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Crawl Space Code — Vent Ratios, Vapor Barriers & Going Unvented

A crawl space is either vented or it is not, and the code treats those as two entirely different assemblies. Vented means a ratio and openings near the corners; unvented means a sealed ground cover plus a way to condition the air.

Requirement	Model Code Standard	Section
Vent ratio	Vented crawl spaces need 1 sq ft of net free ventilating area for each 150 sq ft of under-floor area	IRC R408.1, R408.2
Reduced ratio	The ratio drops to 1 sq ft per 1,500 sq ft where the ground is covered with a Class I vapor retarder	IRC R408.2
Opening placement	One ventilation opening must fall within 3 ft of each corner of the building; the model code sets no minimum number of openings beyond that	IRC R408.1
Opening covers	Openings may be covered with material whose openings have a least dimension of 1/4 in. or less	IRC R408.1
Unvented option	An unvented crawl space needs a continuous Class I vapor retarder with joints lapped at least 6 in. and sealed, carried at least 6 in. up the stem wall, plus one of four air strategies: continuous exhaust or conditioned supply at 1 cfm per 50 sq ft, treatment as a supply plenum, or dehumidification sized per the manufacturer	IRC R408.3
Access	Access openings are at least 18 in. by 24 in. through the floor, or 16 in. by 24 in. through a perimeter wall, and may not be located under a door	IRC R408.4
Ground clearance	The 18 in. clearance to the bottom of floor joists and 12 in. to the bottom of girders is a wood-durability rule, not a ventilation rule	IRC R317.1

Worth knowing

The ratio appears twice in the model text, once in R408.1 and again in R408.2, which is why published citations for the same number disagree. Citing both is safer than picking one.
R408.8 is newer and easy to miss. In climate zones 1A, 2A and 3A below the warm-humid line, air-permeable floor insulation needs a Class I or II vapor retarder tested at 1.5 perms or less by ASTM E96 Procedure B. Crawl spaces built to R408.3 are excepted.
There is no mandatory crawl-space sump in the 2021 IRC. The closest provision requires drainage where the groundwater table rises to within 6 in. of the crawl-space floor.
The dehumidifier sizing figure that circulates as code — so many pints per day per 1,000 sq ft — is a local handout interpretation. The code says to size per the manufacturer.
Washington deviates hard here: the state base ratio is 1/300, black polyethylene ground cover is required in every crawl space, and R408.8 was not adopted.

Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.

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