Understanding the control layer’s role in condensation management

Sarking and other pliable building membranes can be installed as part of an external wall or roof for increasing thermal performance, weatherproofing or providing ember protection in bushfire prone areas. As these control layers can influence condensation management, it is important that the right control layer is used to help manage the risks.

What is a ‘control layer’? 

The NCC defines a control layer as any continuous layer that is installed for one or more purposes of air, water, vapour or thermal control. This includes pliable building membrane and sarking-type material but excludes waterproofing membranes complying with AS/NZS 4858. 

This definition is broad and essentially captures any sarking, reflective insulation or flexible material that forms part of an external wall or roof. It can also capture rigid insulation materials. 

Control layers are adopted as a strategy for compliance with the Housing Provisions to achieve waterproofing, energy efficiency or as a means of restricting ember entry in bushfire prone areas. They can also be specified voluntarily. 

Control layers - vapour permeance requirements 

Control layers located between the external cladding and the primary insulation layer have the potential to restrict vapour transfer between the internal and external environment. 

To prevent control layers from increasing the condensation risk in a building where installed, a minimum level of vapour permeance must be achieved in walls and roofs. 

This permeance obligation is climate zone dependent. In tropical climates (e.g. zones 1 and 2) materials with lower permeance are needed to limit outdoor moisture being driven inward. Higher permeance is required in colder climates (e.g. zones 6, 7 and 8). View an interactive climate zone map.

Walls

The minimum level of vapour permeance required in walls is based on the type of external wall construction and climate zone.

Table 1: NCC Table 10.8.1 (explanatory): Vapour permeance classes

A pliable building membrane or a sarking-type material installed as a control layer in an external wall must also:

• comply with AS 4200.1, which sets out the material requirements; and 
• be installed in accordance with AS 4200.2. 

Control layers within a roof

Control layers are generally installed in a roof below the roof covering as a sarking or part of a roofing blanket. In most cases, the ventilated roof space is underneath the control layer in a pitched roof. Therefore, vapour permeable materials are not prescribed. 

However, in climate zones 6, 7, and 8 where the control layer is installed immediately above the ceiling insulation in a cathedral or raked type ceiling parallel to the roof plane, it must be a Class 4 membrane to enable vapour permeability. 

Vapour permeability in accordance with AS 4200.1 is generally expressed as a class corresponding to the rate of water vapour that penetrates a membrane under a standard test. The result may be described by manufacturers in terms of micrograms per newton second (μg/N.s) (Figure 3) or its corresponding class (Figure 4). AS 4200.1 provides the bounds of different permeance classes.

Note: control layers may also be subject to other NCC requirements such as flammability limits, so it is important to understand all the requirements that apply for a given application.

Members should ensure that regardless of the reason a control layer is installed, the relevant permeance is met.

When do the changes take effect?

The timeline for adoption of NCC 2025 is determined by the states and territories. Visit our NCC 2025 hub for the latest updates as they become available. 

Need help understanding these changes?

The HIA Guide to Condensation Management is currently being updated and will be available in the coming months to provide further support to practitioners using the new condensation management provisions in NCC 2025.