Attic Fan Effectiveness

Many homes have large attic fans to remove hot air from the attic. By exhausting hot attic air, they reduce the heat that is transmitted down through the ceiling into your home. However, attic fans are expensive to install and they use substantial electricity. Attic fans usually have only a small effect on the heat gain through the attic. Most solar heat falling on the roof radiates from the roof deck to the ceiling and the fan can’t affect this radiation heat gain. If the attic fan is controlled by a thermostat, it may be running 12 hours a day or more costing you more money than they’re worth. But in most homes, increased attic insulation, attic air sealing, and effective passive ventilation are more effective and less expensive than attic fans.

Attic insulation and sealing attic air leaks are the best ways to reduce heat transfer from the attic in summer. The attic insulation should be at lease a foot thick and have an R-value of 35 to minimize heat transfer from the attic. Air leaks should be sealed before additional insulation is installed.

Passive attic ventilation employs passive attic vents to remove heat from your attic. Attic vents should be distributed high and low and also across the roof to encourage natural circulation. Continuous soffit and ridge ventilation is ideal.