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

What this diagram shows

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.