Air, Vapour & Fire in Wall Wraps
Not all wall wraps are created equal. Different users have different requirements and expectations for what they want their wrap to do.
In most cases we should, and can expect modern nonwoven textile wraps to control:
- Water – Prevent rainwater from entering the building during construction before cladding is installed. This protects insulation, internal services, and finishes that aren’t designed for exposure, reducing risks to both occupant health and structural integrity.
- Air – After construction, uncontrolled air movement can worsen moisture problems, enable water ingress, introduce unwanted noise, increase heat loss/gain, and allow the spread of fire and smoke.
- Water Vapour – Condensation and high humidity can cause mould, rot, and long-term damage to structural elements.
- Fire Safety – Wraps should not contribute significantly to combustibility in the building envelope. Some glass/aluminium laminates are classified as non-combustible under the NCC, while vapour-permeable and foil laminate wraps are generally of low flammability, not non-combustible.
Sometimes we may be forced to compromise on some of these control layers, but we also need to pay attention to the minimum requirements in the National Construction Code (NCC).
NCC 2019: Improved Clarity for Combustibility Compliance
The 2019 update to the NCC clarified which materials can be used in areas requiring non-combustible construction. Key excerpts from Clause C1.9 include:
(a) In Type A or B buildings, external walls (including all components like facade, framing, and insulation), lift pit floors, and certain internal fire-rated walls must be non-combustible.
(b) Non-loadbearing shafts must be non-combustible if they span more than two storeys in relevant building classes.
- (c) A loadbearing internal wall and a loadbearing fire wall, including those that are part of a loadbearing shaft, must
comply with Specification C1.1.(d) Exemptions include gaskets, caulking, sealants, termite barriers, laminated glass, glazing thermal breaks, and damp-proof courses.
(e) Permitted materials where non-combustibility is required include plasterboard, fibre-cement sheeting, bonded laminated materials (if each layer is non-combustible and adhesives are thin), and sarking-type materials ≤1mm thick with a Flammability Index ≤5.
The NCC 2019 update resolved long-standing conflicts where non-combustibility requirements clashed with the need for air, water, and vapour control—especially in external wall assemblies.
The Challenge of Wrap Performance: Vapour vs Airtightness
There never has been a wrap product available that meets the NCC requirement for non-combustibility, while at the same time as being an effective air and water barrier, combined with sufficient vapour permeability for use in cool and temperate climates.
Current products on the market today that are deemed non-combustible are made from pure aluminium foil laminated, an adhesive layer and a woven glass cloth. Aluminium Foil when reinforced can be very airtight, but these products are also extremely vapor tight (Incapable of allowing moisture vapour to escape/permeate to the outside).
To improve vapour permeability, some manufacturers perforate the foil—creating a trade-off. These perforations compromise water and air barrier performance. After all, nobody sells umbrellas with holes in them.
On the other side of the equation, with aluminium foil type wraps, if the holes are so small and infrequent, these types of products still struggle as a vapour permeable underlay. Like single glazed glass windows, aluminium foil is a non-porous material, and it is prone to condensation forming on its surface. Over the past year, some manufacturers have published questionable vapour permeance values, with figures shifting from plausible to implausible and back again.
Polyolefin Textiles: A Better Option
Well established polyolefin textile air and watertight vapour permeable wraps can provide a far superior vapour permeability performance without compromising other important features.
If following a deemed to satisfy path using NCC 2016, it will require some compromise of water tightness, air tightness or vapour permeability. The only realistic option to date has been to insulate externally the non-combustible sarking, use a non-combustible vapour permeable sheathing board, or follow a performance solution / fire engineered pathway.
Water Barrier Classification: AS 4200.1
Looking at the requirements of a water barrier under the AS4200.1: 1994 code for sarking you can see that almost all perforated aluminium-based materials are unhelpfully classified as:
“Water Barrier – Unclassified”
6.4 Water barrier – The water barrier classification shall be neither high nor unclassified. The resistance to water penetration shall be determined, as required, by the method described in AS/NZS 4201.4. as follows:
(a) High The material shall only be classified as high if it passes the test,
(b) Unclassified When the material is not classified as high, it shall be unclassified.
In terms of AS4200.1 : 1994 classification, if a datasheet is reporting a product to be “unclassified”, it means it has not passed the water barrier test which is a simple 10cm standing head of water test.
The current version of AS4200.1: 2018 is more helpful to the user and the product will now be defined as:
“Water Barrier – Non-water barrier”
5.3.5 Water control classification
The water control classifications shall be determined as follows;
(a) Water barrier – if the membrane passes the test specified as AS/NZS4201.4.
(b) Non-water barrier – if the membrane fails the test specified in or has not been tested to AS/NZS 4201.4.
How then have things changed for classification of vapor permeance?
Vapour Control Requirements
In the 1994 version of the code there was no classification of vapour permeance. Products were either a low, medium or high classification as a vapour barrier. Just because a material wasn’t a particularly fantastic vapour barrier does not imply that it is sufficiently vapour permeable.
The 2019 NCC Condensation Management DTS requirements for climate zones 6,7 & 8 now requires a vapour permeable membrane, In the updated 2017 version of A4200.1 vapour barrier and vapour permeable membranes using table 4. See below. (Southern Australia and Alpine areas)
We should only be seriously using class 4 products with a vapour permeance of >1.14µg/N.s Class 3 seems like a bit of a holdover from the old building code to cover the old “perforated” foil wraps Perforated sarking).
When considering that a typical paper faced plasterboard has a permeance of around 2.0µg/N.s then it is a good rule of thumb to ensure the external wrap is at least as permeable (>2.0µg/N.s) as the interior plasterboard lining. In order to control moisture successfully inside a wall system the vapour permeability of materials is increasingly vapour permeable toward the outside of the wall system.
TABLE 4
VAPOUR CONTROL MEMBRANE (VCM) CLASSIFICATION
Vapour permeance (see Note)
| Class | VCM Category | Min. (μg/N.s) | Max. (μg/N.s) |
|---|---|---|---|
| Class 1 | Vapour Barrier | 0.0000 | 0.0022 |
| Class 2 | 0.0022 | 0.1429 | |
| Class 3 | Vapour Permeable | 0.1429 | 1.1403 |
| Class 4 | 1.1403 | No max. | |
| ASTM-E96 Method B Wet Cup - 28 C 50%RH | |||
NOTE: Vapour permeance is the inverse of vapour resistance. It shall be calculated as follows:
Vapour permeance μg/N.s = 1/(Vapour resistance MN.s/g)
Almost all building wraps are technically combustible, but the caloric value (the heat produced by the complete combustion) of wraps is quite small compared to the volume of materials that are non-combustible in a wall system.
According to NCC2019 building wraps/membranes that are less than 1mm thickness with a flammability index of no greater than 5 can be used wherever a non-combustible material is required. This means you can now have your cake and eat it. For applications using NCC 2016, in most cases, your only solution is to discuss with your fire engineer about a performance solution.
Using Vapour Permeable Wraps at the Bottom of wall detailed with air tightness.
Getting an airtight barrier at the bottom of a brick veneer wall system using damp-course as your air barrier, with an overlap from a vapour permeable wrap. If you have any questions, please contact us.
