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Vapour-permeable membrane

A vapour-permeable membrane is a wall or roof wrap that lets the cavity dry outward, used in cooler NCC climate zones to manage condensation. High AS/NZS 4200 permeance.

By Chalkline. Last updated 25 May 2026. Verified 25 May 2026.

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A vapour-permeable membrane is a pliable building membrane that readily lets water vapour pass through it, so a wall or roof cavity can dry to the outside. It is the vapour-open end of the sarking spectrum, the opposite of a vapour barrier (which blocks vapour to keep moisture out).

What makes it “permeable”

Vapour permeability is measured under AS/NZS 4200.1 as a vapour permeance class. The vapour-open membranes sit at the high-permeance end, Class 3 or Class 4 (Class 4 being the most permeable; products like Tyvek HomeWrap are Class 4). A Class 1 membrane, by contrast, is a vapour barrier. (The European “Sd” rating describes the same vapour-open property a different way; in Australia the AS/NZS 4200 class is the reference.)

Why and where you use one

In a cooler climate, warm moist indoor air pushes outward through the wall, and if it reaches a cold surface it condenses inside the cavity, the cause of hidden mould and timber decay. A vapour-permeable membrane lets that moisture escape outward rather than being trapped. That is why the NCC condensation-management provisions require a vapour-permeable membrane (or an equivalent measure) in the cooler climate zones (broadly zones 6 to 8); see NCC condensation management for the detail.

The flip side: in a warm, humid climate, the moisture drive is inward, and a lower-permeance (vapour-barrier) membrane can be the right call. So the membrane is chosen to the climate zone, not by default.

For a builder

  • Match the membrane to the climate zone. Vapour-permeable in cool/cold zones; the wrong (vapour-closed) membrane there traps condensation in the wall.
  • It is not automatically a reflective barrier. Most vapour-permeable wraps are non-reflective; if you also want a reflective/insulating face, that is a different product (foil sarking).
  • Install per AS/NZS 4200.2 and the product guide. Laps, sealing, and orientation matter; a poorly lapped wrap leaks air and water regardless of its permeance.
  • Common products: Tyvek HomeWrap, Sisalation Vapawrap, Pro Clima, and similar vapour-open wraps.

Also known as: vapour permeable wall wrap, vapour-open membrane, breathable membrane.

See also

Last updated: 2026-05-25. Verified: 2026-05-25. Quarterly review for currency.

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