Vapour control layer (VCL)
A vapour control layer (VCL) is a low-permeance layer on the warm side of a wall that slows moisture entering the cavity, paired with a cold-side permeable membrane.
Ask Chalkline about this →A vapour control layer (VCL) is a low-permeance layer installed on the warm (inside) side of a wall or ceiling to reduce the amount of indoor moisture diffusing into the cavity, where it could condense on cold surfaces and rot the framing. It is the warm-side counterpart to the cold-side vapour-permeable membrane, which lets the wall dry outward. The two work as a pair in a cool-climate wall: slow the moisture getting in, and let whatever does get in dry out.
A VCL is a role, not one product. It can be foil-backed plasterboard (the common Australian product), a dedicated low-permeance membrane, or a vapour-control paint. Its permeance is classified under AS/NZS 4200, where a Class 1 membrane is effectively a vapour barrier. Three terms get used loosely and are worth keeping straight:
| Term | Where | Job |
|---|---|---|
| Vapour control layer | Warm (inside) side | Slows indoor vapour entering the wall or roof |
| Vapour-permeable membrane | Cold (outside) side | Lets the wall dry outward, sheds water |
| Vapour barrier (DPM) | Under the slab | Blocks rising damp from the ground |
Getting the side right is the whole game. Put a low-permeance layer on both sides of the insulation and you seal moisture into the wall with no way out, which is how cool-climate walls rot. In NCC climate zones 6 to 8 the condensation management provisions drive these choices. See foil-backed plasterboard and vapour permeance.
Also known as: VCL, vapour retarder, warm-side membrane.
Category: Building science / Moisture control.
Related
See also
References
- AS/NZS 4200.1 Pliable building membranes and underlays, Standards Australia (verified 2026-05-28)
- ABCB Condensation in Buildings Handbook (verified 2026-05-28)
Last updated: 2026-05-30. Verified: 2026-05-28. Quarterly review for currency.