch cover that seals the entire attic stairwell off from the area above.

5. Long time reader here and armchair energy/hvac nerd. What about the comments you see online at big box stores from buyers of attic fans. A LOT say they have reduced their electric bills or that the stairs is cooler?? I see how they could work but for me I doubt they will be much of a help as I live in Roanoke Va.. even with days approaching 95 my design temp is supposedly still only 89. Interestingly we have ~1300 CDD and ~4500 HDD here.

   Are there actually any studies done on this? I have never ran across a study with a fan off and a fan on. Also what if your HVAC unit, duct and ceiling is sealed? Does it always make no sense to install a fan? I’m trying all my options but my electric bills average $200 a month and I’m only paying 11.4 cts per KWh for a house that is 1128 sqft upstairs and a 600sqft basement.

   1. Shae, yeah, a lot of people have perceptions of improvement because they invested and want to believe it works. I’m sure we’ll get some of those comments in this article, too, because it happened with the previous articles. I’ve already had one on our Facebook page.

      Some of those people may actually see improvement. If they have ducts in the attic, poor attic insulation, and good air sealing, cooling the attic will have a bigger effect on the cooling load inside the home. If instead you have a lot of insulation and an airtight ceiling, lowering the attic temperature will have little effect on heat transfer through the ceiling. Take the ducts out of the attic, and the benefit of lower attic temperature is pretty much gone completely. And then there’s the cost of running the fan.

      The Florida Solar Energy Center did a lot of research on attic ventilation back in the ’80s and ’90s. Unfortunately they tested attics with only R-19 insulation. Here’s a link to their literature review of attic ventilation:

      Literature Review of the Impact and Need for Attic Ventilation in Florida Homes
      https://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

      1. I’m trying to create a somewhat “climate controlled” attic space so we can store things up there that could possibly be damaged from excessive heat/moisture. I was looking at solar powered attic fans to reduce the heat in the attic to bring it “closer” to bearable temps in the attic. What would you suggest as an option in this circumstance?

6. Allison Thank you for mentioning whole house fans. Our research in the Central Valley Research Project showed great results in the reduction of needed cooling when they were used. Unfortunately for people who live in climates with hot nights, they don’t help since flushing the house with nighttime air does not cool the mass of the structure, sorry folks.
   One thing that became clear to me from that project was that the additional attic venting needed for a whole house fan ( to let the air out of the pressurized attic) is detrimental when winter comes and the attic is cooler and the heating load is increased because of the venting.
   The solution is straight forward, have your whole house fan vent to the outside rather than the attic.

7. Another great topic. First of all, I always seal, insulate and condition the attics in the homes I build, just so I don’t deal with all of these issues. That being stated, ridge vents with soffit vents always puzzled me with hip roof systems. The total footage of soffit vents is always several times more than footage of the hip ridge. How is that supposed to work?

8. We live in Houston and have always have had problems with the A/C.
   Besides the heat we have always had problems hi humidity.
   We have replaced all the windows with appropriate windows, replaced the ducts with a higher r-rating and had them sealed. Seer 16 4 ton A/C and r 36 insulation.
   We do have a powered attic fan.
   Recently we installed a continuous soffit vent around the entire house that has ridge vents for years but the soffit vents were plugged.
   Several problems have come up, after installing the soffit vent the entire garage ceiling came down. It seems the blown in insulation absorbed enough moisture that it’s weight increased enough to bring the ceiling down.
   We have had a whole house dehumidifier installed. It took the interior air quality company 5 trips to get it working well enough that with the a/c running and the dehumidifier running we can get the indoor humidity below 50%.
   As the dehumidifier discharges 95 degree air into the living room the a/c has to run even more.
   After reading the first article we turned off the powered attic fan.
   I didn’t seem to make any difference.
   We have calked and sealed for 20+ years. Had blower door tests done, duct leakage tests done and the house has a large electricity bill.
   I’m thinking that the house was constructed so poorly ( built in 1980)
   that we will never find all the problems. The A/C service man helpfully turned the powered attic fan back on.
   Is there any hope that we can have a 75 degree home with 50% or less relative humidity in our climate zone?

   1. David, You didn’t give the size of the house or give the results of the blower door or duct blower test. Are you on a slab or a crawlspace? Though in 1980s its probably a slab. Sounds like you’ve done a lot of things right, but there’s something missing. Any mold issues? High person occupancy in the house? Doing an extraordinary amount of cooking? Lots and lots of plant watering? Doesn’t make sense that the garage insulation would absorb enough moisture to collapse the ceiling unless the material was really cold and there was a condensing issue. With all the caulking and sealing have you addressed the ceiling?

