Attic vent area math — 1/150 vs 1/300, and why net free area is not the vent size

Required vent area = attic floor ÷ 150 (or ÷ 300 when balanced + a vapor retarder in cold zones), split half intake / half exhaust. Size on Net Free Area — the open area after the screen — not the vent’s gross size.

A worked example of how much attic ventilation the code requires. The flat attic floor area is divided by 150 to get the minimum net free ventilating area: a 1,500-square-foot attic needs 10 square feet, or 1,440 square inches, split evenly into 720 square inches of intake and 720 of exhaust. The reduced 1/300 ratio halves that but only with a balanced layout and, in cold climate zones 6 to 8, a warm-side vapor retarder. A detail shows that a vent’s net free area is far smaller than its gross opening because the screen and louvers block roughly half — always size on the printed net free area, not the hole size. — Source: 2021 IRC R806.2

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A worked example of how much attic ventilation the code requires. The flat attic floor area is divided by 150 to get the minimum net free ventilating area: a 1,500-square-foot attic needs 10 square feet, or 1,440 square inches, split evenly into 720 square inches of intake and 720 of exhaust. The reduced 1/300 ratio halves that but only with a balanced layout and, in cold climate zones 6 to 8, a warm-side vapor retarder. A detail shows that a vent’s net free area is far smaller than its gross opening because the screen and louvers block roughly half — always size on the printed net free area, not the hole size.

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    <img src="https://remodelcalculators.com/api/figure-embed/attic-ventilation-nfa-ratio" alt="A worked example of how much attic ventilation the code requires. The flat attic floor area is divided by 150 to get the minimum net free ventilating area: a 1,500-square-foot attic needs 10 square feet, or 1,440 square inches, split evenly into 720 square inches of intake and 720 of exhaust. The reduced 1/300 ratio halves that but only with a balanced layout and, in cold climate zones 6 to 8, a warm-side vapor retarder. A detail shows that a vent’s net free area is far smaller than its gross opening because the screen and louvers block roughly half — always size on the printed net free area, not the hole size." width="800" height="470" style="width:100%;height:auto;border:0" loading="lazy">
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