Solar Powered Attic Fan
Summary: Consider an attic fan if your attic gets as "hot as blue blazes." There are solar attic fans available. They work great on a sunny day. But the solar powered attic fans do not work on cloudy days or when the sun goes down. However, the attic still has hot air inside at sunset.
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.
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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Comments:
Kathie 05 Mar 2008, 14:47
I am confused on your take regarding the attic fans. Are solar better than
electric? You stated you don't recommend attic fans and to use vents, but
in reading your solar attic fans you make it sound like there great. Do
solar fans not suck air conditioned air from your house into your attic
space & not cause safety problems by back-drafting combustion gases in a
house living space?
AsktheBuilder 09 Mar 2008, 08:49
Kathie,
I think you better read the column above once more. You make it sound like I am in love with solar fans. I clearly state that there are many cons associated with them. If you are worried about pulling air conditioned air and back drafting, then that means you have really poor intake vent in the soffit or lower portions of the roof. In my PAV column that you reference, I devote lots of space to the importance of this incoming air. You want the ideal vents in my opinion? Go read ALL of my past columns on Turbine Vents.
Gordon Wagner 30 Apr 2008, 13:55
Nothing wrong with that response at all. Just information.
I love the idea of solar-powered fans, but I don't think I could afford enough of them to provide the air flow I want.
Randall McMurphy 30 Apr 2008, 20:05
I have a small cape with two small rooms upstairs. There is no attic, just
a small space above the two rooms with blown-in insulation. There is only a
small hatch, not even big enough for a person to crawl through to get up
there. The two rooms upstairs get very hot when the outside temp gets above
75ish. I have two gable vents. There really is no way to get up there to
wire-in an electric fan. Do you think one solar fan would make a difference
in these rooms? I hate blasting the A/C all day to keep them livable.
Jason 09 May 2008, 13:04
Randall,
If the attic heat is making THAT much of a difference in the interior (even at very moderate outdoor temps), my first question would be about the R value of the current blown-in insulation. I don't have a very accurate mental image of exactly how much space you have available, but a better use of money might be to blow in/have blown in some more insulation, which should be very feasible even with the tiny space. If that doesn't make a significant difference in, or completely solve the temp. transfer (which it WILL, if the current R val. is low, and there is enough space to blow more in), then go for the solar vent. (also, does your roof have soffits? Putting in soffit vents and some sort of passive exhaust vents (ridge, turbine, etc.), might be cheaper and just as effective. Gable vents by themselves just don't cut it.
Norm Wilson 03 Jun 2008, 08:15
for ventilation, which is better, powered gable vents or powered roof
vents?
I have a cape, which has basically a crawl space versus an attic. I am leaning towards the gable vent because I want to avoid breaching the roof with a vent. There is already an existing passive gable vent on the house now.
Jason 03 Jun 2008, 15:21
Don't know the true answer to that, but I (who also have gable vents) put
in a roof-mounted because I figured IF there was a breeze, there's a
possibility, depending on the breeze direction, that a gable-mounted fan
could be blowing ineffectively AGAINST the breeze, whereas with a
roof-mounted, it doesn't matter. Not sure how much of a real-world
difference it would make, but it made up my mind...
Trace 03 Jun 2008, 22:38
Hello,
My attic is around 2000sq with the highest point around 11 feet tall. I checked the depth of the pink blown in insulation and it is between 4 - 8 inches. What should the depth be for a house in the dallas / Fort Worth area? I also currently have an electric attic fan and two spinners. I would like to remove those and place in several dc fans with one central solar panel on the back of the house. Any manufactures that you can suggest? Also is radiant barrier paint as good as they claim or just another scam? Trace
John Patrick 08 Jun 2008, 13:41
I'm thinking about installing a whole house fan in my cape style house.
Unlike the houses described above, mine has no attic or crawlspace. All of
the whole house fans I've seen are designed for installation into an attic.
Has anyone come across a whole house fan designed for installation into
the upstairs living spaces. Obviously the fan itself has to be mounted
outside on the roof.
Thanks! John
ralph skinner 08 Jun 2008, 23:24
Be nice.
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