Air Vent recommends 1 square foot of attic ventilation for every 150 square feet of attic floor space divided evenly between intake vents at the roof’s edge or in the overhang or undereave and exhaust vents at or near the peak of the roof. Here's what the formula looks like for a 1200 square foot attic space: 1200 divided by 150 = 8 square feet of attic ventilation. Then divide that number by 2 to provide half the ventilation for intake and half for exhaust. Thus, 8 divided by 2 = 4 square feet of attic ventilation for intake and 4 square feet of attic ventilation for exhaust.

The final step is to figure out how many vents would be needed to provide 4 square feet of attic ventilation. To do this let's start by converting the number to square inches by multiplying by 144. Thus, 4 x 144 = 576 square inches of attic ventilation for intake and 576 for exhaust. Air Vent ridge vents provide 18 square inches of Net Free Area per linear foot. To determine how many feet of ridge vent would be needed the formula looks like this: 576 divided by 18 = 32 feet of ridge vent. A typical 8" x 16" undereave vent provides 56 square inches of Net Free Area per vent. To determine how many undereave vents would be needed, the formula looks like this: 576 divided by 56 = 10.2 (which can be rounded up to 11).

If using Air Vent’s The Edge™ Vent, ProFlow Vented Drip Edge or continuous soffit vents simply double the amount of needed ridge vent linear footage because all of those intake vents provide 9 square inches of Net Free Area per linear foot.