Vapour barrier: definition and role under a screed

What is a vapour barrier?

A vapour barrier is a membrane that slows the passage of water vapour through a building element. Its role is not to stop liquid water but the vapour carried in the air, which naturally migrates from warm, humid zones towards cold ones.

Without it, that vapour can reach a zone where the temperature causes it to condense, generally within or beneath the insulation. The water then accumulates with no way of escaping.

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Vapour barrier or damp-proof membrane?

Both are membranes, often in polythene, and they are regularly confused on site. Their function is nonetheless different.

The vapour barrier blocks vapour coming from inside the building. The damp-proof membrane blocks moisture rising from the ground by capillary action. The first is placed on the warm side of the insulation, the second beneath it, between the damp substrate and the rest of the build-up.

On a ground floor over soil or over a crawl space, it is most often the second that is at stake.

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Why continuity decides everything

A membrane is worth only its continuity. The sheets must overlap, the overlaps must be sealed, and the membrane must be turned up around the perimeter against the walls.

A membrane punctured by a nail, torn where a conduit passes through, or simply laid edge to edge without overlap loses most of its effect. Vapour takes the defect, and it condenses behind the membrane, in exactly the place where water is least wanted.

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What happens when it is missing?

Moisture accumulates in the insulation, whose thermal conductivity degrades: the stated performance is no longer achieved, with nothing visible to show it. Over the longer term, that trapped moisture encourages defects in the floor covering and odours.

With an anhydrite screed the stakes are more direct still: calcium sulphate tolerates prolonged moisture from the substrate badly. See our thermal insulation solutions.