Vapour permeance (AS 4200.1 classes)
Vapour permeance is the rate water vapour passes a material. AS 4200.1 Class 1 (barrier) to Class 4 (permeable). NCC requires Class 3-4 in wall sarking.
Ask Chalkline about this →Vapour permeance is the rate at which water vapour passes through a building material under a given pressure difference, expressed in micrograms per Newton per second (μg/N/s) or in older units of “perm” (US) or “MNs/g” (SI metric). AS/NZS 4200.1:2017 classifies pliable building membranes into four vapour permeance classes (Class 1 to Class 4). The NCC 2022 Housing Provisions Part 10.8 (introduced for condensation management) sets Class 3 or Class 4 permeance requirements for wall sarking in certain climate zones, to prevent interstitial condensation forming inside the wall cavity (verified 2026-05-16).
AS 4200.1 vapour permeance classes:
| Class | Permeance (μg/N/s) | Common name | Where used |
|---|---|---|---|
| Class 1 | 0.022 maximum | Vapour barrier (impermeable) | Under slab on ground, where vapour from soil must be blocked |
| Class 2 | 0.022-0.142 | Vapour-impermeable | Some roof sarking in cold climates |
| Class 3 | 0.142-1.140 | Vapour-resistant (semi-permeable) | NCC-compliant wall sarking in mixed climates (zones 4-5) |
| Class 4 | > 1.140 | Vapour-permeable | NCC-compliant wall sarking in cold climates (zones 6-8); “breathable” |
(Permeance values are nominal; refer to AS 4200.1:2017 for the actual test method and ranges.)
Why permeance matters: interstitial condensation:
Water vapour migrates through wall and roof assemblies under temperature and humidity differentials. If the vapour meets a cold surface inside the wall cavity, it condenses to liquid water. The liquid then:
- Rots timber framing within 12-24 months.
- Saturates insulation, dropping R-value.
- Promotes mould growth on the cold side of the cavity.
- Damages plasterboard and paint on the internal face over years.
The mitigation: let the vapour through to the outside, by using a vapour-permeable sarking (Class 3 or 4). A vapour-impermeable sarking (Class 1 or 2) on the wrong side of the wall traps the moisture, accelerating decay.
NCC 2022 Part 10.8 requirements (commenced staggered by state from May 2026):
| Climate zone | Wall sarking permeance | Reason |
|---|---|---|
| Zones 1-3 (warm-humid, tropical) | Class 1 or 2 acceptable in some applications | Vapour drive is inward; impermeable membrane on the outside protects from external humidity |
| Zones 4-5 (mixed, e.g. Sydney, Brisbane) | Class 3 (vapour-resistant) | Allows enough vapour through to escape; sufficient resistance to dampen interior humidity |
| Zones 6-8 (cool to alpine, e.g. Melbourne, Canberra, Hobart) | Class 4 (vapour-permeable) | Vapour drive is outward in winter; membrane must let vapour out to avoid trapping in walls |
State commencement (verified 2026-05-16):
- Vic 1 May 2026.
- NSW, QLD delayed past 2026; typical commencement late 2026 to 2027.
- Other states: rolling adoption.
The “right-side” rule:
A vapour-impermeable membrane (Class 1 or 2) installed on the wrong side of the insulation will cause condensation. The simplified rule:
| Climate type | Vapour-impermeable side |
|---|---|
| Cold-winter dominant (Melbourne, Canberra) | Inside face of the wall (the warm side) |
| Hot-summer dominant (Brisbane, Darwin) | Outside face of the wall (the warm side) |
| Mixed (Sydney, Perth) | Use vapour-permeable (Class 3) on both sides; avoid impermeable membranes if possible |
For most Australian residential, the simplest rule is: use Class 3 or 4 sarking on the outside of the wall framing. Class 3 in mixed climates, Class 4 in cold.
Common defects:
- Class 1 plastic builders’ film used as wall sarking: classic 1990s installation that NCC 2022 specifically addresses. Class 1 traps vapour in the cavity; rot, mould, R-value loss.
- Reflective foil with vapour permeance assumed but not checked: many traditional foil sarkings are Class 2 (effectively impermeable). Need to confirm AS 4200.1 class on the supplier label.
- Wall sarking installed inside-out (vapour-permeable side facing the wrong way): some membranes are directional; check arrows on the sheet.
- Penetrations not taped: a single penetration (electrical outlet, pipe) breaks the vapour-control plane; condensation forms at the gap.
- No vapour-control strategy at all: relying on “the walls will breathe somehow”; rarely works, particularly in zones 6-8.
Vapour permeance vs vapour barrier:
| Concept | Definition |
|---|---|
| Vapour permeance (this) | Property of a material: how much vapour gets through |
| Vapour barrier | Material with very low permeance (Class 1); deliberately blocks vapour |
| Vapour retarder | Material with moderate-low permeance (Class 2); slows but doesn’t block |
| Vapour-permeable membrane (Class 3 or 4) | Material that lets vapour through; protects from liquid water but not vapour |
A wall sarking in modern AU residential is typically Class 3 or 4: a vapour-permeable membrane that sheds liquid water (rain, condensation) but lets vapour escape.
Builder takeaway:
- For NCC compliance in cold and mixed climates, specify Class 3 or 4 wall sarking.
- Use the supplier’s AS 4200.1 class on the label as the compliance proof.
- Tape all penetrations (services, openings) to maintain vapour-control continuity.
- For under-slab vapour control, Class 1 is correct (block soil vapour entirely).
Also known as: permeance class; AS 4200 class; vapour transmission class; WVT rating (water vapour transmission); breathability rating.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.