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Foil-backed plasterboard: the vapour-control wall lining

Foil-backed plasterboard is plasterboard with a foil layer on the back, acting as a vapour control layer on the warm side of the wall. When to use it under NCC H4.

By Chalkline. Last updated 28 May 2026. Verified 28 May 2026.

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Foil-backed plasterboard is standard plasterboard with an aluminium or metallised polyester foil layer bonded to the back face. The foil acts as a vapour-control layer (VCL) on the inside (warm side) of a wall or ceiling, reducing the amount of indoor moisture that diffuses into the cavity and condenses on cold surfaces. The branded product most commonly named in Australia is Knauf Vapour Panel, with similar foil-backed boards available from CSR Gyprock and other manufacturers (verified 2026-05-28, Knauf). It is one tool in a condensation strategy, not the NCC default; the relationship to NCC 2022 Part H4 is more nuanced than it looks.

What it is

Foil-backed plasterboard is a composite product:

  • Standard plasterboard core (gypsum between paper liners), the same as any other lining sheet.
  • Foil layer on the back face: aluminium foil or metallised polyester, bonded during manufacture.
  • Standard front face: the inside of the room is finished and painted the same as ordinary plasterboard.

The foil layer’s low vapour permeance is what differentiates it from regular plasterboard. Where ordinary plasterboard is broadly vapour-open, foil-backed sheets are effectively vapour-resistant.

Where it fits in a wall

Building physics for condensation control in a heated indoor space follows a simple principle: warm indoor air carries more moisture than cold outdoor air. As that air moves through a wall, it cools. If it reaches its dew point inside the wall cavity or on the back of the cold-side cladding, the moisture condenses, wets the timber and insulation, and ultimately causes mould, fastener popping, and decay.

The two tools the NCC and building physics use to manage that risk are:

  • Vapour-permeable membrane on the outside (the cold side), so moisture that does get into the cavity can escape outward rather than being trapped. NCC 2022 Part H4 makes this a deemed-to-satisfy requirement in climate zones 6, 7, and 8 (all of ACT, Tasmania, alpine areas, and parts of southern VIC, NSW, SA, and WA), per the live NCC condensation management summary.
  • Vapour-control layer on the inside (the warm side), so less moisture enters the cavity in the first place. This is where foil-backed plasterboard sits.

The NCC’s prescriptive path emphasises the outside membrane. Foil-backed plasterboard on the inside is a designer’s add-on chosen when the assembly, climate, or occupancy makes inward vapour drive a particular risk: a heated living space in a cold zone, a high-humidity room, or a wall with sensitive insulation.

When a builder sees it specified

Common situations where the architect or designer calls for foil-backed plasterboard:

  • Cold-climate (Zone 6/7/8) walls where the design uses classical “vapour control on the warm side” detailing.
  • Bathroom or kitchen walls in cold zones where indoor humidity peaks (paired with the right wet-area substrate).
  • Walls with foil-faced insulation where the lining ties in with the insulation system’s vapour strategy.
  • Specific manufacturer system specs (e.g. a wall system warranted only with a foil-backed lining).

It is not a substitute for the vapour-permeable membrane on the outside; those two layers do different jobs at opposite ends of the wall.

Installing it as a vapour-control layer

The foil only works if it is continuous and sealed. Specific install points (verified 2026-05-28, Knauf, Siniat):

  • Seal every joint with foil-backed jointing tape or an airtight sealing tape so the VCL is unbroken across the wall.
  • Seal penetrations (powerpoint cut-outs, downlight rings, service entries) with airtight collars or tape; an unsealed penetration is a hole straight through the VCL.
  • Mind the order of layers: the foil sits on the back of the board, facing the cavity. Some assemblies require the foil to face the cavity air space; check the system spec.
  • Mechanical fixing as per ordinary plasterboard: screw pattern and stud spacing to the manufacturer’s table.

A poorly sealed foil-backed lining performs little better than ordinary plasterboard; the value sits in the airtightness of the layer, not just the foil itself.

Why it matters for the builder

Foil-backed plasterboard is a specification-driven product: a builder rarely chooses it on their own. The architect or NCC condensation report calls it out, and the builder needs to do three things right:

  • Order to the spec, not substitute ordinary plasterboard.
  • Install with airtight detailing, not just standard lining fixing.
  • Sequence with the rest of the condensation strategy: external vapour-permeable membrane, sarking, insulation order, ventilated cavity.

Substituting the wrong board or skipping the joint seal is a defect that does not show up at handover; it shows up two winters later as mould or sagging.

References

See also

Last updated: 2026-05-28. Verified: 2026-05-28. Quarterly review for currency.

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