skip to Main Content

Is It Ever Helpful to Use a Powered Attic Ventilator?

Powered Attic Ventilator

It’s time to revisit one of my favorite topics:  the powered attic ventilator.  Ten years ago I wrote an article titled Don’t Let Your Attic Suck – Powered Attic Ventilators Are a Bad Idea.  It’s got 169 comments and would probably have over a thousand if spammers didn’t force me to close comments on older articles.  In 2014, I wrote a followup titled The #1 Reason Powered Attic Ventilators Don’t Help.  It’s got 103 comments.  Even as old as they are, those two articles have been viewed 15 thousand times already this month.

With the heat wave in the Pacific Northwest this past week, a lot of people are looking for help with cooling.  (If that’s mainly what you’re looking for here, see the Related Articles section below for some good links.)  Powered attic ventilators seem to be one option they’re looking at.   But I also get questions on this topic from time to time, so today I’ll explore the issue of whether there’s any situation where a powered attic ventilator might be justified.

Recap of powered attic ventilator problems

You can go back and read the previous articles for more details—and he hundreds of comments—but here’s a quick rundown of the problems I described.

  1. They pull air from wherever they can get it, including conditioned air from the house.  Consequently, some of that cooling they provide to the attic is from your air conditioner.
  2. They can suck moist, moldy air up into the house from a basement or crawl space.
  3. They can backdraft a water heater, furnace, or boiler.  Backdrafting can lead to incomplete combustion, which can lead to carbon monoxide in the air your breathe.
  4. They’re a solution to the wrong problem.  The heat gets into the attic by radiating down from the hot roof deck.  Using a fan to address radiant heating is like thinking you can’t get a sunburn on a breezy day.

Oh, and if your goal is to save money on your cooling bill, they probably won’t do that either.

Questions from readers

Since the earlier articles are closed for comments, I get questions through email and readers filling out the Contact Us form here on the website.  If you’re not immersed in the world of building science, it can be a confusing issue.  You may read through the articles and comments and believe your situation is unique and maybe a powered attic ventilator would be good for your house.  Here are some of the questions I’ve received from readers about their situations followed by my answers.

The installers of the new HVAC system told us we “may” need to have a thermostatically controlled exhaust fan installed in order to protect the air handler from excessive heat. It’s June 11th, 92 deg today, and the temperature in the [attic] crawlspace is already well over 100 deg. May I ask your opinion on this specific instance?

~ Steve W., Pennsylvania

Steve’s HVAC company thinks he needs a powered attic ventilator to protect his air conditioner from attic heat.  An unconditioned attic is certainly a bad place to put your heating and cooling system, but a powered attic ventilator will do little to extend its life.  I’m not a big fan of radiant barriers because they’re cost effectiveness is marginal, but if an attic air conditioner really needed to be protected from heat, you’d do better to reduce the radiant heat by putting a radiant barrier above the unit.  A powered attic ventilator doesn’t address radiant heat and will reduce the attic air temperature only a bit.  The air around the AC will still be hot.

What if a 2nd gable fan was installed that drew air from the outside located at the other end of the home opposite of the gable fan that sucks the air out?

~ Carl S., California

Balancing the air flow into and out of the attic is an attempt to eliminate problem #1 above.  Even if the two fans are moving equal amounts of air into and out of the attic, though, there’s no guarantee that the exhaust fan isn’t pulling some air from the house and the supply fan isn’t pushing some hot attic air into the house.  Air takes the path of least resistance.  In addition, more fans means more electricity use.

We do not have ac so those fans sucking the hard earned cold air out of the house isn’t an issue. There is no vapor or reflective barrier on the attic ceiling, just wood and nails. During the day our house stays relatively cool with all the windows closed. But come around 7pm until about 11 it’s is hotter inside than out (all windows open now) and from a thermal camera it looks like the attic is dumping stored heat down into the house. Would it be worth it in my case to rig up 3 (or more) of these fans to 3 of the attic gables in my 1,300 sq ft, 1 story home?

