Do You Need an Attic Fan If You Have a Ridge Vent?

Purchasing an electric or solar attic fan may not provide the savings that homeowners expect. Installing roof ridge vents and more attic insulation are better options, experts say. Adding roof ridge vents can reduce summer air conditioning costs, prevent roof mildew and rot and keep your roof in better shape longer.

Americans spend about $1,900 per year to heat and cool their homes, says Sarah Kirby, a North Carolina Cooperative Extension Service professor and housing specialist. "Consumers spend approximately 44 percent of their total energy costs on heating and cooling their homes," Kirby says. To reduce that expense, some homeowners buy an electric or solar attic ventilator. This fan moves 1,000 or more cubic feet of air per minute, creating low pressure in your attic. Electric attic fans, however, often increase energy consumption and cancel out any savings, unless the attic is not insulated at all.

By installing more roof vents, such as a ridge vent, you increase the possibility that most attic air will come from outside, says Tim Carter, a residential contractor in Cincinnati. If you have few vents in your attic, most of the air will be drawn from inside the house. A fan may actually suck air-conditioned air from your home into the attic. If the air conditioner is working at the same time as the attic fan, cool air from your living space is being wasted in your attic, seeping through the attic access panel, drywall and holes in the interior wall top plates where plumbing pipes and electrical wires feed into the attic. When the University of Central Florida's Florida Solar Energy Center tested homes, they found that air leaking from a home's duct system can cause negative pressures within the home's interior when the air handler runs. This might draw hot air from the attic into the air-conditioned interior, especially in Sunbelt state homes built on concrete slabs. Most air leakage in these homes is through the ceiling. Hot, humid air outdoors can also enter the house, forcing your air conditioner to work even harder.

Instead of installing an attic fan, Carter recommends using ridge vents on your roof. After installing a ridge vent, a homeowner might feel a temperature change, says Jerry Alonzy, a handyman working in Canton, Connecticut. The prevailing winds and the amount of shade your house gets will also affect your results. A ridge vent installed in a very hot attic will have more effect than in a slightly cooler one. Ventilation decreases moisture year round, reducing roof mildew and rot, Alonzy says.

If you live in Florida, though, where relative humidity tends to increase at night, a vented attic may attract even more humid air, say Danny S. Parker and John R. Sherwin of the Florida Solar Energy Center. Many Southern homes have the cooling system's air handler based in the attic, usually poorly insulated with the greatest temperature difference of any location of the cooling system. It also has the greatest negative pressures, causing leakage into the unit.

To reduce humidity, the center recommends checking the building's duct system and air pressure when some doors are closed, replacing an inefficient air conditioner and turning off an attic exhaust fan. Turning off the exhaust fan reduces building pressure, causing a large decrease in the ventilation rate and indoor relative humidity, the center says. Some homeowners are attracted to solar fans because they can operate for 50 years without needing repairs. They don't use electricity and are easier to install, but can be expensive. Prices start at $500 for a 1,200 square foot home, not including installation. When the Solar Energy Center tested solar fans, however, it reported that if your attic is already well-insulated, the cost reduction is only about 6 percent, or 460 kilowatt hours each year. Since most older homes have only 3 to 6 inches of insulation, beefing up the attic's insulation might be the best solution.
The federal Department of Energy recommends up to R-49, or about 16 inches of fiberglass insulation, for most attics.