At what temperature should I set my attic power vent fans to keep my attic cool? I live in a hot and humid climate, where temperatures are often in the 90s. The roof gets full exposure to the sun throughout the day.
Attic ventilators generally turn on between 90 and 110 degrees Fahrenheit. That said, attic fans are not the best choice for cooling attics, especially if you have central air conditioning. The reason is that attic fans depressurize attics and can rob air conditioned air from the main body of the house, greatly decreasing efficiency. It essentially pulls that cooled air through cracks and crevices in the attic floor, and also through holes that allow for wires and pipes.
A much better cooling option is continuous ridge and soffit venting. Basically, you cut a slot in the top ridge of the roof and put a vent right over it. As the wind blows over the roof, it depressurizes the ridge and sucks the moist warm air from the attic. Everything you want to vent from the attic gets sucked out of that ridge through the depressurization that happens through the normal wind cycle.