The attic can reach 150 F on a hot summer afternoon – double the ambient temperature! That puts a huge load on the air conditioning. You can add insulation to the attic floor (which also reduces heat transfer in the winter). Heat moves in direct proportion to the temperature difference on either side of the insulated partition. An attic fan can cut the temperature difference in half and hence reduce the roof load on the AC by half.
There are many kinds of thermostat-controlled attic ventilators. The thermostat turns on the fan when, say, the attic temperature reaches 100 F, after which time the fan draws cooler outside air into the attic. The easiest attic ventilation fan to install is a gable attic fan. It’s best to locate the thermostat away from the outside wall, but interior to the attic where the temperature tracks the average attic temperature.
An alternative installation is a roof-mounted attic fan. Heat rises, so the roof-mounted fan pulls the hottest air from the attic. However, any insert into the roof structure risks more leaks than a gable-mounted fan. Solar-powered roof ventilation fans incorporate a small solar panel to operate the fan. A solar unit saves the expense of running an electric cable to the fan.