How Do I Calculate Vent Size Required in Attics?

Vented Air Recommendations

• The Asphalt Roofers Manufacturing Association recommends a minimum of 1 square foot of free flow ventilation to 150 square feet of attic. If your attic has good soffit vents and exhaust vents on the roof ridge, or if it contains vapor retarding insulation such as fiber glass blankets or batting with paper facing, then 1 square foot of free flow ventilation to 300 square feet of attic may be adequate. To ensure a smooth flow of air through your attic, divide your vents equally between intake and exhaust vents. Fan-driven ventilators remove hot air efficiently, but they can cost more money to run them than you save in cooling bills.

Calculating Free Flow

• Calculate free flow venting requirements according to the free vent area (FVA) of your vents. FVA is open space of vents minus louvers and screen mesh. You’ll find the FVA listed on the tag of commercial vents.

Roof Architecture

• The architecture of your house can affect the number of and kinds of vents you should have. Flat roofs vent less efficiently than steep roofs and thus require more vents. The size of the soffits can determine the specific size of soffit intake vents. Soffit vents are commonly marketed in two sizes -- 4-by-8-inch vents fit between studs spaced 16 inches apart, while 14-by-22-inch vents fit between studs spaced 24 inches apart. Soffit vents 4 inches wide and 96 inches long also are available. Space your soffit vents equally around the sides of your house, including the corners.

Vent Effectiveness

• More vents mean a cooler attic and a cooler house. Studies conducted by Texas A&M University show that outside temperatures of 95 to 97 degrees F can cause attic temperatures of 150 to 160 degrees F. If the outside temperature is 95 degrees F, air vents that remove the entire volume of interior hot air once a minute will drop the attic temperature to 101 degrees F. If air vents remove half that amount of air, the interior temperature will drop to about 106 degrees F.