Glossary · Insulation & windows
Vapour barrier / vapour retarder
Also called: vapour control layer · polyethylene vapour barrier · 6-mil poly · perm rating · perms
A layer that slows water vapour 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 vapour barrier on the warm side, often polyethylene in Canada, slows it. Retarder is the more accurate term, because materials are rated by how much vapour 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 vapour control for your climate can trap moisture and rot walls, so auditors check it before recommending insulation, especially in basements.
In Canada
Interior polyethylene is the traditional Canadian vapour barrier, but NRCan advises using it in basements only when they're proven dry; otherwise use a breathable 'smart' membrane or the airtight-drywall approach.