      1. It is a slab,1863 sq ft.
         I don’t seem to have the blower door tests results.
         We have several horrible hip walls in the attic that we have had problems with in the past as well as several areas of the attic that are not accessible. Three adults, not a lot of cooking, all vents go outside, 2 bath vents and dryer vents.
         One indoor plant.
         Garage ceiling came down in July, it was around 100degrees and 80-90% rh that week.
         We sealed everything we could find, ceiling fan electrical boxes and light fixtures are all sealed. Replaced attic stairs with one with a gasket, and zip up insulated attic door cover with radiant barrier.
         The house is brick on three sides, east, west,south.
         The west wall brick gets to hot to comfortably touch and radiates heat into the master bedroom until around 11pm when it starts to cool down.
         Before the dehumidifier was installed we had mold growing in the toilets, bread would mold in a couple of days but no sign of mold in the structure that we have found.
         After the dehumidifier, the rh stays between 45-55%. But it runs almost all the time may-sept the a/c has been checked by several different companies to make sure it’s operating correctly.
         Our electric bills are a bit high because of the A/c and dehumidifier running.
         Hi humidity and temp aggravates my asthma. I just can’t figure out why we don’t get a better moisture break on the A/C.
         Looking at the service guide and such I think the dehumidifier was designed for basements in the upper Midwest, it works here, but not well.

         1. There’s not anything obvious left standing out to me that could be causing the high moisture issues. You said the AC unit has been checked several times . Do you know the air temp entering and leaving the unit and the temp at the supply air registers and return grille?

            1. I believe there are no vapor barriers in the house.
               I know there is little to no insulation.
               I also believe the dehumidifier would work better if it was set up as a return connected to the supply type instead of how is currently ducted.
               I’m having a A/C service company out next week to go over it again.
               I’ll have them check the split temp again and post it.

         2. On a house that’s been audited with a Blower Door test, duct leakage test, and then air sealing work done, I would think that 4 tons is too much capacity on 1800 sq ft.
            An oversized A/C doesn’t dehumidify as well as a right sized unit. You could possibly adjust the fan speed lower, to remove more moisture from the inside air due to a colder evap coil, and longer run times at the lower indoor airflow. Just measure total airflow, and don’t go below around 350cfm per ton, so 1400 cfm total.

9. Before you rip one out know this:
   1) the model at Home Depot uses 1/3 of the energy of the one at Lowes.
   2) in some homes the PAV was part of a system to keep the whole home dry, and it is possible to have INCREASED CRAWLSPACE HUMIDITY as a result of deleting the “high-negative-effect” of those fans.
   3) the 14” round hole with the 6” round obstruction (fan) of a broken or disabled PAV provides more free area than 25’ of Ridge vent.
   4) in townhomes the first 2-4 feet of ridge vent is disabled or outlawed due to fire spread, so a 22’ wife townhome may only have 14-16’ of actual ridge vent.
   5) when we disable those fans after detailed weatherization we like to use the old hood as an exhaust penetration, installing collars and reducers to accommodate a 6” exhaust duct.

10. Exhaust fans in commercial warehouse buildings located in Dallas, TX.
    We put the exhaust fans in commercial buildings, hoping it lowers the temperatures enough that warehouse workers don’t melt. Are we spinning our wheels?
    Different topic. Does spraying 1.5″ closed-cell foam on 2×6 exterior walls and then spraying open cell on top of the closed create any problems? Thanks. jay jones

    1. Jay, no, you’re not spinning your wheels. Exhaust fans do keep attics in houses cooler. In the case of homes, though, they can create other problems (e.g., backdrafting), don’t address the main way attics get hot (radiation), and probably use more energy than they save on cooling the house. As long as you’re not air conditioning the warehouse, it would be a good use of an exhaust fan for cooling.

       No, there’s no problem with spraying open-cell spray foam on top of closed-cell. It’s actually a less expensive way to get the full R-value you need while using the closed-cell for better air sealing and vapor control.

11. First article I have read but I find it very interesting. I bought a cottage 3 years ago in northern Wisconsin. I sm in the process of putting on a new roof. The roof has 2 of these power vent units.
    We have no AC at this time but do plan to add in the future. This is a A-frame house with a finished basement.
    There is a large vent way up by the peak of the ceiling. My thought is the power vents pulled hot air out of the attic and also the house. The past owners had a window AC unit so they disconnected the vents. With the roof the contractor suggested we keep the vents in others have said remove. The contractor also plans to eliminate the soffit vents since they are only 4″ wide and in his words providing very little good for air movement. I being one who has to rely on a contractors word as I know little about ventilation. But I’m not sure he is correct on this one. Your thoughts? And thank you for the very interesting article I plan to look for more.

