What Size Attic Fan Do I Need?

How Attic Ventilation Works

• Two components make up an attic ventilation system: air vents and intake vents. The air vents are at the peak of the roof so that hot air -- which rises -- escapes through them. Intake vents are located under your home’s soffits. These vents allow fresh air to enter from outside. Proper ventilation is important all year. In the summer, an attic fan will reduce the temperature in your attic by up to 25 degrees, and in the winter, it will prevent ice dams from forming in the gutters and damaging your roof.

Attic Fans

• The job of an attic fan is to circulate air through your attic; it pulls cool air through the intake vents and forces hot air out of the house. A potential downside to installing an attic fan is that it can increase your cooling costs. If you do not completely seal off the attic from the rest of your house, the fan can draw air-conditioned air from inside the house, forcing your air conditioner to work harder.

Calculating Fan Size

• According to the University of Missouri Extension, an effective attic fan changes the air once per minute. Manufacturers measure fan capacity by the number of cubic feet of air the fan moves per minute (CFM). To find the right size fan for your attic space, first determine its volume. Do this by multiplying the width by the length by the height of your attic space. If your attic is 30 feet wide by 40 feet long by 8 feet high, you need a fan capable of moving at least 9,600 CFM: 30 x 40 x 8 = 9,600 cubic feet.

Calculating Intake Needs

• Your attic fan can only work efficiently if enough new air is cycled into the attic. To find the area in square inches of the intake vents your attic needs, divide the fan’s CFM by 300 and multiply the result by 144. According to “Contracting Business” magazine, you need a minimum of 1 square foot of intake for every 300 CFM. Without sufficient fresh air flowing into the attic, the fan works harder and is less energy-efficient.