Vapour barriers for residential builds: under-slab and envelope membranes

TL;DR

Vapour barriers in residential construction are a different product class from sarking and foil-reflective insulation, although the three categories overlap. The pure vapour barrier role: stop water-vapour migration through a building element (typically a slab or a wall) without serving as the primary water-shedding or insulating layer. The two volume Australian applications: under-slab polyethylene vapour barrier (mandatory under AS 2870:2011 and ABCB Housing Provisions Part 4.2 for slab-on-ground residential, minimum 0.2 mm thick orange or black polyethylene), and envelope vapour barrier / vapour control layer (used in cool-climate walls and roofs to balance the building’s drying capacity; classified under AS/NZS 4200 as Class 1 low-permeance membrane). For under-slab, the volume products are Fortecon orange film, BlackJack black film, and equivalent 0.2 mm reinforced polyethylene from Bidim or specialist supply. For envelope vapour control, the products overlap with foil sarking (which is itself Class 1 vapour barrier). The two job-killers: punctures and tears in the under-slab barrier (every penetration, every plumbing stub, every footing edge must be sealed; bare areas become moisture-transmission points), and using a Class 1 vapour barrier in a cool-climate wall where Class 3 vapour-permeable is needed (traps internal moisture in the wall cavity, framing rots within years).

What it is

A vapour barrier is a thin film material with very low water-vapour permeance: it blocks the migration of water-vapour molecules through it. In residential construction, vapour barriers serve two distinct purposes:

Under-slab vapour barrier: stops ground-water vapour rising from the soil into the concrete slab. Without it, ground moisture migrates through the slab into the interior, causing rising-damp problems, floor-covering failure, and elevated indoor humidity.
Envelope vapour control: regulates moisture migration through walls and ceilings. In cool climates, internal humid air condensing on the cold side of the wall cavity causes mould and timber rot. A vapour barrier on the warm side of the insulation stops the migration before it reaches the cold side.

The dominant residential vapour barrier in Australia is polyethylene film, typically 0.2 mm thick, supplied in coloured rolls (orange is the most common; black, blue, and clear variants exist) so it can be visually distinguished on site from other membranes.

Under-slab vapour barrier

Standards requirement

AS 2870:2011 and ABCB Housing Provisions Part 4.2 require a vapour barrier under all residential concrete slabs on ground. The specific requirements:

Item	Requirement
Material	Polyethylene film, 0.2 mm (200 micron) minimum thickness
Colour	Distinctive (orange most common, also black) so it’s visible on site
Lap	Minimum 200 mm overlap at joints
Sealing at laps	Sealed with tape or proprietary jointing tape per manufacturer’s instructions
Penetration sealing	Sealed around every penetration (pipe stub, conduit, dowel)
Edge condition	Turned up at slab edges to the top of the slab
Damage check	Visual inspection before pour; no holes, tears, or punctures

The barrier sits below the slab but above any structural blinding or capillary break gravel. Typical layer sequence (bottom to top):

Compacted sub-base (sand or gravel blinding)
Vapour barrier (orange polyethylene, 0.2 mm)
Slab steel reinforcement
Concrete slab pour

Volume products

Brand / product	Type	Where used
Fortecon (Fielders / Bradford-related)	Orange polyethylene, 0.2 mm reinforced	Volume residential under-slab
BlackJack	Black polyethylene, 0.2 mm reinforced	Alternative residential under-slab
Bidim DPM	Damp-proof membrane (polyethylene)	Under-slab vapour barrier supply
Bostik / Fosroc Methylene	Specialised vapour barriers including chemical-resistant versions	Specialist or contaminated-soil applications
Specialist contaminated-soil vapour barriers	Higher-spec (4 mm reinforced PE, butyl-modified)	Sites with hazardous-vapour migration risk (former industrial, asbestos)

For standard residential, the 0.2 mm orange polyethylene Fortecon-equivalent is the default.

Install requirements

Rule	Why
Lap minimum 200 mm	Provides envelope continuity at joints
Lap sealed with tape	Stops vapour bypassing at laps
Wrapped around penetrations (pipe boots, conduit collars)	Maintains envelope at every service entry
Turned up at slab edge to top of slab	Continuity at the slab perimeter
No tears or holes	Holes are direct vapour-migration paths; patch with tape before pour
Inspect immediately before pour	Last chance to find puncture damage from steel placement, foot traffic, or formwork

Special cases

Situation	Recommended vapour barrier
Standard residential slab on dry soil	0.2 mm orange polyethylene
Slab on aggressive or contaminated soil	Specialist barrier with chemical resistance
Slab on a high-water-table site	Thicker (0.3 to 0.5 mm) or reinforced product; consider waterproof tanking
Slab above suspect ground (former industrial, asbestos, methane risk)	Specialist barrier with documented contaminated-soil rating; engineer review
Polished concrete floor (decorative, finished)	Higher-spec barrier (0.3 mm reinforced) to ensure no slab-side moisture appears as efflorescence

Envelope vapour control

In cool climates (NatHERS climate zones 6, 7, 8: Melbourne, Adelaide hills, Canberra, Hobart, Ballarat, alpine), the building envelope needs careful vapour control to prevent interstitial condensation: water vapour from internal humid air migrating into the wall or roof cavity, where it condenses on the cold side and saturates the framing or insulation.

