Posted by: SherlockHomesSY | 6 Aug 2011

Why Not to Use Powered Roof Ventilators


I’m in the unenviable position of having to tell a builder the house they’re building can’t be certified for one of a variety of green building programs.  I have no problem doing it when the reason is solid.  However, enforcing a provision I don’t understand is difficult to do.  Such was the case with powered attic ventilators (PAVs).

This episode started when a new Georgia homeowner, originally from the tropics, decided to require the builder of a green energy home to install powered roof vents.  Unfortunately, I found them installed when I did the required pre-drywall inspection.  It’s obvious the homeowner did his homework and I applaud him!  He knows ventilation is used to cool the attic and remove moisture in the winter.  A review of the available literature on line shows this to be the widely held view.  Unfortunately, he, like most Americans, thinks more is better.  Now I have to explain why it’s not wise to use powered roof vents!

No PAV Policy. I have to say no to powered roof ventilators because the program requirements say so.  I could hide behind the rules and just refuse to certify the house with no further explanation required.  I don’t like to be treated that way and try to refuse to treat others that way too.  The “I’m the mommy, that’s why” argument never did sit well with me.

Typical Reasons. There are three well known valid reasons for saying no to powered roof ventilators.  I’ve known these arguments for years.  The Natural Florida Retrofit and Advanced Energy Corporation Applied Building Science Center reported results of their research in 1994, The Unplanned Impacts On Houses By Powered Attic Ventilators.

1.    They can backdraft combustion appliances
2.    They can pull conditioned air through holes in the ceiling into the attic
3.    They use more energy than they save

In this case, there are no combustion appliances to backdraft, the ceiling will be sealed tighter than a drum, and a solar powered PAV is installed.  What’s the problem?

Principles of Attic Ventilation. I went on-line in search of white papers.  I found a very solid article about attic ventilation by Air Vent, Inc. published in Dec 2010: Principles of Attic Ventilation:  A Comprehensive Guide to Planning the Balanced System for Attic Ventilation.  Sure enough, they convincingly argued for the use of a powered roof vent.  My next step was to discover the flaw in their argument.  I didn’t have to search long only because I got help on where to start looking.

Preferred Practices. Before revealing what I learned, I need to discuss what the preferred options are to avoid using a powered roof ventilator.  In general the principles are as follows:

1.    Use balance ventilation between the soffits and ridge
2.    Provide adequate ventilation openings
3.    Use a ridge vent rather than a gable vent
4.    Install a radiant barrier
5.    Use lighter color roofing materials
6.    Air Seal the attic ceiling
7.    Seal all ductwork in the attic to make it leak as little as possible

I did all this to my house and I have a much cooler attic.  Currently, I’m sitting in my office during the hottest three weeks of the year in Georgia without air conditioning supplied to the room.  I could not be in this office without the room air conditioner running full blast in years prior.  (Another major reason is that I fixed my attic kneewalls too.)  Even then, I had to abandon my office after 4 PM.

Informal Survey. I found it interesting to see the results of this informal study someone did just a few days ago.  Someone measured the temperature of attics with various ventilation strategies on a 105° F day.

1: Ridge vent house: 155° F
2: Power vent house: 145° F
3: Attic turbine house: 145° F
4: Radiant barrier stapled to underside of roof: 125° F

In researching the topic of attic ventilation in the past, I figured out the building code rules worked well for roof pitches up to 6/12.  After that, a different level of thinking is required.  I’ve been in several houses since then with roofs with greater roof pitches that had ridge vents and continuous soffit vents.  However, their attics are incredibly hot.  I knew not to recommend powered attic ventilators, so I frequently recommended turbine vents instead—with great success!

Ridgeless Roofs. However, I often wondered what I would say to a homeowner with a hipped roof, especially for a house that didn’t have much in the way a ridge and the roof pitch was greater than 6/12.  It’s a known fact that a hip roof withstands high wind events better than houses with gables.  Structurally, I prefer them.  I can certainly recommend turbine vents and static vents (turtlebacks), but they may not be enough.  The folks at Air Vent advocate the use of a powered attic vent in this case.  I’d opt for a foamed roofline instead, which takes the question of ventilation completely off the table.

No Cooling Savings Evidence. After spending the day researching, I came across this finding: houses with powered roof vents don’t perform any better in terms of heating & cooling costs than houses that don’t.  Perhaps the informal survey reinforces the point.  I had to ask, Why?

I found this in an on-line article reprinted by permission: Danger Above, Brendan Reid, Contracting Business, March 1998

•    3. Even with a perfect installation, PAVS still don t work significantly better than passive ventilation. Scientists at the Florida Solar Energy Center have proven that infrared radiation is how heat primarily gets from the underside of the hot roof sheathing into the insulation on the attic floor below. The invisible radiant energy waves pass right through the attic air without warming it, and are absorbed by the top surface of the insulation. Once the top of the insulation is hot, it in turn transfers heat to the air. It turns out that instead of the hot attic air heating the insulation, it s actually the insulation that s heating the air. Removing the hot attic air at this point is like shutting the barn door after the horse has escaped.

Case Closed. The argument for using a powered attic vent to reduce cooling costs is flawed.  There is little benefit to increasing the air exchange rate beyond what occurs naturally through passive vents.

In Conclusion

Back to the house that started this investigation.  The house has a 12/12 pitch roof with a great passive ventilation system on it already.  I’ve already told you that all the traditional arguments for not using a powered attic vent have been answered.  This house is in Atlanta, Georgia, so ice damming will never be an issue.  In my experience here after inspecting over 1000 houses, I’ve never seen an attic severely moisture damaged by moisture venting into the attic.  It’s also obvious that an attic with a fan is somewhat cooler.  So I’m left with the argument of reducing cooling cost by using the PAV.

Here’s what I’m going to do for this house.  I’ll recommend replacement of the powered attic vents with turbine vents or complete removal.  The house may need the benefit of turbines on really hot days, but I don’t think so.

Here’s my recommendation for all new houses where the roofline is not foamed.  Add a radiant barrier, especially if the roof pitch is over 6/12.  If there is any ductwork, I’ll strongly encourage it under all conditions.

I now have 3 reasons to recommend radiant barriers.  First, they lower the HERS Index by up to 3 points.  Second, my attic is cooler in the hottest part of the year.  Finally, cooling off the attic after the radiant energy has heated the attic ceiling below is pointless.  The message is to stop the radiation in the first place with a radiant barrier.

Oh yeah, I’m not going to certify the house with operational powered attic vents present!


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