Not all attic fans are outfitted with a humidity setting, which requires that the ventilation fan unit have an internal humidistat. Homeowners who have a standard attic fan can install an independent humidistat that turns on the fan at a set humidity level.
Most attic fans with a humidity setting allow the homeowner to adjust the allowable humidity percentage. Seventy percent humidity is a common attic fan setting, but the appropriate percentage depends on the temperature as well as the existing humidity.
Homeowners who purchase an attic fan with a humidistat often live in moist and humid climates. By encouraging air circulation when the humidity rises, the fan reduces the chance of mold and mildew growth that could weaken the house's underlying structure.