Glossary · Insulation & windows
Vapor barrier / vapor retarder
Also called: vapor control layer · polyethylene vapor barrier · 6-mil poly · perm rating · perms
A layer that slows water vapor moving through walls and ceilings, to keep it from condensing inside the structure.
What it means
In cold climates, warm indoor air holds moisture that tries to diffuse outward through walls; a vapor barrier on the warm side, such as polyethylene sheeting or the kraft facing on batts, slows it. Retarder is the more accurate term, because materials are rated by how much vapor they let through (perm rating), from almost none to quite a lot. Air leakage moves far more moisture than diffusion, so the air barrier matters even more.
Why it matters in an audit
The wrong vapor control for your climate can trap moisture and rot walls, so auditors check it before recommending insulation, especially in basements.
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| US vapor retarder classes (IRC, as summarized by DOE Building America) | Class I below 0.1 perm (polyethylene, sheet metal, foil); Class II 0.1–1.0 (kraft-faced batts, some vapor-control paints); Class III 1.0–10 (latex or enamel paint) | www1.eere.energy.gov ↗ |
| Where interior vapor control is required (US) | Class I or II required in climate zones 4C–8, with exceptions; not required on framed walls in zones 1–4B; avoid Class I or II interior layers in hot-humid climates | www1.eere.energy.gov ↗ |
In the United States
US codes use the term vapor retarder, classify materials by perm rating, and set requirements by IECC climate zone.