Attic Ventilation Calculator

How It Works

Proper attic ventilation serves two critical purposes: in summer, it expels trapped heat that can reach 150 degrees F and make your AC work harder; in winter, it allows moisture to escape before it condenses and causes rot or mold. Building codes require a minimum ventilation ratio of 1:150 (1 square foot of Net Free Area for every 150 square feet of attic floor). With a vapor barrier and balanced intake/exhaust vents, you can use the less restrictive 1:300 ratio. Balanced ventilation means roughly equal intake (soffit vents) and exhaust (ridge vents). This creates a natural convection current: cool air enters at the soffits, absorbs heat as it rises, and exits at the ridge. Never block soffit vents with insulation.