12. Hello,
    A.C’s are everything here in the Southwest. Time to concentrate on heat recovery units off the condenser. For water heating…of all sorts. No one cares about the attic, just put an attic tent if you’re concerned.

13. Allison, I agree that a hot attic isn’t a problem if the surface between it and the conditioned space is well sealed and insulated. The “problem” is the cool night that follows the hot day. In the Boston area, with inadequate attic ventilation, the water held by hot air during the day condenses on the cool roof deck at night and mold follows. It’s more than a sensible heat, energy efficiency issue.

    1. I the thermal barrier is tight & the ventilation is poor, just how much humid air gets into an attic?
       Mold doesn’t follow one night’s condensation, the next day does. If it’s a sunny day doesn’t that drive the moisture back out of the roof decking?

       1. On hot humid days, I believe the vapor drive would be towards the interior of the attic.

14. Whatever we are doing building we are ultimately looking for performance. There’s often any number of prescriptive paths, hurray for those who find the best one, but we all aren’t starting from the same place, so I’m most interested in making sure I get “to the top of the mountain”.
    Of all the reasons to ventilate an attic, it’s my opinion that the primary reason is to lower its temperature in hot weather.
    In a passively ventilated attic the air flow from the eaves to the high outlets is hardly noticeable if the wind isn’t blowing & on hot days in many places, it doesn’t blow much!) Air temps in those attics tend to stratify, being hotter at the peak but still very hot just above the insulation.
    A mechanically vented attic can lower attic temperatures much better, that’s why they have existed for the past 50 or more years. The problem is obviously that lowering attic by sucking the air out of the living space is……what’s the technical word, oh yeah; stupid.
    Insulation is working against a temperature differential. I seldom see passively vented attics with summer temperatures under 120 degrees. If one is trying to maintain 75 degrees inside the living space the heat gain at the ceiling is about the same as the heat loss in winter when the attic is 30 degrees. If the attic temperatures are 140 degrees in summer which I often see the corresponding heat loss in winter would require 10 degrees in the attic. So it’s common sense to realize that the closer we can bring the attic temperature to the ambient temperature of the summer air outside the house, the better.
    Sadly a tight lid is still a rarity in existing homes, but it can be done & should be done on every house being built no matter how an attic is ventilated.
    The idea of mechanical ventilation makes sense, but like anything else doing it wrong doesn’t!
    When using an attic exhaust fan a tight lid is critical, but what’s also critical that there is enough (better yet, more than enough) exterior air (‘make up air’) intake capacity.

    Really is that so hard?

15. It seems to me that a whole-house fan would do an even “better” job of creating appliance backdrafts and pulling crawlspace or basement air into the living space than attic exhaust fans.

    1. A whole house fan is used with some open windows at night (when it is used) so the pressure in the house is very near neutral. If the windows were closed then you are right the potential for backdrafting is high. With respect to crawlspace air, my thought is to put a low watt fan exhausting from the crawlspace with all the old crawlspace vents closed. This pulls a little air from the house, which has a lower moisture content and dries the crawlspace, making it a space that mold does not favor.

16. I have a 1.5 story tudor bungalow. The Unico air handler is in one of our attic “cubbies” on the 2nd floor.
    The attic space is insulated with open cell spray foam.
    The second floor cools ok, but sometimes there is a musty smell, and I wonder if it’s due to lack of ventilation in the attic. The HVAC techs have not found any leak issues with the handler, and suggested the smell was normal and that I should just put an air freshener in there. Not the answer I was looking for. I wondered if adding venting would help, but it would be difficult to do given the insulation on the rafters, and this post makes me rethink that idea. Your thoughts?

    1. Larissa, it’s going to be hard to deal with several small attics instead of one big one. A powered attic ventilator would definitely be the wrong approach, though. Here’s another article I wrote on using a small exhaust fan to control humidity in spray foam attics:

       Is It Ever Helpful to Use a Powered Attic Ventilator?

17. To help answer the question presented by the article, I think there is definitely something to be said for powered attic ventilation for areas with a combination of high heat and humidity. While I agree it is the wrong solution to the problem taken at the article’s chosen scope limited to “heat” and discrediting the marketing behind attic ventilation products (which isn’t hard to do). If your attic has passive ventilation but still creeps above 20-30 degrees over ambient in 100 degree summers with high humidity (90%+), your enemy isn’t heat–it’s moisture, especially if your air handler is up there. 130+ degree air can hold more moisture than you think, and all that moisture stays inside the attic when it finally cools during the night. As the hot attic air cools at night, the relative humidity increases. Since heat is no longer being used to “move” the air through the passive vents in the attic, the relative humidity will continue to rise with a much higher dew point than outside air. Given that, a powered ventilation to replace the cubic volume of the attic per 1-1.5 hour ran at a few intervals of the day wouldn’t be a bad investment. Your attic will still be hot, but moisture would no longer be a concern.

