I have been thinking of ways to keep my home cool this summer, and a friend suggested I install a solar-powered attic fan to pull the heat out of my attic. Will this help?
Common sense suggests that removing heat from the attic over the living space should keep the house cooler. Since homes get hot during the day from the sun beating down on the roof, which in turn heats up the attic, why not use a fan powered by the sun the very thing that is making the attic hot to keep the attic cool?
However, a recent study by Danny Parker and John Sherwin of the Florida Solar Energy Center on the impact of solar-powered attic fans on cooling energy used in houses in Florida found that mechanical attic ventilation was not as effective as had been theorized.
The study showed that the use of a solar-powered attic fan reduced annual cooling energy by only about 6 percent in a house with R-11 attic insulation. That number dropped even further when the ceiling insulation was increased.
So why isn't attic ventilation more effective? According to the FSEC study, heat transfer from an attic to the interior of a residential building is dominated by radiant heat gain from the hot roof decking, not from hot air in the attic. Consequently, controlling attic heat gain through measures other than attic ventilation is preferable and more cost effective.
The FSEC study noted that increasing attic insulation, adding a reflective radiant barrier to the underside of roof trusses or installing a light-colored roofing material is more effective a than increasing attic ventilation.
Art Krenzel, a professional engineer with Phoenix Technologies in Battle Ground, also believes that keeping the roof cool is an important factor. He offers another suggestion.
"I use a sprinkler system on my roof during hot days. I have found that if you can keep the bulk roof temperature at 85 degrees or less using evaporating water, you will drive less heat into the living space from the top," he said.
"As far as attic ventilation is concerned, the better the ceiling insulation, the less effective attic ventilation is at reducing cooling load," Krenzel continued. "However, because heat transfer is proportional to the difference in temperature between the attic and the rooms below, any measures that remove the high-temperature attic air will help."
Krenzel's use of evaporative roof cooling may cost less and prove more effective than investing in a solar attic fan.
If you have less than 12 inches of insulation in your attic, adding more can be an effective way to keep your home cool in the summer and warm in the winter.
George Tsongas, mechanical engineer and professor emeritus from Portland State University, also agrees that an attic fan may not be your best bet.
"Several national studies have shown that radiant heat gain from the roof is a larger contributor to the temperature in the house than is the attic air temperature. While attic ventilation can lower the air temperature in the attic, it doesn't seem to have a significant impact on the temperature in the house."
Attic fans may also present a potential back-drafting risk for in-home combustion appliances, he said.
Tsongas also recommends increasing insulation over adding ventilation.
Putting a sprinkler on the roof on hot days or adding more attic insulation may make more sense for keeping your home cool this summer than spending money on a solar-powered attic fan.
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