Building Wrap and the 2019 NCC Revisions

The 2019 revision of the National Construction Code (NCC) will be released and come into effect at the beginning of May. For the first time, guidance will be provided regarding the interaction of climate zones, wall construction, and condensation risk.

The new provision states:

(a) Where a pliable building membrane is installed in an external wall, it must —
    (i)    comply with AS/NZS 4200.1; and
    (ii)    be installed in accordance with AS 4200.2; and
   (iii)    be a vapour permeable membrane for climate zones 6, 7 and 8; and
   (iv)    be located on the exterior side of the primary insulation layer of wall assemblies that form the external envelope of a building.
(b) Except for single skin masonry or single skin concrete, where a pliable building membrane is not installed in an external wall, the primary water control layer must be separated from water sensitive materials by a drained cavity.

To interpret this the new provision requires the NCC’s new definition for ‘pliable building membrane’:

a water barrier as classified by AS/NZS 4200.1.

Some have taken this to mean that building wraps installed in all houses in climate zones 6, 7 and 8 must be water barriers, but a careful reading of the provision reveals that this is largely incorrect.

The first problem is that ‘pliable building membrane’ has an existing definition under Australian Standards, and includes both water barriers and non-water barriers. Water barrier is simply a property classification.

The second problem arises when a brick wall is built in accordance with (b) as listed above. If it is single skin there are no requirements for water control or vapour control at all (one might wonder why it’s even mentioned). This is also true when there is a drained cavity separating the ‘primary water control layer’ and internal ‘water sensitive materials’ – e.g. a brick veneer wall and a timber stud frame

But, just because the NCC doesn’t need a pliable building membrane, it doesn’t mean that it’s not a good idea to install a sarking-type material, the definition of which has been in place for decades:

Sarking-type material means a material such as a reflective insulation or other flexible membrane of a type normally used for a purpose such as waterproofing, vapour management or thermal reflectance.

Sarking-type materials are recommended for all framed housing. Reflective membranes such as Ametalin SilverSark® , our advanced foil laminate for air and water control, will contribute to the system R-value of the wall when installed facing an air cavity. Ametalin Brane® VHP is our Class 4 vapour permeable wrap and will provide a barrier to liquid water while allowing the passage of water vapour, helping to manage the risk of mould and moisture damage.

The take-away message from the new NCC provision: if such products are installed in lightweight construction in accordance with (a), they must be water barrier membranes. Almost everywhere else, micro-perforated reflective membranes are suitable to be used in most Australian climate zones to manage condensation.