How much vapor barrier do I need for my crawl space?

Floor area plus at least 6 inches up every wall, plus 6 inches of overlap at each seam. A 40 by 30 ft crawl space with 12 ft rolls needs about 1,310 sq ft before waste.

What thickness of vapor barrier is best?

The IRC defines the retarder by class, not mil. Thicker liners resist tears from foot traffic. The 6 mil figure is the polyethylene the IRC names under concrete slabs.

Should seams be taped?

For an unvented crawl space, yes: R408.3 says joints shall overlap 6 inches and be sealed or taped.

Can I install a vapor barrier myself?

Laying a ground cover is common DIY work. Fix standing water and any rotted wood first, and wear protection in a dirty crawl space.

Crawl Space Vapor Barrier Calculator: Rolls and Tape

Which crawl space design is this for?

Vented crawl spaces. The IRC asks for 1 sq ft of vent per 150 sq ft of under-floor area, and allows 1 per 1,500 sq ft where the ground is covered with a Class I vapor retarder. A ground cover here cuts moisture from the soil.

Unvented crawl spaces. R408.3 requires a continuous Class I vapor retarder with joints lapped 6 inches and sealed or taped, and edges extending at least 6 inches up the stem wall and sealed, plus a conditioning method. That is encapsulation; price it with the encapsulation calculator.

Half of each is neither. Sealing vents with no conditioning, or a torn liner under open vents, is the version that grows mold. The crawl space vents guide covers the choice.

Questions this calculator answers

How much vapor barrier do I need for my crawl space?: Floor area plus at least 6 inches up every wall, plus 6 inches of overlap at each seam. A 40 by 30 ft crawl space with 12 ft rolls needs about 1,310 sq ft before waste.
What thickness of vapor barrier is best?: The IRC defines the retarder by class, not mil. Thicker liners resist tears from foot traffic. The 6 mil figure is the polyethylene the IRC names under concrete slabs.
Should seams be taped?: For an unvented crawl space, yes: R408.3 says joints shall overlap 6 inches and be sealed or taped.
Can I install a vapor barrier myself?: Laying a ground cover is common DIY work. Fix standing water and any rotted wood first, and wear protection in a dirty crawl space.

Sources and methodology

Figures dated 26 October 2026. Last reviewed August 16, 2026.

International Residential Code R408.3, unvented crawl space: vapor retarder laps, wall extension and the 70 pints per 1,000 sq ft dehumidification option (2018 edition text) (International Code Council, read via UpCodes, retrieved 2026-10-26. Model code. Your jurisdiction adopts an edition and may amend it.)
International Residential Code 2024, Chapter 4: R406.1 parging, R408.2 vent ratio of 1 sq ft per 150 sq ft, R408.4 access and R408.5 debris removal (International Code Council, read via UpCodes, retrieved 2026-10-26)
International Residential Code 2024, Section R506: slab thickness of 3 1/2 in, fill limits, 4 in base course and 6 mil vapor retarder (International Code Council, read via UpCodes, retrieved 2026-10-26)

Crawl space encapsulation calculator Price a sealed system.
Crawl space vents Open, closed or sealed.
Crawl moisture check Is your design working?
Crawl space repair Structure before sealing.

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Crawl space vapor barrier calculator