Look at the cold facts: Can a solar-powered attic fan do the tough, hot job?

Ask the Builder

August 03, 2008|By Tim Carter | Tim Carter,Tribune Media Services

I know I need an attic fan to cool my hot attic. But there are many different types available. Should I consider a solar-powered fan? What type of attic fan do you use? How much air do I need to move through my attic to make a significant difference in the temperature? How important are soffit vents in this system?

Attic fans are effective tools to help lower excessive attic temperatures. The first thing to realize is that it takes large amounts of air moving through an attic to create significant cooling. Farmers who raise chickens can attest to this, as the buildings and barns that house chickens are usually equipped with large fans that move tens of thousands of cubic feet per minute (CFM) of air. Even then, it is almost impossible to drop the temperature to that of the air temperature outdoors.

Think of the dynamics of what is happening on a blazing summer day as the sun is pounding relentlessly on your roof. The temperature of the shingles soars. They can easily approach a temperature of 150 F or possibly more. This heat then transfers to the roof-framing materials. The trusses, rafters and roof sheathing get very hot. All of these things radiate heat, much like the coals of a fire or a hot radiator.

The heat that is released is infrared energy that passes through air and starts to heat up solid objects like the insulation and anything stored in the attic. To cool these things down, the heat needs to be transferred to something else that can store the heat. The only thing available is the water in the air that is moving through the attic.

I am testing a solar attic fan. I have two identical units installed in my attic. The day before I installed them it was a cloudless day and the attic temperature at noon was 129 F. When both fans are operating at noon, the temperature has never been below 128 F. Each fan is supposed to move 800 CFM of air. The fan blades do spin rapidly, and I have good soffit intakes, as well as windows in my attic that allow air to freely enter the attic space.

The trouble with solar attic fans is they stop working as soon as a cloud blocks the sunlight. On partly cloudy days, the temperature can be beastly hot, and the fan blades simply stand still for much of the time. After the sun goes down, the attic can still be stiflingly hot, and there is no hope of air movement because the sun is not powering the fans.

Powered attic fans that get a constant supply of household electric will move lots of air. But you need quite a few to feel a difference. Many have thermostats so they will turn on and off automatically. This is a handy feature that saves energy.

You can install wind-powered turbine vents as well. When the wind blows hard, they can exhaust lots of air. But because the performance is tied to wind speed, you can't count on these to work at peak performance in the hottest part of the day.

You need great soffit vents to supply the attic with all of the replacement air. Powerful fans that are exhausting air from your attic work best when they can get ample amounts of makeup or replacement air from outdoors. The lack of soffit vents may cause the fans to draw air up from the inside of your home. If you are running air conditioning, this can cost you dearly, as your air conditioner will work harder to keep your home comfortable.

Expert home builder and remodeling contractor Tim Carter has 20 years of hands-on experience in the home industry. If you have a question, go to askthebuilder.com and click on "Ask Tim."

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