If you are specifying a wall wrap in Australia, the classification matters more than the brand. Here is what the classes mean and when each is required.
AS 4200.1 classifies vapour control membranes by vapour permeance, measured in micrograms per newton-second and tested to ASTM E96 Procedure B at 23°C and 50 percent relative humidity.
| Class | Category | Range (µg/N.s) |
|---|---|---|
| Class 1 | Vapour barrier | 0 to 0.0022 |
| Class 2 | Vapour barrier | 0.0022 to 0.1429 |
| Class 3 | Vapour permeable | 0.1429 to 1.1403 |
| Class 4 | Vapour permeable | Above 1.1403 |
Classes 1 and 2 are barriers: they stop vapour. Classes 3 and 4 are permeable: they let it through at increasing rates.
Note that these classes have nothing to do with NCC building classifications. A Class 1 building and a Class 1 membrane are unrelated concepts that unfortunately share a naming convention.
Class 4 is roughly eight times more permeable than the Class 3 threshold. That is not a marginal upgrade. A Class 3 membrane at the bottom of its range passes a small fraction of the vapour that a Class 4 membrane does.
Where that matters is drying capacity. A wall that gets wet, whether from a construction period rain event, a plumbing failure or ordinary vapour diffusion, has to dry faster than it wets or the moisture accumulates. In a cold climate the outward drive is strong and continuous, so the assembly needs a membrane that can keep up.
Under NCC 2022, for Class 1 buildings, sole-occupancy units of Class 2 buildings and Class 4 parts of buildings, where a pliable building membrane is installed in an external wall on the exterior side of the primary insulation:
Climate zones 4 and 5 require not less than 0.143 µg/N.s. Class 3 or Class 4 both satisfy this.
Climate zones 6, 7 and 8 require not less than 1.140 µg/N.s. Only Class 4 satisfies this.
Climate zones 1, 2 and 3 are not covered by these deemed-to-satisfy permeance requirements, because the vapour drive in hot humid climates is different and often reversed.
Specifying Class 3 on a zone 7 project is a compliance failure, not a judgement call. It is also a performance failure, because the assembly will not dry at the rate the climate demands.
Mostly, and in cooler climates it is generally the safer choice. A higher permeance membrane gives the wall more drying capacity, which is margin against the things that go wrong on site.
The situations where more permeance is not automatically better are in hot humid climates, where the design question is not “how do we let vapour out” but “how do we manage vapour coming in.” In climate zones 1 and 2, a highly permeable external membrane paired with an air conditioned interior and no ventilated cavity can move humid outdoor air toward a cold internal lining. The answer there is usually a ventilated cavity and a considered assembly rather than a permeance number.
There is a second property that gets confused with vapour class: whether the membrane is classified as a water barrier under AS 4200.1.
Not all vapour permeable membranes are water barriers. If the membrane is providing weather protection during construction, before cladding goes on, it needs to be. NCC 2022 continues to allow non-water-barrier vapour permeable sarking in external walls where there is a drained cavity, such as brick veneer.
Check both properties on the technical data sheet. They are separate classifications and a product can be Class 4 vapour permeable without being a water barrier.
Four things, all of which are on the technical data sheet:
If a supplier cannot give you the permeance figure, that is an answer in itself.
