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10/06/2010

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Energy-Saving Tips

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Why Power Attic Ventilators Are Such a Bad Idea

This is a guest post by Allison A. Bailes III, PhD, founder of Energy Vanguard.

power-attic-ventilator Trade shows can be both educational and infuriating, and there’s one product I see at a lot of trade shows that fits easily into the latter category: power attic ventilators. The variety of this device that’s been popular lately is the solar-powered attic ventilator. Using the sun to make the electricity for this fan, however, makes it only marginally better than its grid-powered cousin.

Oh, yes, power attic ventilators will probably keep your attic cooler, and that means you’ll have less conductive heat transfer across your ceiling. The problem is that a significant portion of the cooling in your attic will be provided by your air conditioner. So you spend money to buy the fan, to run the fan if it’s not solar, and then your air conditioning bill goes up, too.

How can that be, you ask? Isn’t it supposed to pull that blazing hot air from the attic and send it outside, replacing it with much cooler outdoor air that gets pulled in through the soffit and gable vents? In marketing theory, yes. Building science shows a different result, however.

What really happens is that when that power attic ventilator runs, it’s going to pull air from wherever it can find it. Since air takes the path of least resistance, some of it will most likely be coming from the conditioned space in your home. So basically what you’re doing is air conditioning your attic. The longer the fan runs, the more conditioned air it pulls into the attic.

If you have a perfectly air-sealed ceiling, you’re not going to have this problem, of course. The reality, however, is that few ceilings are leak-free. Since air needs only a pressure difference and a pathway to move, and your ceiling probably has plenty of pathways, it’s best not to enhance any pressure differences that will increase air movement into or out of your home.

In other words, don’t install that power attic ventilator. If you have some installed already, disable them so they never run.

If you’re tempted to buy one because it’s solar powered and won’t increase your electricity bill, go back and read what I just said. These things won’t save you any money. Even if they’re solar, they’ll still suck the conditioned air out of your house and make your bill higher, not lower.

Image: Allison A. Bailes III, PhD

  • Ed H. Wednesday, October 06, 2010

    So what about a house that isn't air conditioned?  At worst, it would act as yet another outside fan, right?

  • Leo Wednesday, October 06, 2010

    What if you only run the AC occasionally?
    For example in our house we only turn the AC on when the temperature and the humidity is too much to bear. Would a power attic ventilator reduce the number of days we have to resort to the AC?

  • Chris Wednesday, October 06, 2010

    This is only true if you have and use air conditioning.  If you don't then this is likely a good way to help keep you house cooler in the summer.

  • solarfan Wednesday, October 06, 2010

    unfortunately I have to disagree on this from my own experience. My home is 26 years old with 3 whirlybirds on top. I decided to install a single 16" 18v solar fan under one of them exhausting the hot air since I live in texas. After a month my home was much cooler and my electric bill did go down. Since this was doing so well I installed a second fan and dropped the temps in my attic as well as my home which is all brick. My thermostat stays on 80 degrees in the summer months with a nnice 77 degree average temp in the home with cieling fans running so from my own experience I would say this is not the same results everyone would have.  

  • Ryan Wednesday, October 06, 2010

    I could see if they were talking about "whole house fans" where it's a direct fan from the living area to the attic area, but if it's a solar powered fan that just pulls from the soffit vents, I would like to see the difference in airflow from the soffits (assuming they're correctly cleaned, and maintained) vs. the 'leaky ceiling'.

    More important to me, since you're Seattle based, lets talk about all the houses that don't have central air... If the living quarters are not air-conditioned, then it's pulling outside air from the soffits as well as air from the living quarters, all it's doing is increasing the air flow in the living quarters... How is this a bad thing? We've got a window air-conditioner for summer months, and it rarely runs. Decreasing our attic temp will mean that it runs even LESS often, and we're still increasing our airflow for the living area of my home. Not a bad product if you ask me...

  • Patrick Wednesday, October 06, 2010

    This article seems incomplete.  It assumes that everyone is running an air conditioner any time the attic needs additional ventilation.  Does an power attic fan make sense if I'm relying on summer breezes to cool my living space?

    What if I have *one* room that is cooled via a window unit and there is finished living space above it?

  • Kurt Schorsch Wednesday, October 06, 2010

    thank you for clearing that up.

     

  • Chuck Thursday, October 07, 2010

    Do you have data to support this?  How dependent is it on older/newer building codes?

  • dns Thursday, October 07, 2010



    "Since air takes the path of least resistance, some of it will most likely be coming from the conditioned space in your home. So basically what you’re doing is air conditioning your attic. The longer the fan runs, the more conditioned air it pulls into the attic."

    You don't know what you are talking about !

    The path of least resistance for air infiltration is through either your soffit venting, rooftop vents or gable vents and the powered vents work well !

    Why do we insulate - to stop infiltration !

    What does the drywall stop - air infiltration  ?

    You need to go get some more degrees !

  • Allen Thursday, October 07, 2010

    I've heard this argument before. I think it would be true with a giant fan that created a large vacuum, but I can't see most solar fans doing that.  In addition most attics have vents around their parameter, and as you say, the air takes the path of least resistance.  If you have big holes in your ceiling, maybe it would suck your AC out of your house, but in 99.99% of the cases, these vents would provide a much easier air path then the air in your ceiling would.

