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), the 50/50 intake/exhaust split, and the soffit-vent count and ridge-vent length.
Formula and source
Required 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.
IRC R806 attic-ventilation rule (by name).
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