Missing vapour barrier (pare-vapeur): what it is, how serious, what it costs

In winter, the air in the house is warm and holds moisture. If it gets into a wall or the attic, it meets the cold and the moisture condenses, sometimes as frost. The vapour barrier, « pare-vapeur » in a French report, stops it: a membrane, usually a polyethylene sheet, installed on the warm side of the insulation, just behind the drywall.

Because it is hidden by finishes, the inspector only sees it in a few places: in the attic, in an unfinished basement. Reports say “vapour barrier missing”, “not continuous” or “not verified”.

A missing or torn vapour barrier lets moisture build up in the insulation and the wood. The signs: frost or stains on the attic wood in winter, compressed or damp insulation, mould. Over time, the framing can rot.

With no sign of moisture, it is not urgent. It becomes so when the report notes condensation, mould or darkened wood in the attic.

Repairing the vapour barrier in one spot: $200 to $450, or $500 to $1,200 for an area. Installing a continuous vapour barrier over a section of the attic ceiling: $800 to $1,800.

It is often fixed at the same time as the attic insulation.

These are ballpark figures for Quebec, to be confirmed with quotes.

1. Where did you see the vapour barrier missing or torn?
2. Was there frost, black stains or damp wood in the attic?
3. Does the unfinished basement have a vapour barrier on the floor and walls?

Does your report mention the vapour barrier? Upload it: in five minutes, I tell you what comes first, what to have checked and what to ask the seller.

No. The vapour barrier, inside, stops household moisture. The air barrier, outside under the siding, stops the wind while letting the wall dry.