Soffit Vent and Ridge-Vent Count from Required NFA
Soffit vents and ridge-vent feet from the required net free area, split balanced intake / exhaust: total NFA = attic area / ratio x 144; intake = exhaust = total / 2; soffit vents = ceil(intake / per-vent NFA); ridge = ceil(exhaust / ridge NFA per ft). A 1,500 sf attic at 1/300 needs 720 in^2 of NFA (360 each way), so 14 soffit vents at 26 in^2 and 20 LF of ridge at 18 in^2/ft; without a vapor retarder the 1/150 ratio doubles it to 28 vents and 40 LF. The ratio comes from the IRC (attic-ventilation gives the NFA); the product NFA comes from the manufacturer.
Formula and source
total_nfa_in2 = attic_area_sf / vent_ratio x 144; intake_nfa_in2 = exhaust_nfa_in2 = total_nfa_in2 / 2; soffit_vents = ceil(intake_nfa_in2 / soffit_vent_nfa_in2); ridge_lf = ceil(exhaust_nfa_in2 / ridge_nfa_per_ft_in2).
Attic vent-count identity by name (required NFA split balanced, then soffit vents and ridge feet over the product NFA); first-principles count arithmetic.
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