Roof Ventilation Calculator

1. Enter your attic floor area. Measure the total attic footprint in square feet. For multi-level homes, calculate each attic space separately.

2. Select vapor barrier status. If a proper vapor barrier is installed on the warm side of the ceiling, the code-required ventilation ratio drops from 1:150 to 1:300. Most newer homes have one; older homes often do not.

3. Choose your ventilation system. A balanced soffit + ridge system is the gold standard. The calculator splits required ventilation 50/50 between intake and exhaust for balanced airflow.

4. Check ridge vent NFA. The default is 18 sq in per linear foot, which is standard for most ridge vents. Check the manufacturer spec sheet for your specific product.

The IRC requires a minimum net free ventilation area (NFVA) to prevent moisture buildup and heat accumulation in attic spaces. The calculator determines the total required NFVA and splits it between intake and exhaust vents.

The 144 multiplier converts square feet to square inches. Continuous soffit vents typically provide 10-18 sq in per linear foot; this calculator uses 15 sq in/ft as a conservative estimate. Always check the actual NFA rating on your specific vent product.