Which side of the wall does vapor barrier go?

On the warm side (interior in cold climates). In hot humid climates, vapor barrier goes on the exterior or is omitted entirely. Climate zone determines placement.

Do I need vapor barrier under a concrete slab?

Yes. IRC requires a minimum 6-mil vapor retarder under all concrete slabs on grade to prevent ground moisture from wicking up.

Can vapor barrier cause mold?

Yes, if placed on the wrong side of the assembly. Incorrect placement traps moisture inside the wall. Consult local code for your climate zone.

Insulation

Moisture

$0.05-$0.20 per sq ft

Vapor Barrier: Moisture Control for Walls & Crawl Spaces (2026)

Prepared by Lumberama's Building Materials Team

|Last updated July 25, 2026|Sources & methodology

Vapor barriers are polyethylene sheeting or coatings that restrict moisture vapor from passing through building assemblies where it could condense and cause damage. The most common type is 6-mil polyethylene film used in crawl spaces, under slabs, and on warm-side wall surfaces in cold climates.

Key Features

Controls moisture vapor diffusion; 6 mil polyethylene standard; Crawl space and wall applications; Under-slab moisture protection; Required by code in many climates

Specifications

Thickness: 6 mil (0.006 in) minimum

10-20 mil for crawl spaces. Permeance: less than 1 perm (Class I). Material: polyethylene or cross-laminated HDPE. Roll sizes: 10x100 ft

20x100 ft.

Applications

Crawl space ground cover; Under concrete slabs; Cold-climate wall assemblies; Ceiling vapor control; Around foundation walls

Installation Overview

In crawl spaces, overlap sheets 12 inches and tape seams. Run 6 inches up foundation walls and secure. Under slabs, lay over gravel base with 6-inch overlaps, taped. On walls in cold climates, staple to studs on the warm side of insulation.

Pros & Cons

Pros

✓ Prevents moisture damage; Very affordable; Easy to install; Code-required in many areas; Multiple thickness options

Cons

✗ Can trap moisture if placed wrong side; Punctures easily; Incorrect placement causes problems; Not needed in all climates; Can promote mold if misused

Top Brands

Stego

Americover

Husky

Code Requirements

Vapor retarder per IRC R702.7. Class I vapor retarder on warm side in climate zones 5-8 (marine 4). Ground cover per IRC R408.2 in crawl spaces. Under-slab per IRC R506.2.3.

Buying Tips

Use 10-20 mil for crawl spaces where foot traffic may puncture thinner material. Know your climate zone before deciding on wall vapor barriers. Tape all seams for best performance.