  • Dom Thursday, October 07, 2010

    Good evening Allison,

    I've just read your article pertaining to the use of attic fans.

    We are located in Quebec, Canada where a vapor barrier and at least 8" of insulation is installed, in our attics, with this situation do you still feel that a fan in the attic is a waste.

    Thanks

    Dom

  • Sean Tuesday, October 12, 2010

    This is misleading. Here's what really happens:

    If you DON'T install an attic fan, your attic will get super hot and all that trapped heat will radiate down (your celings will feel very hot), thereby causing your airconditioners to run more. So even if the attic fan did such some of your air conditioning (which it won't if you have proper ventilation in your attic for airflow), the fact that it's getting rid of all that trapped heat means that you will end up using less air conditioning at the end of the day.

  • Steve Monday, November 01, 2010

    As an HVAC contractor, if you're sucking air out of your living space with your attic fan, you have serious other issues than that fan. Also, roofing contractors will tell you, your shingles will last much longer with a cool attic.

    Like most of these 'self-help' programs, the experts are not hired by what they know, or by real-life experience, but by who they know.

  • Debra Monte Wednesday, November 10, 2010

    To dns,

    There is a lot more to building science and pressures than the average person realizes.  Allison has a lot of both knowledge and experience, so there is a lot of validity in what he stated.  I only wish that he would reply to these posts, to clarify.  However, you have gotten a lot of the building science confused.  To quote you: "The path of least resistance for air infiltration is through either your soffit venting, rooftop vents or gable vents and the powered vents work well !"

    Yes, this should be the case, and most of the time it works this way, (sometimes to a lesser degree) provided all conditions are optimal.  That being if your attic is unconditioned and your ceiling below the attic is properly sealed off and insulated, which includes even small openings and cracks, gaps around the attic access hatch or pull down stairs, for lighting, recessed lights, electric boxes, vents, flues and drain waste vents, etc...  All these leaks add up to more than we realize and we end up heating our attic space in the winter and having to keep our AC running more in summer.  Adding more ventilation to an attic that already has ridge vents along with soffit vents and gable vents can cause additional air to pulled through these gaps and cracks.  But, what is worse is that in the winter, the heat is pulled up into the attic space because air will move from heat to cold and from high pressure to low pressure, all in an effort to normalize it.

    Next you state: "Why do we insulate - to stop infiltration !"  This is not why we insulate.  Insulation is for the thermal boundary and works to decrease heat transfer.  It doesn't actually stop air movement, however, depending upon the type and amount of insulation, it can slow down the infiltration rate.  Fiberglass insulation that is dirty is a sign of infiltration in that area, it filters the air as it continues to pass through it.  So, we insulate to slow down or decrease the transfer of heat.

    Next you state: What does the drywall stop - air infiltration  ?  Drywall may stop some of the infiltration because of it's make up and density, but it is not considered a true air barrier, it works in part as a barrier, not alone.  You have to ensure that the drywall is properly installed and taped and has at least two coats of mud.  However, there will be gaps all around the edges, particularly at the bottom of the wall where it is standard building practice to leave a gap of at least an inch and more in many cases.  Remember the drywall and the insulation are just parts of the whole system.  We have to remember that a house is a system and what goes on in one area, will affect another.  There are vapor barriers, thermal barriers, air barriers.  So, dependent upon the region in which you live, vapor barriers are used in different positions and different amounts and types of insulation are used.  If you dense pack cavities with cellulose you can achieve not only a high R-value (decrease the heat transfer the higher the R-value), but you can minimize the air infiltration, not stop it.  Spray or injected foam will stop the air infiltration (closed cell, which has highest R-value and acts as a triple barrier against air, water/vapor, heat, and insect/pollutants) provided it is also applied properly and has no gaps. But there is so much more to it.... much more to learn and do...
    Our buildings are far from tight, they are mostly leaky and that is what is costing us more money and having a negative impact on the environment.

    You lastly stated: You need to go get some more degrees ! 
     I believe you were being sarcastic with this line, but getting an education, whether it is in a classroom, online, from experience or other's experience, is important and it's just as important for us to share the knowledge and experience.  Sign up for energy efficiency blogs and take a course or two in building science.  

    I have taken these courses, I am a Home Inspector and a certified Building Analyst and Envelope Specialist through BPI, the Building Performance Institute.  I use a blower door system with a manometer to test the amount of air leakage there is in a building and I use an infrared camera for thermal imaging to find and document the leaks and other problems in the building.  Each home is different, but I have yet to find a home that is not leaky, with exception of a newly constructed energy efficient building.

    I suggest you have a professional energy audit, we can all learn more.  

  • AirierIndia Wednesday, November 17, 2010

    Airier, Industrial ventilator, Eco ventilator, Air ventilator, Air ventilator, Wind turbine ventilators, Turbo ventilator, Turbine ventilators, Rooftop ventilator.

    http://www.airier.com/

  • pattyinmidwest Saturday, January 15, 2011

    i just read this for the first time. my electric and gas bill was 497.00 this month! i need insulatio you think? basement sills. i have radiator heat. dirty insulation in attic space. learned alot already. this site will be a favorite

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