Attic Ventilation Net Free Area
The net free vent area an attic needs (the IRC 1/150 rule, or 1/300 balanced with a vapor retarder).
Example
You enter
- Attic floor area (sq ft) 1500
- Code ratio 150
- Soffit vent NFA (sq in) 9
- Ridge vent NFA (sq in/ft) 18
You get
- Nfa (sq in) 1440
- Soffit vents 80
- Ridge vent 40.0 ft
Checked against published worked examples from: Gibraltar Building Products (Air Vent formula); Lomanco.
Details, formula, and sources
ScopeThe 50/50 intake/exhaust split, and the soffit-vent count and ridge-vent length.
Formula or tableRequired NFA = floor area / divisor; x 144 to sq in; split half intake / half exhaust; vents = ceil(intake / per-vent NFA); ridge ft = exhaust / per-foot NFA.
Source and editionIRC R806 attic-ventilation rule (by name).
Free accessIRC available to read free at codes.iccsafe.org; vent NFA on the product label.
What governsEstimate. AHJ and licensed professional govern.
API field namesattic_floor_area_sqft, ratio, intake_vent_nfa_sqin, ridge_nfa_per_lf_sqin, nfa_sqin, intake_vents, ridge_lf
- Ratio 1/150, reducible to 1/300 with a balanced split + vapor retarder (advisory)IRC R806