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.
Example
You enter
- Attic floor area (ft²) 1500
- NFA ratio denominator (1/N) 300
- NFA per soffit vent (in²) 26
- NFA per foot of ridge vent (in²/ft) 18
You get
- Total nfa (in²) 720
- Soffit (intake) vents 14
- Ridge (exhaust) vent 20 LF
Details, formula, and sources
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.
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.
The count arithmetic is public first-principles; the 1/300 or 1/150 ratio is IRC and the product NFA comes from the manufacturer.
Estimate. AHJ and licensed professional govern.
Field names used by the API: attic_area_sf, vent_ratio, soffit_vent_nfa_in2, ridge_nfa_per_ft_in2, total_nfa_in2, soffit_vents, ridge_lf
- Vent ratio 1/300 with a vapor retarder / balanced venting, else 1/150 (IRC R806)IRC
- Product NFA per soffit vent (~26 in^2) and per foot of ridge (~18 in^2/ft) from the manufacturervent manufacturer