Q&A / 

Solar Powered Attic Fan

DEAR TIM: My attic space gets as hot as blue blazes. I saw an advertisement for a solar-powered attic fan and that seems like a great way to remove heat and reduce my carbon footprint. Do these fans really work, and are they hard to install? Will the fan significantly reduce the temperature inside my attic? What are the pros and cons to these solar attic fans? Brad L., Phoenix, AZ

DEAR BRAD: I installed a solar-powered attic fan last year to see how well it would work. The installation went like clockwork. I had decided to do the work early in the morning while it was cool on the roof. Within an hour after installing it, the sun hit the solar panel and the fan blades started to spin. It was almost magical to see the solar attic fan work.

You are a pretty clever guy to get the sun to do double duty. It makes perfect sense to make the sun cool your attic space, since it is the source of the problem in the first place. As for reducing your carbon footprint by not using electricity from a power plant, I can't tell you if the solar fan you might buy will be that environmentally friendly. The manufacturing process used to make your solar fan might actually be quite carbon positive. But in any event, you are doing the right thing by trying to use a solar-powered fan.

This solar panel produces enough electricity to power a spinning fan in the round ventilator on the roof behind the panel. But the fan only works when the panel is basking in sunlight. PHOTO CREDIT: Tim Carter

This solar panel produces enough electricity to power a spinning fan in the round ventilator on the roof behind the panel. But the fan only works when the panel is basking in sunlight. PHOTO CREDIT: Tim Carter

There are pros and cons to solar-powered attic fans. The biggest pros are they work for free using the suns rays, and they do exhaust hot air. The solar fans do not require the services of an electrician to connect. They are also very quiet.

But after seeing my solar-powered attic fan work, I have a list of cons. My fan only works when it is getting direct sunlight on the solar panel. If a cloud drifts through the sky blocking out the sun, the fan stops spinning immediately. As the sun sets, the attic is still hot and my solar fan stops spinning.

The fan I have moves 800 cubic feet of air per minute (CFM) when the sun is shining directly on the solar panel. If the angle of the sun is lower in the sky (morning and late afternoon) or the sun's rays are trying to cut through thin clouds or haze, the fan spins slower moving less air. The single solar-powered attic fan I have has not lowered the temperature in my attic at all. I took precise before and after temperature readings.

To significantly reduce the temperature in your attic, you need thousands and thousands of CFMs of air moving through the attic space. What's more, this air needs to continue to move through the attic space after the sun sets to remove the residual heat from the roof framing lumber, roof sheathing, roofing materials and the attic insulation. Yes, the insulation in your attic gets very hot during the day and then holds that heat long after the sun sets.

If you are going to go solar, I urge you to by several solar-powered attic fans. You will need them. Furthermore, consider buying ones that have a solar panel that can be located away from the actual fan. My solar fan has this neat feature. This allowed me to put my solar fan on the rear portion of my roof so you don't see it from the street, while the solar panel is on the part of my roof that faces due south.

You want the solar panels located on the roof where they will not be shaded by trees, and where they will get direct solar power from Noon until sunset. This is the hottest part of the day, that part where you need the fan blades spinning at full speed.

Solar-powered attic fans require a hole to be cut in the roof, possibly two if you purchase one that has the remote-panel location feature. If you do not know how to properly flash these fan housings into the roof shingles, then you should hire a qualified roofer. It is not hard to do the work, but there are very important steps that must be followed to have a leak-proof installation.

When installed properly on a roof with standard asphalt shingles, solar-powered attic fans do not require caulk, roofing cement or any other product to prevent leaks. A great roofer will cut the shingles and lace the fan housing into the shingles so that rain will stay outdoors where it belongs.

If your attic space is over 1,800 square feet, you will need enough solar fans to move 8,000 CFM. You need that amount of air, if not more, to get any sort of cooling benefit from the fans. Intense sunlight can create heat faster than one or two small fans can cool an area. If you want to see how to properly cool attic spaces, visit a chicken farm. These farmers use giant fans that move tens of thousands of CFM of air that keep the chickens alive.

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