Vapour barrier requirements

Are you using the correct material?

A vapour barrier is required by the National Construction Code (NCC) Volume Two under slab-on-ground construction for all Class 1 buildings and Class 10 buildings where the slab is a continuation of the Class 1 slab.

The information below has been prepared to assist practitioners to understand the requirements related to vapour barriers and damp-proofing membranes and what product technical information you should be looking for to ensure the products you are using comply with the NCC and relevant Australian Standards.

NCC requirements regarding vapour barriers

As stated above, a vapour barrier is required by the NCC under slab-on-ground construction for all Class 1 buildings and Class 10 buildings where the slab is a continuation of the Class 1 slab.

The NCC nominates the required material and thickness and the impact-resistant level for the barrier and the resistance to puncture and moisture penetration and also that the barrier be continuously branded for easy identification.

The NCC also references AS 2870 for residential slabs and footings, which also provides information relating to the required impact resistance and thickness of the vapour barrier and moisture penetration resistance.

The Standard goes on to provide methods to determine the required properties of the material, including the average thickness of the film and how to determine the required impact resistance for medium and high impact and the vapour permeance test.

There is also a requirement to determine the resistance to puncture and moisture penetration that relates to penetration resistance to falling aggregate that will occur when the slab is poured.

The NCC requires a vapour barrier be a nominal 0.2mm thick (or 200 µm, which is the thickness in microns) polyethylene film, be medium impact resistant and be branded continuously ‘AS 2870 Concrete underlay, 0.2mm medium impact resistance’.

It may not always be possible to determine this just by the packaging on the vapour barrier and damp-proofing membrane, if indeed it does have packaging. 

SA and NSW variations

South Australia and New South Wales have variations to the NCC provisions that require a damp-proofing membrane as described in the variation, to have ‘high impact resistance’ rather than the medium-impact resistance required in other states and territories.

It also must be branded continuously “AS 2870 Concrete underlay, 0.2mm high impact resistance”.

This is most important to consider if you work ‘cross border’. You should not just assume that the requirements are the same everywhere.

In areas prone to rising damp and salt attack, AS 2870 recommends the use of damp-proofing membranes also under slabs.

NCC materials compliance requirements

Compliance with the NCC is required not just for the installation of vapour barriers and damp-proofing membrane – the materials must also comply. It’s not the case that you can do one and ignore the other you – must comply with both to satisfy building legislation.

Manufacturers and suppliers generally have an obligation to supply materials that are to be used in the residential building industry that are fit for purpose and, where required, comply with the NCC.

Even so, you should understand what is required under the NCC and ensure the products you use comply. This is set out in the evidence of suitability provisions of the NCC.

These provisions are listed at A5.0 - A5.2 (formerly A2.1-A2.2) in Volume Two.

NCC evidence of suitability provisions

Part A5 of each Volume of the NCC contains the documentation of design and construction provisions. 

A5.2 lists the types of evidence that can be used to demonstrate that the materials, products, forms of construction and design are fit for their intended purpose and meet the requirements of the NCC.

One such method is a product technical statement. This statement provided by the manufacturer will specify the product, how and when it can be used and the performance requirements that it satisfies, and limitations or conditions on the use of the product and testing data or certification information.

In purchasing vapour barrier materials, it is important to access and check the technical data sheet that should accompany the product for the properties of the material, compliance with relevant requirements of AS 2870. You should also ensure you check the suitability for the impact-resistance rating. 

These technical data sheets can generally be sourced directly from the product suppliers or manufacturer. The relevant building surveyor or certifier may also request this information as part of the building approval documents.

Also talk to your supplier to ensure you are receiving the correct material and ask them to provide the documentation that should accompany the product, such as a technical data sheet.

Be aware that in most circumstances, the builder will have responsibility to the home owner should a product that is installed not meet the requirements of the Standard. Therefore, if there is any doubt about the compliance of a product that has been supplied, you should ask further questions about its compliance prior to purchase or installation.

Further information

For further information on required materials and installation of vapour barriers refer to the BCA part 3.2.2.6 Vapour barriers and AS 2870 clause 5.3.3.3.

An information handbook on the NCC evidence of suitability provisions can be accessed from the ABCB website.

To find out more, contact HIA’s Building Services team.

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