Reducing the heat buildup in the attic or top-floor ceiling space is vital in hotter climates during the cooling season. An attic that is not ventilated can heat up 30 or 40°F above the outside temperature. This increases the thermal load on the building by introducing more heat into the upper floor zones. Vent louvers allow the natural migration of outside air through the attic, reducing the temperature gain in the attic space by half or more. (They are also critical to control moisture.)
An attic ventilation fan can bring the average attic temperature down to near ambient. A thermostat-controlled, two-speed fan can save energy by turning the fan on only when the attic temperature exceeds the outside temperature by 5 or more degrees, and then at the lowest possible fan speed. Another control feature interlocks the fan with the central AC, so it operates only when the building is actually in use.
A forced ventilated attic also has a higher effective R-value (by as much as R-7), which reduces the roof load on the HVAC system. In the winter it is advantageous to have a warmer attic, so it is best to disconnect the vent fan then. This allows the temperature to increase above ambient because of incidental solar radiation on the roof. This eliminates part of the roof load. In some cases, it can even assist the heating cycle by providing a positive input of thermal energy, offsetting the required thermal input by the furnace. The attic should not be completely closed off, however, because that can cause severe moisture problems.
These same principles also apply to any unconditioned spaces that abut a conditioned space, such as mechanical rooms and storage rooms. If these spaces are not adequately ventilated they add to the heating and cooling loads of the nearby rooms and cause inconsistent and uncomfortable conditions for occupants.