Crawl Space (Under-Floor) Ventilation Net Free Area

ScopeIRC R408.1 wants 1 sq ft of NET FREE area per 150 sq ft of under-floor area, with an opening within 3 ft of each corner -- but covering the ground with an approved Class I vapor retarder AND arranging the openings for cross ventilation drops it to 1/1500, a TEN-fold cut, because the ground is where nearly all the moisture comes from. A 1,500 sq ft crawl needs 1,440 sq in and 29 vents bare; with a retarder it needs 144 sq in and just 4, at which point the CORNER rule governs instead of the area. Net free area is product data off the vent and is far less than the rough opening. The base ratio is an input because jurisdictions amend it -- Washington adopts 1/300, not 1/150. The unvented conditioned crawl space of R408.3 is a different assembly and is not sized here.

Formula or tablerequired net free area = under-floor area / ratio, with ratio = the base denominator (IRC 150) or 1500 when an approved Class I vapor retarder covers the ground (R408.2 exception 1); with the retarder, openings placed for cross ventilation lift the corner rule (exception 2), leaving at least two openings; sq in = sq ft x 144; vents by area = ceil(required sq in / net free area per vent); vents required = max(vents by area, corner count).

Source and editionIRC R408.1 / R408.2, under-floor space ventilation. Washington's adopted WAC 51-51-0408 amends the base ratio to 1/300, which is why the base denominator is an editable input here rather than a fixed 150.

Free accessThe ratio, the 1/1500 exception, the corner rule, and the permitted opening coverings are short prescriptive provisions reproduced in state adoptions and building-department handouts; the 1/1500 exception language and the covering list were read from Washington's adopted text. No table is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namesfloor_area_sf, base_ratio_denominator, vapor_retarder, vent_net_free_sqin, corner_count, ratio_used, required_sf, required_sqin, vents_by_area, vents_required, provided_sqin, surplus_sqin, alt_required_sqin, alt_vents

• Base ratio 1 sq ft per 150 sq ft of under-floor area; editable, as jurisdictions amend itIRC 2024 R408.2 (unchanged from 2021); WAC 51-51-0408 uses 1/300
• Vapor-retarder reduction 1/1500 where an approved Class I vapor retarder covers the groundIRC 2024 R408.2 exception 1
• Corner rule an opening within 3 ft of each corner, a floor on the vent count, waived with the retarder when the openings cross-ventilate (then at least two)IRC 2024 R408.2 and exception 2
• Net free area product data off the vent, much less than the rough openingstated scope limit
• Covering minimums hardware cloth 0.035 in wire or heavier; wire mesh no finer than 1/8 incode list of permitted coverings
• Unvented alternative R408.3 conditioned crawl space is a different assembly, not sized herestated scope limit