Advice provided by: David M. Pratt, P.E., CEM, LEED AP, MWH

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In this article, I'm going to address solar powered attic fans.

Before getting to solar attic fans, specifically, it's important to note that there is some debate on the pros and cons of attic fans, in general, whether they are powered by the sun or normal household power. I want to briefly touch on those before I go into tips on purchasing solar powered attic fans.

Research done back in the mid 90s by Advanced Energy Corp had uncovered issues with power attic ventilators such as excessive moisture, combustion safety and depressurization issues due to drawing conditioned air out of the house because of poorly sealed and insulated attic floors. Drawing conditioned air from the house to the attic also obviously uses more energy.

Also, the Florida Solar Energy Center conducted research (http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/) around the same time that concluded solar attic fans had relatively low return on investment, only reducing the annual energy usage by a measily 460 kwh/yr.

That said, I think we can take those case studies with a grain of salt due to their limited sample size and geographical limitations.

In my opinion, for the most part a solar powered attic fan should be a good investment. Lowering the temperature in your attic space should reduce your overall energy consumption, and as long as your attic floor is properly sealed and insulated you shouldn't have any problems with drawing conditioned air from the house to the attic.

Fans are sized by their air flow in cubic feet per minute, or CFM. In order to properly size your attic fan, you have to first determine the volume of your attic. A good rule of thumb is that your attic needs 10 air changes per hour. The volume of your attic (L*W*H) in cubic feet multiplied by the number of air changes per hour divded by 60 equals the number of CFM required.

If this is too complicated, the Home Ventilating Institute (http://www.hvi.org/resourcelibrary/HowMuchVent.html) says to mulitple the square footage of your attic space by 0.7, and that is how many CFM you need. So if you have a 1000 SF attic, you need a 700 CFM fan. Add 15% if you have a steep roof, and another 15% if you have a dark roof.

HVI also recommends dividing the amount of CFM required by 300 to determine the soffit intake vent area.

I would also recommend that you find a vendor that can include a battery with the fan, because without one, the fan stops working every time the sun goes behind a cloud.

In addition, I recommend installing a thermostat so the fan only works when you want it to work. This will increase the life of the fan.

In terms of installing a solar attic fan, whether this is something you can do yourself all depends on your home repair skill level. If you've never done any type of roofing, I wouldn't recommend it. Leave it to the pros. But if you're pretty handy and have done some roof repair before, go for it.

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