The vapour barrier rule (warm-side application)

In cool-climate building science:

Place the vapour barrier on the warm side of the insulation (the inside face of the wall)
Place a vapour-permeable membrane on the cold side (the outside face, behind the cladding)

This combination lets vapour escape outwards while preventing inward migration. NCC 2022 Volume Two Specification 37 sets the condensation management provisions for zones 6 to 8.

Class 1 vapour barrier products

AS/NZS 4200.1:2017 Class 1 (low water-vapour permeance, vapour barrier) products:

Foil-faced kraft sarking (Sisalation): Class 1 typical
Reinforced polyethylene wall membrane: Class 1
ProClima Intello: smart vapour retarder (Class 1 at humid conditions, more permeable at dry)
DuPont Tyvek HomeSeal SD: specific vapour-retardant variants

For warm humid climates (zones 1, 2, 3) the vapour-barrier placement is reversed: barrier on the outside (or no barrier), vapour-permeable membrane on the inside. The wall is built to dry inward away from the humid outside air. A condensation assessment per NCC Specification 37 is the design basis.

Vapour barrier vs sarking vs DPC

These three products overlap; clarification:

Product	Primary function	Where used
Vapour barrier (this entry)	Stop vapour migration	Under slab; cool-climate warm-side wall
Sarking	Water + vapour + wind barrier under cladding	Roof, wall behind cladding
DPC (damp-proof course)	Stop rising damp in masonry walls	Between brickwork courses
Foil-reflective insulation	Reflect radiant heat with air gap	Roof and wall thermal envelope

A single physical product can serve multiple functions. A foil-faced sarking is simultaneously a Class 1 vapour barrier and a thermal radiant insulation; the product chosen is the one that ticks the requirements box for the specific layer.

Common defects and on-site issues

Under-slab barrier torn or punctured during steel placement: very common; the reo placement crew walks on the barrier with screwdrivers and bar chairs in pockets. Inspect immediately before pour; patch any holes with tape.
Lap sealing missing or inadequate: 200 mm lap is the AS 2870 requirement; less than that or unsealed lap creates an open join through which vapour can pass.
Vapour barrier missing entirely from slab: a defect that’s invisible after pour and very expensive to remediate. Inspection at pre-pour stage is the only safeguard.
Wrong polyethylene thickness: 0.1 mm or 0.15 mm film used instead of 0.2 mm. Non-compliant with AS 2870.
Vapour barrier omitted around pipe and conduit penetrations: water vapour migrates through the penetration. Use pipe sleeves and tape; never leave a hole.
Wall vapour barrier on cold side in cool climate: reverses the design intent; traps condensation in the cavity. Confirm zone and barrier placement before install.
Vapour barrier in warm humid climate: traps external humidity in the wall cavity. Use vapour-permeable membrane instead.
Sealant incompatibility with PVC pipe sleeves: some pipe-boot sealants react with PVC. Use product-recommended sealant.

Pricing (2026 indicative, ex-GST, supply only)

Product	Per square metre or roll
Standard 0.2 mm orange polyethylene (Fortecon equivalent)	$0.80-1.50/m2
Reinforced 0.2 mm polyethylene (black or reinforced versions)	$1.50-2.50/m2
0.3 mm reinforced (polished concrete grade)	$2.50-4.00/m2
Specialist contaminated-soil barrier	$8-25/m2 (project-specific)
Vapour barrier lap tape (50 mm roll, 50 m)	$20-40
Pipe boot / penetration sealing kit	$15-40 per kit

For a typical 200 m2 residential slab, the under-slab vapour barrier supply is approximately $200 to $500 (a small line item relative to overall slab cost).

Standards and references

Standards Australia, AS 2870:2011 Residential slabs and footings. https://store.standards.org.au (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 ABCB Housing Provisions Part 4.2 Footings, slabs and associated elements. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions/4-footings-and-slabs/part-42-footings-slabs-and-associated-elements (verified 2026-05-13).
Standards Australia, AS/NZS 4200.1:2017 Pliable building membranes and underlays. https://store.standards.org.au (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 Volume Two Specification 37 (Condensation management). https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two (verified 2026-05-13).