Friday, September 28, 2007

Attic Fan Covers

During the winter months (and in summer when air conditioning is used), an attic exhaust fan represents a potential energy loss because it is essentially a large, uninsulated hole in the ceiling. Since standard fan louvers do not insulate or seal tightly, a cover should be constructed to airseal and insulate this hole. The cover may be installed from the attic side (if attic access is easily available) or from the house side. Covers for both sides could be included in excessively hot or cold climates. Homeowners must remember to remove cover(s) before operating the fan and to replace cover(s) during seasons when the fan is in operation.

Attic Fan Air Flow

In addition to sizing a whole house fan correctly, it is important to ensure that all penetrations between the attic and living space are sealed and that the attic is properly ventilated. A central hallway, or a stairway in a two-story house, is the most common location.

Attic Fan

Sizing Your Attic Fan

Determining the amount of airflow in cubic feet per minute (cfm) that the whole house fan should provide involves a simple calculation. Multiply the total gross square footage of the house (include upstairs area)by the ceiling height (typically 8 feet). Select a fan that delivers between one-half to one times that amount of cfm at 0.1" static pressure. For example, a 25' x 40' one-story home is 1,000 square feet and would need an 8 x 1,000 x 1/2 = 4,000 cfm fan or better. A two-speed unit that delivers 4,500 cfm at the high setting (240 watts) and 3,200 cfm at low (120 watts) should be adequate.

Selecting Your Attic Fan


A two-speed fan permits the entire house to be ventilated quickly on high speed (such as when the occupants first arrive at home) and then provides gentle air circulation at the lower, quieter speed. A variable speed unit offers more flexibility in selecting the desired air movement.


Control may be a simple on/off pull or wall switch, a multi-speed rotary wall switch, or a timer that automatically shuts off the fan at pre-selected time intervals.


Dampers or louvers typically operate automatically whenever the fan is on. Motorized dampers are available but are not necessary if the louvers are correctly installed and maintained.


A direct drive unit has its fan blades attached directly to the motor shaft. This type of unit is usually less expensive to buy and operates at a higher speed than its belt-driven counterpart. A belt-driven unit typically features a motor driving a slower-moving,larger-diameter fan with four or more blades. It may be quieter, but will require maintenance of the pulley and belt.

Save Money with an Attic Fan

Operating a properly sized 2-ton air conditioner with a seasonal energy efficiency ratio (SEER) of 10 in Atlanta, Georgia, costs over $250 per cooling season (1,250 hours), based on 8.5¢/kWh, or roughly 20¢ per hour of runtime.

A large 18,000 Btu/h window unit air conditioner with an energy efficiency ratio (EER) of 8.8 costs more than 17¢ to operate for one hour. By contrast, a whole house fan has a motor in the 1/4 to 1/2 hp range, uses 120 to 600 watts, and costs around 1¢ to 5¢ per hour of use.

Why Use an Attic Fan?

An attic fan is a simple and inexpensive method of cooling a house. The fan draws cool outdoor air inside through open windows and exhausts hot indoor air through the attic to the outside. Running an attic exhaust fan whenever outdoor temperatures are lower than indoor temperatures will cool a house.

In summer, the air inside a home is heated during the hot part of the day. During the
morning, late evening, and night, the outside air is often cooler and can be used to replace the inside air. Operating the attic exhaust fan at these times will cool interior materials. As daytime temperatures rise, the attic exhaust fan can be turned off. The cool materials (along with ceiling or circulating fans, which create an additional cooling effect) will help keep the interior more comfortable.