At face value, attic exhaust fans make a lot of sense: if your attic is too hot, you force more air through it to cool it down. To be efficient, you use a solar-powered attic exhaust fan. When the sun is shining and heating up your attic, that’s when the photovoltaic panel wired to the exhaust fan powers the fan. Pretty slick.
But there is a catch: why is your attic “too hot?” It is probably because living space under the attic is uncomfortable…from the less-than-well insulated and air sealed ceiling that separates the attic from those rooms. If you don’t have a continuous air seal at the ceiling plane, then your solar-powered attic exhaust fan can pull conditioned air into the attic—now that will cool it down! It can be worse than that: if that attic fan is depressurizing living space that has atmospherically-vented gas appliances or a problem with radon, you may have moved from wasting energy to indoor air quality problems.
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Field research supports these scenarios. Work done in 1995 by John Tooley and Bruce Davis of Advanced Energy Corp (as reported in Home Energy, “Drawbacks of Powered Attic Ventilators”) revealed depressurization issues and associated energy, moisture and combustion safety problems. And field research done by the Florida Solar Energy Center (FSEC) specifically on solar-powered attic ventilation (“Performance Assessment of Photovoltaic Attic Ventilator Fans”) concluded that the approximately $850 installed cost of the system yielded relatively modest cooling energy savings and an unfavorable payback over more than twenty years.
The bottom line? Proceed with caution. If you have a well-sealed and insulated attic floor and either no HVAC equipment in the attic or all of the HVAC equipment and ducts in the attic are well sealed, then you could install solar-powered attic ventilation with some cooling energy benefit and little potential for safety or indoor air quality problems. For far too many homes, this is a mighty big “if.”
Joe Lstiburek of Building Science Corp. puts it this way:
“In order for the fan to work the air needs to come from the outside and not be pulled from the house so this means that the attic ceiling needs to be airtight. If the attic ceiling is airtight you don’t need the fan. Your money is better spent on something else.”
For most homes, it will be “greener” to take the money you would have spent on the PV-powered attic ventilation and upgrade the air sealing and insulation in your attic and on your HVAC system.