A vapour barrier and a vapour permeable membrane are not two grades of the same product. A barrier holds moisture out and holds it in. A breather lets the wall dry outward. For external walls, NCC 2022 sets a minimum permeance class in climate zones 4 to 8, and that requirement decides the choice more often than anything else on the datasheet.
In short
- Barrier or breather is the first decision, and getting it wrong traps water in the wall.
- NCC 2022 clause 10.8.1 sets a minimum vapour permeance class for external walls in zones 4 to 8.
- Reflective or not is a separate decision. A foil face needs an air gap, a non-reflective breather does not.
- Duty rating matters as much as permeance. Light Wall duty is a classification, not marketing.
- An untaped wrap is not a weather barrier. The tape is part of the product.
Sarking, wall wrap, building membrane, breather, vapour barrier. The words get used loosely, which is how a wall ends up with the wrong one. This guide separates the decisions that actually have to be made.
Two families, not a scale
Everything on the shelf sits in one of two families, and they do opposite things with moisture.
| Vapour barrier | Vapour permeable membrane | |
|---|---|---|
| What it does with vapour | Stops it passing through, in both directions | Lets it pass outward while stopping liquid water coming in |
| Where the moisture ends up | On the cold side of the barrier, wherever that is | Out of the wall |
| Right answer when | The build up is designed to keep vapour out of a cavity entirely | The moisture load comes from inside and the wall needs to dry |
| Wrong answer when | Moisture can get in behind it and has no way out | The design specifically requires vapour to be blocked |
Our own product notes put it bluntly for the barrier family: it holds moisture out, and it also holds moisture in. That is the right answer in some wall build ups and the wrong answer in others, and it is why the breather ranges exist alongside them rather than replacing them.
In an Australian house the moisture load usually comes from inside, from showering, cooking, washing and drying. That is the argument for letting a wall dry outward, and it is why the code went the way it did.
The classes, and what the NCC asks
AS/NZS 4200.1:2017 classifies pliable building membranes by vapour permeance, from the tightest barriers through to the most permeable breathers. Higher class number means more permeable.
NCC 2022 Housing Provisions clause 10.8.1 then sets a minimum for external walls:
| Climate zone | Minimum for an external wall membrane |
|---|---|
| 4 and 5 | At least Class 3, not less than 0.143 µg/N.s |
| 6, 7 and 8 | Class 4, not less than 1.14 µg/N.s |
Two things about that table are worth saying plainly. It is written for external walls, not roofs, even though the numbers get quoted for roofs constantly. And it is a minimum, so a membrane can sit well above it. Our Trade Select VapourTech Wall is a Class 4 membrane with a vapour permeance of 3.764 µg/N·s, which is high even within Class 4.
Read the clause against your own build up and climate zone rather than trusting a summary, including this one.
Reflective or not, a separate decision
This is the point that saves the most confusion, because people treat it as the same choice and it is not.
Permeability is one axis. Reflectivity is another. A membrane can be:
- Reflective and a barrier. The traditional foil sarking. Most foil faced products sit here.
- Non-reflective and permeable. The modern wall wrap.
- Reflective and permeable. Less common, and worth knowing exists. AIR-CELL Permishield XV 70 is Class 3 permeable at 0.300 µg/N·s while still being a water barrier to AS/NZS 4201.4, with 97 per cent reflectance on the reflective face.
The practical consequence is about installation, not performance on paper. A reflective face only does anything with an air gap in front of it. Press it against the cladding or the frame and the reflective performance is gone, and you have paid for a facing that is not working.
A non-reflective breather has no such requirement. Trade Select VapourTech Wall has an emittance of 0.58 and 0.56 on its two faces, category NN, meaning it does not work by reflection at all. There is no air gap to preserve, so it can sit directly against the frame. It is also non-conductive, unlike a foil wrap, which removes one hazard around wiring.
So: decide the vapour class from the code and the build up, then decide reflective or not from whether the design actually provides a cavity.
Duty rating and strength
Permeance is not the only classification on the datasheet. AS/NZS 4200.1:2017 also sets duty ratings, and a membrane rated Light Wall duty is not the same product as a medium duty laminate even if the permeance suits.
The numbers underneath are worth a glance because they tell you how the material will behave on a windy frame before the cladding goes on. Trade Select VapourTech Wall records a ball burst of 215 N against a 200 minimum, an edge tear of 189 N in the machine direction against a 45 minimum, and 0.0 per cent shrinkage against a 0.5 per cent limit.
Where a membrane has to survive being walked over on a battened roof, or left exposed for a while during construction, the duty rating is the thing to check rather than the permeance.
The tape is not optional
A wall wrap only becomes a weather and air barrier once the joins and penetrations are sealed. Untaped, it is a sheet of material with gaps in it. This is the most commonly skipped step on site and the one that most often explains why a wrapped wall still leaks air.
The same applies to roofs in cooler climates. BlueScope's guidance is to seal the overlaps with appropriate tape so the membrane works as a vapour control layer rather than only as a thermal one.
Match the tape to the membrane, since adhesion varies between substrates. Double sided tape joins laps without a visible strip, flexible sealing tape follows penetrations and corners where a rigid tape will not, and air barrier tape seals to the frame or to adjoining materials. Some products carry their own tape strip, for example the integrated tape version in the Enviroseal range, which is faster and neater on a long run.
