AS 4200 (Pliable building membranes): the wrap and sarking standard

In plain English

The AS/NZS 4200 series sets the product standard and the installation rules for pliable building membranes: the flexible sheets that builders fix under roof cladding (sarking) and over wall framing (wall wrap) on Australian residential and small-commercial work. The two parts are:

AS/NZS 4200.1:2017 sets the product specification including vapour permeance classes and water-shedding performance.
AS 4200.2:2017 sets the installation rules including laps, tie-down and continuity around penetrations.

Both parts are called up by NCC 2022 Volume Two Housing Provisions Part 10.8 (Condensation management) (verified 2026-05-15 against ncc.abcb.gov.au). Part 10.8 specifies which vapour class of membrane is acceptable in which climate zone, and is the regulatory reason a builder cannot drop a Class 1 (very low permeance) foil-faced sarking into a cool climate zone wall and expect the certifier to pass it.

The single most important thing builders need to know: the membrane class is climate-zone-specific. The standard does not say “use any sarking you can buy”; it requires the right class for the assembly and climate.

What it requires

Membrane vapour classes (AS 4200.1)

Class	Vapour permeance (µg/N.s)	Common product type	Use case
Class 1	≤ 0.0022	Foil-faced low-permeance sarking (e.g. RFL foil)	Reflective insulation use, hot climates; restricted in cooler zones under NCC 10.8
Class 2	> 0.0022 to ≤ 0.143	Older non-breathable wall wraps	Generally not specified on new compliant residential work
Class 3	> 0.143 to ≤ 1.14	Reinforced foil sarkings with limited permeance	Niche use
Class 4	> 1.14	Vapour permeable membranes (e.g. ProClima Solitex, Bradford Thermoseal Vapaseal, James Hardie HardieWrap)	The default for cool-climate walls and roofs under NCC 10.8

Class 4 vapour permeable membranes allow water vapour to escape outward through the wall or roof assembly while still shedding bulk liquid water inward. This is the management strategy NCC 10.8 prefers for cool-climate residential walls (zones 6, 7, 8) and is increasingly default specification in zones 4 and 5.

Installation (AS 4200.2)

Lap dimensions specified by application (horizontal vs vertical, roof vs wall).
Continuity at penetrations: pipes, fixings, junctions sealed.
Tie-down requirements where the membrane forms part of the secondary weatherproofing layer.
Cap detail under cladding where ventilation cavity (drained cavity) is required by NCC.

What it doesn’t cover

Rigid air barriers and rigid sheathing. Those are separate products (e.g. fibreboard sheathing, OSB, magnesium oxide boards) under their own standards.
Insulation product specification. Bulk and reflective insulation is under AS/NZS 4859.
Roof underlay for tile vs metal. Both use AS 4200-class membranes but the specific class and installation rules vary; the manufacturer specification overlays the standard.
Site work documentation. The standard sets what the product is and how it goes up; the framing inspection sign-off is the certifier’s job under EP&A and equivalent state regulations.

Practical implications

Climate zone determines class. The certifier will check the sarking and wall wrap product called up against the NCC 2022 Part 10.8 schedule for the project’s climate zone. Substituting a Class 1 foil for a designed Class 4 vapour permeable membrane in a Melbourne (climate zone 6) wall is a non-compliance and a frame inspection fail.
Heat-pump sweat and shower steam are why this exists. Modern residential houses are tighter, run heat-pump hot water and reverse-cycle aircon, and produce more interior moisture vapour. Without a Class 4 permeable membrane the vapour condenses inside the wall cavity, wets the insulation, and creates the mould-and-rot defects that drove NCC 2019 and 2022 condensation reforms.
Cold-climate wall builds are now cavity-drained. The combined effect of AS 4200 Class 4 and NCC Part 10.8 is that residential walls in zones 5, 6, 7, 8 now generally need a drained cavity behind the cladding, with the membrane facing into the cavity, weep holes at the base, and a clear path for vapour to ventilate.
Brand substitution is non-trivial. Two products both marketed as “vapour permeable” can have very different permeance values and water-resistance ratings. The specification should call up the AS 4200 class plus the specific manufacturer / product or an equivalent with matching certified values.
Older detail libraries are out of date. Standard wall details from before NCC 2019 routinely show Class 1 foil. On any current build, the wall detail needs an AS 4200 cross-check before construction begins.

Source link

AS/NZS 4200.1:2017 product page, Standards Australia (verified 2026-05-15)
AS 4200.2:2017 product page, Standards Australia (verified 2026-05-15)
NCC 2022 Volume Two, Part 10.8 Condensation management, via ABCB (verified 2026-05-15)

References

AS/NZS 4200.1:2017, Pliable building membranes and underlays, Part 1: Materials (verified 2026-05-15)
AS 4200.2:2017, Pliable building membranes and underlays, Part 2: Installation (verified 2026-05-15)
NCC 2022 Volume Two Housing Provisions, Part 10.8 (Condensation management) (verified 2026-05-15)