    1. In coastal NC, in the evenings and overnight, with the temperature drop, RH maxes out and there is condensation everywhere. Wouldn’t the fan be pulling in that saturated air as well?

18. Chad, I have not checked your calculations, but there are other reasons to not have the attic ventilator. I was standing next to a natural gas furnace checking for CO. All was good, very low. Then the attic ventilator went on, the meter went off scale and I reached over and turned off the gas. Felling very bad, I walked into the front yard and barfed my guts out.
    Some houses are very connected between the attic and the mechanical room. As I am sure you know a depressurized mechanical room is a bad thing.

19. Middle Tn – we used 2 PAV to replace all our ridge vents when we got a new roof – roughly 5300 sf house – 3 stories plus high pitch attic / roof. Choppy roof that had very short LF of ridge vents. Our 2nd story duct work is in the attic, getting hot… PAV may not help with the radient heat, but if its 110 degrees instead of 145 than I figure it’s better. My first goal was to reduce the heat entering from the attic into 2nd story, and reduce the heat entering the attic ductwork thru convection or radiant. Wrapping the ductwork or foaming/ sealing the attic was not cost efficient for me. A $350 attic fan may be good enuf – for me. It may be a wash cost wise, but if my AC can ‘keep up’ on 100 degree days upstairs, I’m happy. PS – house has good intake ventilation in the soffits. Sometimes an analog truck is better than a Tesla…good luck.

20. NW Arkansas resident. 5 yr old custom home w HVAC in attic with ducts suspended half way between the roof deck and floor. can’t put in radiant barrier per a contractor who just put in r42 due to some insultation settling/rafter exposure (it has helped some).

    Was going to put a solar fan in but now not so sure. HVAC can’t keep up when its 90 and sunny w temp set at 75. It’ll run 45 min/hour between 3 and 7p. Ducts are insulated (not sure if its r6 or r8) and i suspect radiant/ambient heat is killing the cold air.

    What can i do to improve the efficiency? Wrap the air handler? Is there better duct insulation?

21. My elderly friend lives in southern California in a 2700 sf ranch style, open beam house with 12 foot gabled ceilings, no attic. There is no insulation. The interior temp of the ceiling surface can exceed 100 degrees on a hot day, essentially a 2700 sf whole house radiator. She has 2 central hvac units, but hot air gets trapped at the high points of the ceiling. She lives on social security and can’t afford to replace the roof to add insulation., and insulating the ceiling is not an option. I was going to suggest a whole house fan to evacuate the (blazing hot) air trapped in the area at the peak of the gable, but your writings condemning attic fans while praising whole house fans leave me wondering why a whole house fan would not cause the same problems as an attic fan, eg removing the cooled air, causing backdrafts etc.?

22. Hi Allison, great article!

    But we’re still debating if we should install an attic fan in the dormer of our cape cod. It gets really hot up there in the summer. We have gable vents on each side of the house. We just reinsulated one of the rooms in the dormer with rockwool (ceiling is not covered with drywall yet). We don’t have central AC, but we’re considering adding a mini split system.

    Does it make sense to add an attic fan in this case? What about a radiant barrier instead?

    Thanks!!

23. Accepted practices for home ventilation changed, of course, with the introduction of Central AC. Prior to that, double hung windows, operable transoms over doors, whole house fans (I am from Michigan and had them frequently), etc. A “breathing” house was called normal.
    Without AC, attic fans make perfect sense to me as there is no risk of pulling cooled air up into the attic.
    Obviously, the issue created by AC is the temperature differential and dew points. Now we have to keep the warm air from ever meeting up with the cool air. I.E. very tight houses.

24. My one-story Montana house has the typical A-frame attic, unused for storage and never accessed, under an asphalt-shingle roof. No AC, no ductwork, no venting into the attic. It’s sealed from the rest of the building with only a 3’x3′ tightly-fitted trapdoor allowing access. Doesn’t it make sense to ventilate the attic in the summer to keep the air cooler and carry off some of the radiant heat generated by the sun on the asphalt? A fan in some electrical devices like a microwave oven cools the works by circulating cool air around the heat-radiating elements–tubes, heat sink fins, etc. Why doesn’t the same heat-transfer effect apply to cool air carrying off radiant heat in an attic? My attic fan recently went dead and I’m thinking of replacing it. Good idea?

    1. I’m sure that someone already asked this, but I couldn’t find the answer. So does this mean we’re better off closing the vent altogether? What should we do with the existing power ventilator?

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