Which one for which job
| Situation | What it points to |
|---|---|
| External wall, climate zone 6, 7 or 8 | Class 4 permeable, because the code sets it as the minimum |
| External wall, climate zone 4 or 5 | At least Class 3 permeable |
| Wall with no cavity behind the cladding | Non-reflective breather, because there is no air gap for a foil face to work into |
| Wall with a designed cavity and a radiant heat problem | A reflective product, with the bright face into the cavity |
| Tile roof | Tile sarking. It stops wind driven rain, dust and embers reaching the roof space through the tile gaps |
| Metal roof, condensation risk | A breathable roof sarking, or a blanket with a facing, draped with air space above and below |
| Exposed ceiling with acoustic material above | A perforated facing, because a solid one reflects the sound the absorber was installed to take |
| Membrane left exposed for a while during construction | Check the duty rating, not the permeance |
For where a membrane sits against bulk insulation rather than instead of it, see blanket, sarking or batts. For the condensation reasoning behind all of this, see condensation in metal roofs.
Bushfire
Bushfire attack level is a separate qualification again, and it is not implied by either permeance or duty.
Several products in the Bradford Thermoseal range are suitable for BAL 12.5 through to Flame Zone when installed to specification. In the breather family, the Enviroseal roof and wall range is suitable for all BAL regions, which is broader than most membranes and makes it a straightforward choice on a rated site.
The qualifier matters: these ratings apply to the product installed to specification, as part of a system. A compliant membrane in a non-compliant detail is not a compliant roof.
What goes wrong
- Treating barrier and breather as interchangeable. They do opposite things with vapour.
- Using the wall permeance numbers for a roof. NCC 10.8.1 is written for external walls.
- Pressing a reflective face against the cladding. No air gap, no reflective performance.
- Skipping the tape. An untaped wrap is not an air or weather barrier.
- Choosing on permeance alone. Duty rating decides whether it survives the build.
- Assuming a BAL rating comes with the product. It comes with the installed system.
We supply it and we install it
AWBM carries vapour permeable membranes, foil sarking, wall wrap, roof sarking and the tapes that go with them across Australia. Wrap and sarking are work we install as well as supply, along with batts. Tell us the climate zone, the build up and whether there is a cavity, and we will tell you which membrane the job actually needs.
Or browse building membrane, wall insulation and roofing blankets.
Frequently asked questions
What is the difference between a vapour permeable membrane and traditional sarking?
A traditional foil sarking is a vapour barrier: it stops moisture passing in either direction. A vapour permeable membrane lets water vapour pass outward so the wall can dry, while still stopping liquid water getting in. They are different tools, not different grades.
Does the NCC require a vapour permeable wall wrap?
For external walls, NCC 2022 Housing Provisions clause 10.8.1 sets a minimum vapour permeance: at least Class 3, not less than 0.143 µg/N.s, in climate zones 4 and 5, and Class 4, not less than 1.14 µg/N.s, in zones 6, 7 and 8. Check the clause against your own build up.
What do the vapour classes mean?
They are the classification in AS/NZS 4200.1:2017 for how readily a membrane lets water vapour pass. A higher class number means more permeable, so Class 4 is a breather and the lower classes are barriers.
Does a reflective wall wrap need an air gap?
Yes. A reflective face only works against an air cavity. Pressed against the cladding or the frame it contributes very little, so the detailing decides whether you get what you paid for.
Can a membrane be both reflective and vapour permeable?
Yes, though it is less common. AIR-CELL Permishield XV 70 is Class 3 vapour permeable at 0.300 µg/N·s and a water barrier to AS/NZS 4201.4, with 97 per cent reflectance on the reflective face.
Do I have to tape the joins?
Yes. A wrap only becomes an air and weather barrier once the laps and penetrations are sealed. Untaped it is a sheet of material with gaps in it, and this is the most commonly skipped step on site.
What is a duty rating on a building membrane?
A classification in AS/NZS 4200.1:2017 covering how robust the membrane is, separate from its vapour permeance. Where a membrane is exposed during construction or walked over on a battened roof, the duty rating matters more than the permeance.
Which membrane suits a bushfire area?
Several Thermoseal products are suitable for BAL 12.5 through to Flame Zone when installed to specification, and the Enviroseal breather range is suitable for all BAL regions. The rating applies to the installed system, not to the roll on its own.
Can AWBM tell me which membrane my wall needs?
Yes. Send the climate zone, the wall or roof build up and whether there is a cavity, and we will work out the class and duty the job needs and quote it, supplied or installed.
Sources
- AS/NZS 4200.1:2017, Pliable building membranes and underlays, materials, for vapour permeance classes and duty ratings
- AS 4200.2:2017, Pliable building membranes and underlays, installation requirements
- AS/NZS 4201.4, Pliable building membranes and underlays, water barrier requirements
- NCC 2022 Housing Provisions, clause 10.8.1 Condensation management, external walls
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- Vapour permeance, emittance, duty ratings, strength figures and BAL suitability from the CSR Bradford and Kingspan product data for the products listed above
General information. Vapour permeance requirements vary by climate zone and by building element, and BAL suitability belongs to the installed system rather than the product alone, so confirm the current NCC provisions and the manufacturer documentation for your own project before selecting a membrane.