Vapour impermeable

Vapour impermeable describes a material or layer, closed-cell foam, foil, polythene, that water vapour effectively cannot diffuse through. It is useful as a deliberate vapour barrier on the warm side of a wall, but a condensation risk if it is placed where the wall actually needs to dry.

Building materials sit on a spectrum of vapour permeability, from vapour-open (a breather membrane that lets moisture diffuse through) to vapour impermeable (a barrier that stops it). The role of an impermeable layer is to keep moist indoor air from diffusing into the wall and condensing on a cold surface inside it. To do that job it has to be on the warm-in-winter side of the insulation, and there must not be a second impermeable layer on the cold side trapping moisture between them.

The danger is putting an impermeable layer in the wrong place. A wall generally needs to be able to dry in at least one direction. Sandwich the framing between two impermeable layers, or put the barrier on the cold side, and any moisture that gets in (and some always does) cannot escape, so it accumulates, condenses, and rots timber or corrodes steel. This is the classic condensation failure that the NCC condensation provisions are written to prevent.

For a builder the practical rule is to know which of your layers are vapour impermeable (sarking type, wraps, foams, foil-faced boards all vary) and to make sure the wall can still dry. Put any vapour barrier on the warm side, avoid a second impermeable layer trapping the cavity, and follow the NCC condensation management requirements for your climate zone rather than assuming “more barrier is better.”

Also known as: Vapour-tight, low vapour permeability.

Category: Building science / Moisture.

Related

• Vapour barriers
• Rigid foam

See also

• Sarking

References

• Vapour barriers (Chalkline) (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.