~ Nate B., Oregon

What Nate really needs is a fan that will do what’s called a night flush.  In dry climates—and yes, the air in the Pacific Northwest is dry on warm summer days—flushing the air out of the house with a fan at nighttime brings in cooler outdoor air and is a great way to cool without air conditioning in that kind of climate.  Don’t try it in Houston in the summer unless you want to grow mold in the house.  To do a night flush, though, you want a whole-house fan, not a powered attic ventilator.  You want to cool the house, not the attic.  This one by Tamarack* (photo below) is a modern version that does a good job and won’t steal heat from the house in winter.

Air-sealed, insulated whole house fan by Tamarack
Air-sealed, insulated whole house fan by Tamarack

Last week I was replacing some of the screens in the soffit vents and hot air was POURING out of them, and it’s not even summer yet! My immediate thought was to install an attic ventilation fans on each end of the house in the exterior walls under one of the gables that are located on opposite ends of the house (the house is laid out like an “H”). I’m reading the explanations you listed for why attic vents aren’t a good idea, but in this situation I don’t know how else we’d get the hot air out of the attic. We do have insulation in the floor of the attic over the house, but the hot air is trapped up there and it seems like we’d do much better getting it out of there. What are your thoughts?

~ Mark K., Florida

Many people seem to think a hot attic is a bad thing.  It’s certainly bad for air conditioners and ducts that might be up there, but it’s OK for an attic to be hot.  If that heat is getting into the house, then you need to do more air sealing and insulating.  That applies to poorly sealed and insulated ducts, too.  (I just wrote about what you can do for ducts in an unconditioned attic.)  Unconditioned attics get hot.  It’s a fact.  Let it be hot up there.  If it’s causing a problem for you down below, there are better solutions than a powered attic ventilator.

The insulation pro is telling me to only add an attic fan (and do not do any additional insulation or touch the old insulation) because adding 14″ of blown insulation is not going to leave enough space in the attic for air flow. I have tried searching the internet but have not had any luck one way or the other about whether or not too much insulation is a bad thing. And there are mixed reviews about an attic fan being a solution. Your article was very informative but I am left with reservations regarding the best choice given the specifics of my attic. I am hoping you are willing to provide a fellow Gator some advice. 😉

~ Erika M., Florida

The best choice, Erika, is not to install a powered attic ventilator. As was the case for your fellow Floridian Mark, it’s OK for the attic to be hot. Also, here’s another idea that’s going to get some readers foaming at the fingers as they type their comments: Even passive attic ventilation doesn’t help much. It doesn’t extend shingle life. It doesn’t save energy in cooling season. It can help with condensation in the attic and ice dams on the roof in cold climates, but Florida doesn’t have that problem.

Is it ever helpful to use a powered attic ventilator?

I’ll admit right up front that powered attic ventilators are helpful for one thing.  If you don’t have air conditioning or natural draft combustion appliances, a powered attic ventilator can keep your attic cooler.  I had someone once tell me that the rubber handles on a wheelchair he stored in the attic stopped melting after he installed one.

But for keeping the house cool, a powered attic ventilator isn’t a good idea.  To overcome the problems I listed at the beginning of this article, you’d need a situation where the following are true:

  • The house doesn’t have air conditioning, or the AC is turned off when the attic ventilator runs.
  • The house has no natural draft water heaters, furnaces, or boilers.

If you don’t have air conditioning, why not just use a whole house fan to cool the house instead of trying to cool the attic? I read the questions readers send to me on this topic and keep hoping to find a case where I can say, yeah, you really don’t have a better option for keeping your house cool than to put in a powered attic ventilator. I’m still waiting.

 

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.

 

* This is an Amazon Associate link. You pay the same price you would pay normally, but Energy Vanguard may make a small commission if you buy after using the link.

 

Related Articles

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

The Best Way to Cool Your Attic

The #1 Reason Power Attic Ventilators Don’t Help

9 Uncommon Tips for Keeping Cool with a Struggling Air Conditioner

Simple Steps to Improve Air Conditioner Performance

 

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

This Post Has 32 Comments

  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?

Leave a Reply

Your email address will not be published. Required fields are marked *

Back To Top