What is a vapor barrier?

A vapor barrier, also called a vapor retarder, is a layer that slows water vapor moving through building materials. DOE lists Class I at 0.1 perm or less. The right class depends on climate and wall cladding; specify the location as well as the material so the wall can dry.

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By InstaBids editorial team with AI research agents. Updated 5 November 2026

What this means for your home project

Read the proposed material’s specification and record its vapor-permeance class before choosing it for the wall. Source: U.S. Department of Energy, Building Science Education, read 2026-11-05.
Compare the proposed foam’s perm rating and the wall finish’s vapor permeability with DOE’s below-grade guidance before accepting the assembly. (source)
Request a written installation scope that identifies how seams, gaps and tears at electrical boxes and attic openings will be sealed. Source: University of Alaska Fairbanks Cooperative Extension Service, read 2026-11-05.
Request the manufacturer’s performance specification for the selected material so the stated vapor resistance can be assessed. Source: University of Alaska Fairbanks Cooperative Extension Service, read 2026-11-05.

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What are some typical vapor barrier materials?

DOE identifies Class I materials and alternatives for below-grade walls; use these distinctions when discussing your assembly (read 2026-11-05). “They include polyethylene plastic sheeting, rubber membranes, glass, aluminum foil, sheet metal, foil-faced insulating sheathings, and foil-faced non-insulating sheathings.” (source) “Instead, use Class II or Class III vapor retarders such as Kraft-faced fiberglass batts in the wall cavity or latex paint on the drywall for below-grade walls.” (source)

What is a perm rating?

UAF explains the measurement behind a product’s rating so you can interpret its stated performance (read 2026-11-05). “If a material has a perm rating of 1.0, 1 grain of water vapor will pass through 1 square foot of the material, provided that the vapor pressure difference between the cold side and the warm side of the material is equal to 1 inch of mercury (1 inch Hg).” (source)

What is the difference between a vapor barrier and an air barrier?

DOE distinguishes the jobs these layers perform; include the intended moisture-control function in your scope (read 2026-11-05). “In new construction, vapor retarders (also sometimes called vapor barriers) can be installed to reduce vapor diffusion into the home or building assemblies.” (source) “The building air barrier consists of an assembly of materials, including sealants that connect those materials, which resist the movement of air between outdoors (including unconditioned portions of the building) and indoors.” (source)

Where should a vapor barrier be located in the wall?

DOE compares climate-based wall placement with Class I restrictions for basement and crawlspace insulation (read 2026-11-05). “Vapor retarders should be located at or near the wall exterior surface in hot and humid climates.” (source) “They should be located at or near the wall interior surface in cold climates (DOE climate zones 5 and higher).” (source) “They should not be used interior to fibrous insulation in basements and crawlspaces as they can trap moisture inside of wall assemblies.” (source)

How do I know if a material is a vapor barrier?

Use DOE’s account of IRC 2012 to interpret product classes before comparing their suitability (read 2026-11-05). “The 2012 International Residential Code describes three classes of water vapor retarders.” (source) “Class I vapor barriers have a vapor permeance of 0.1 perm or less.” (source) “Class II vapor barriers have a vapor permeance of between 0.1 and 1.0 perm.” (source)

Did they do this vapor barrier install right?

UAF’s Alaska guidance shows why installation details deserve attention alongside the material on the label (read 2026-11-05). “A carefully installed, well-sealed 4-mil polyethylene vapor barrier is much preferred to a 6-mil vapor barrier with unsealed seams, gaps, tears at electrical boxes and unsealed attic scuttle openings.” (source)

What factors affect vapor barrier installation cost?

DOE identifies retrofit difficulty and possible exterior treatments as scope considerations, without providing installation prices (read 2026-11-05). “In existing buildings, it is generally difficult to add a vapor retarder.” (source) “In some cases, exterior treatments (e.g., elastomeric paint on block walls, stucco exterior, or siding with panel insulation underlayment) can add significant vapor resistance.” (source)

Sources

Vapor Barriers — U.S. Department of Energy, Building Science Education; published undated; read 2026-11-05.
Understanding Vapor Retarders for Basements — U.S. Department of Energy, Building Science Education; published undated; read 2026-11-05.
Permeability of Common Building Material to Water Vapor — University of Alaska Fairbanks Cooperative Extension Service; published undated; read 2026-11-05.
Managing the Drivers of Air Flow and Water Vapor Transport in Existing Single-Family Homes — U.S. Department of Energy; published 2012-10; read 2026-11-05.