When to Turn on an Attic Fan

With only passive venting to circulate air through an attic, summertime temperatures in the attic could climb to 150 degrees Fahrenheit. Attic fans can reduce peak temperatures to about 90 degrees Fahrenheit, cooling the roof enough to extend the lifetime of asphalt shingles. Attic fans also reduce the load on air conditioning systems, although running the fan usually does not lower total energy costs.

Attic Problems

Excessive attic heat can damage roof shingles.

Poor ventilation in the attic causes problems in both summer and winter. If humid air from the warm interior leaks into the attic in winter, moisture condensing on cold surfaces can cause water damage. Passive venting and adequate insulation should prevent these problems, but humidistats can turn the attic fan on when humidity reaches a chosen level such as 70 percent. Adjustable thermostatic controls turn on the attic fan automatically when summer temperatures exceed preset levels. Fans should turn on at 100 degree Fahrenheit and turn off at 90 degrees Fahrenheit.


Attic fans can reduce the air temperature in an attic, but have little effect on roof temperature. The difference in temperature of the sheathing beneath roof shingles varies only 10 degrees Fahrenheit between ventilated and unventilated attics, according to the University of Wisconsin Extension. Insulation over the attic floor protects rooms below from the temperature swings in the attic. In a house with adequate insulation, attic temperatures place little extra load on the air conditioning system.

Whole-house Fans

Whole-house cooling fans installed in attic floors exhaust house air through existing vents. Whole-house attic fans cool both the living space and the attic. One central fan positioned in the ceiling of a central halfway pulls cool air into the house and pushes hot air out through the attic. Whole-house attic fans require lower outside temperatures for efficient cooling and only work properly with open windows. When outside air temperatures reach uncomfortable levels, homeowners switch off the whole-house attic fan and close up the house to shift to air conditioning.


If the homeowner leaves windows open, the whole-house fan can be set to turn on automatically at a preset time or temperature. The starting point depends on the user's preferences. Setting the thermostat to a starting temperature of 72 degrees Fahrenheit inside the house triggers the system regardless of the outside temperature. When inside temperatures reach 82 degrees Fahrenheit, homeowners often turn the fan system off, close the windows and the fan vent, and switch on the air conditioning instead. Whole house fans should also include temperature-controlled cutoffs to shut the system down in case of fires.