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How to Size Your Attic Vents

Proper attic ventilation is an important aspect of a roofing system. A constant flow of air underneath a roof eliminates moisture buildup, maintains better temperature control in the living area, increases the performance of insulation and may prolong the life of a roof. Passive vents are the most commonly used method of achieving this insulation although you may use fans to supplement these vents for attics where high temperatures or moisture levels are common.

  1. Measure the square footage of your attic floor. Use the measuring tape to calculate the length and width in feet, and multiply these two numbers together.

  2. Divide the result of step 1 by 300. This is the total square footage of ventilation your roof will require. If your attic does not have a vapor barrier installed on the insulation (either kraft paper or plastic), is especially prone to high temperatures or damp conditions or if vents are not equally distributed around the roof, divide by 150 instead of 300.

  3. Convert the result from step 2 into square inches by multiplying by 144. This is the total number of square inches of ventilation recommended for your roof. This number should be equally divided into inflow vents (under-eave vents) and outflow vents (ridge, roof or gable vents). Air naturally flows from low to high roof vents.

  4. For each vent type used, measure the length and width in inches and multiply together to calculate the total square inches of ventilation. Use enough vents to achieve the total calculated in step 3.

About the Author

Andrew Hazleton has been writing on a freelance basis for more than 20 years, and his work has appeared in national, regional and in-house publications. His work has appeared in "Sports Illustrated," "IEEE Spectrum," "Popular Photography" and several newspapers. Hazleton has a Bachelor of Science in engineering from Lehigh University and a master's degree in management from Pepperdine University.