Attic Fans That Make a Difference You Can Feel

Q DEAR TIM: I need an attic fan to cool my hot attic, but there are many 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? -- Pam W., Green Bay, Wis.

A DEAR PAM: Attic fans are effective tools to help lower temperatures. I have used many types of fans and have formulated opinions based on experience and data collected over the years.

The first thing to realize is that it takes massive 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 the animals are usually equipped with massive fans that move tens of thousands of cubic feet per minute of air. On a blazing summer day, the sun is pounding relentlessly on your roof. The temperature of the shingles soars: They can easily approach 150 degrees Fahrenheit. 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 the air and starts to warm solid objects, such as the insulation and anything stored in the attic. For these things to be cooled, the heat needs to be transferred to something that can store it. 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 solar panels get sunlight until mid-afternoon. At that point, my entire roof is in the shade created by two massive trees. Unfortunately, these two fans have not done anything to reduce the attic temperature. The day before I installed them it was a cloudless day and the attic temperature at noon was 129. When both fans are operating at noon, the temperature has never been below 128. Each fan is supposed to move 800 cubic feet per minute of air. The fan blades 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.

Attic fans powered by regular household electricity will move lots of air. But you need to have quite a few operating to feel a difference. Many of these fans are equipped with thermostats so they will turn on and off automatically, a handy feature that saves energy.

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

Tim Carter can be contacted via his Web site,http://www.askthebuilder.com/printer_Submit_Question.shtml.

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