Attic Vent Calculation – On the House

Attic Vent Calculation

By on April 18, 2014
attic ventilation

Question

My home has no eave vents, but it does have three gable vents (14 inches x 14 inches each). The roof is 5-in-12 pitch, has R-30 insulation and covers 1500 square feet. Could you tell me how to calculate the proper amount of attic ventilation?

Clem

Answer

Actually, you can disregard pitch and insulation value for this calculation. Your attic area of 1500 square feet is the base we will work from.Attic ventilation should be equal to 1/150th of total attic area (1,500 square feet divided by 150 = 10 square feet). You’ll want to convert your ventilation requirement to area in square inches (10 square feet times 144 inches = 1,440 square inches.)According to the building code, you need a total of 1,440 square inches of attic ventilation. Your three existing 14 x 14-inch gable vents currently provide only 588 square inches (14 inches x 14 inches = 196 square inches times 3 = 588 square inches).The rest is simple! You need 1,440, you have 588, so adding 852 square inches of venting should do the trick. For example: a common 3-1/2 inch by 22-1/2 inch eave vent has an open area of about 66 square inches. The number of vents needed to bring your attic up to snuff would be 852 divided by 66, or 13 vents.Check with your local building department. Some have fire codes that restrict eave vents from being placed above exterior windows and doors.

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