In 2019, the NCC introduced management clauses to help combat problems associated with condensation in buildings for the first time. In NCC 2022, more measures were included. In this blog, we take a look at the latest condensation management considerations for ‘sarking-type materials’ in external walls.

The issue with condensation

If you think back to science class, you will remember that water naturally exists in three states: gas, liquid and solid.

Indoor and outdoor air always holds a degree of vapour and where the air goes, the water vapour goes. Condensation refers to the transformation of water from invisible vapour to its liquid form when it contacts a cold surface.

In a building, condensation can lead to moisture being trapped in the building envelope where it can lead to all kinds of problems, including:

Structural damage due to rotting materials
Poor performing insulation due to absorbed moisture
Mould that can lead to health problems
Microbial hazards

One reason the NCC is taking a firmer stance on condensation issues is that its increased energy efficiency requirements are leading to buildings becoming more airtight. Without effective strategies, condensation and its associated problems will increase.

Vapour-permeable membranes

Vapour permeance or permeability in the NCC refers to the property of a material to allow water vapour to diffuse through it without the material being chemically or physically affected.

The use of vapour-permeable membranes that allow water vapour to escape is an important part of condensation management in built structures. Vapour-permeable membranes help reduce the degree of condensation and assist with the drying of the structures when it occurs.

In cooler climates, a vapour permeable membrane lowers the risk of condensation by allowing water vapour generated on the inside of the building to escape to the outside. In hot, humid climates, a vapour barrier/retarder lowers the risk of condensation by preventing external water vapour from entering the building.

The permeance of vapour control membranes is tested in a controlled environment, and based on the results then classified as Class 1, 2, 3 or 4. A membrane classified as Class 4 will be more vapour permeable than Class 3. Classes 1 and 2 are classified as vapour barriers and are not vapour permeable.

Please note that these classes are not to be confused with the building classes referred to in the NCC.

In 2019, it became mandatory to install a vapour-permeable membrane wrap on the outside of insulation in certain climate zones. In 2023, the NCC added specific requirements for these membranes.

Condensation management and the NCC

NCC 2019

When condensation management clauses were included in the NCC in 2019, a clause relating to the sarking used on the outside of the primary insulation layer was included.

Volume Two of NCC 2019 introduced a new section titled ‘Condensation Management’, including the clause 3.8.7.2 Pliable building membrane. This clause was broken down into two parts, (a) and (b).

In terms of Clause 3.8.7.2 Part (a), compliance could be achieved by installing a membrane that:

classified as a water barrier in accordance with AS/NZS4200.1;
was installed in accordance with AS4200.2 in all climate zones; and
was vapour permeable in climate zones 6, 7 and 8.

Clause 3.8.7.2 (b) stated that where a water barrier membrane was not installed, a drained cavity had to be present.

Note: The climate zones referred to above, are the 8 Australian climate zones for thermal design determined by the NCC. The climate zones group together areas with similar climates, and range from Climate zone 1 (high humidity summer, warm winter) to Climate zone 8 (alpine).

NCC 2022

In 2022, the NCC took this one step further by introducing further condensation management changes, in particular for Class 1 buildings (including townhouses, detached and semi-detached dwellings), Class 2 buildings (apartment buildings) and residential units attached to commercial buildings (Class 4).

In terms of the Deemed to Satisfy (DTS) requirements for vapour permeance in sarking-type materials in external walls in Volumes 1 & 2 of the NCC 2022, the following now applies:

Building Classes 1, 2 & 4 in Climate Zone 4 & 5 must include a vapour permeable membrane of Class 3 or Class 4.
Building Classes 1, 2 & 4 in Climate Zone 6, 7 & 8 must include a vapour permeable membrane of Class 4.

Note: It is recommended that sarking-type materials are also classified as a water barrier in accordance with AS/NZS 4200.1 to provide weather protection. A water barrier provides protection against water in its liquid form, e.g., wind driven rain and water ingress.

Siniat Weather Defence

Siniat Weather Defence™

Siniat Weather Defence is a rigid air barrier that is used behind facade cladding systems to create a pressure equalised cavity.

It is classified Class 4 vapour permeable and also classified as a Water Barrier in accordance with the equivalent water barrier classifications within AS/NZS 4200.1 ‘Pliable building membranes and underlays, Part 1: Materials and AS 4201.4 ‘Resistance to water penetration’.

Image: Weather Defence is used behind facade cladding systems; Brookvale Public School, Sydney

Architecture & Design Team

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