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Condensation management changes in NCC 2022

With houses and apartments becoming more air-tight, condensation problems have increased for some buildings. NCC 2022 contains additional requirements to mitigate potential condensation issues for houses and apartments.

As buildings become more air-tight to meet energy efficiency requirements, condensation problems have started to appear within some new homes and apartments. 

This is particularly evident in the cooler regions of Australia as well as some parts of the subtropics. In response to this, the Australian Building Codes Board (ABCB) have introduced additional condensation provisions into NCC 2022.

Which building will be affected?

Class 1 buildings including townhouses, detached and semi-detached dwellings and sole occupancy units within apartments (Class 2) and residential units (Class 4) attached to commercial buildings.  

Changes apply to building elements as follows.

  • Increase in vapour permeance to wall sarking within climate zones 4 to 8. 
  • More stringent roof ventilation within climate zones 6, 7 and 8. 
  • Additional exhausting requirements for every climate zone.

Changes do not apply in the following circumstances.

  • Common areas within a Class 2 building.
  • Buildings that are used for commercial, industrial, and minor domestic purposes, best summarised as Class 3 and Class 5 to 10 buildings. 

Wall construction

Where non breathable wall wraps or rigid insulation is used in external walls particularly in colder climates, moisture can become trapped between the internal lining and the external cladding. This can create an environment for mould and/or fungal decay build up. 

This risk can be mitigated by using a vapour permeable pliable building wall membrane for the wall wrap. The Deemed-to-Satisfy (DTS) Provisions listed in NCC 2022 require membranes where installed to have a vapour permeance to the following classifications: 

  • Class 3 (as a minimum) for buildings located in climate zones 4 and 5.
  • Class 4 for buildings located in climates zones 6, 7 and 8.

A membrane that has a higher classification provides better vapour transmission, allowing moisture within the air to pass through the material. Builders and contractors will need to ensure membranes are positioned on the exterior side of the Primary Insulation layer (PIL) when used in external (perimeter) walls. This is commonly the outside of the timber or steel frame that has insulation batts or similar in between the studs.

Where a pliable building membrane (wrap) is not installed, the NCC requires that a drained cavity must be provided between the internal lining and the external cladding.
Single skin masonry, blockwork, tilt-up concrete and the like (where there is no frame and the wall is a single skin type arrangement) is exempt from this requirement. 

Roof construction

Roof spaces that do not have adequate ventilation can accumulate moisture, problems occur when the PIL is in direct contact with either the roof cladding or frame. Condensation can form on the underside of the roof when the inside temperature is warmer than the outside, houses are particularly vulnerable on cold mornings. 

The DTS Provisions listed in NCC 2022 requires buildings located in climate zones 6, 7 and 8 have a minimum 20 mm air gap above the PIL so that roofing elements remain clear of the insulation. This gap must be vented to the outside air via an unsarked tiled roof or through roof vents or similar, like those found on eaves. 

Where sarking is positioned immediately above the PIL, the membrane should be rated as Class 4 and retain a minimum 20 mm clear space above it. This gap is also required above any continuous insulation placed above the PIL or insulation that is used within thermal bridging for metal framed roofs.

Separation of the PIL from the roof cladding and battens is not mandatory when a building is in a bushfire zone nominated as BAL-FZ. This concession also extends to roofs made of structural insulated panels and/or concrete.

Exhaust systems

Some rooms within a house are more prone to condensation including kitchens, bathrooms, sanitary compartments, and laundries. Although these rooms may be serviced by natural ventilation, additional DTS requirements have been included in NCC 2022 to help avoid internal moisture build up.

All exhaust systems installed in kitchens, bathrooms, sanitary compartments, and laundries will be required to discharge air directly out of the building at a designated flow rate and may require continuous operation according to the room its serving.

Importantly, exhaust systems must discharge directly to the outside; they cannot discharge into the roof space whether it is vented or not under the DTS Provisions. 

Where internal kitchens, sanitary compartments, laundries and wet areas are positioned away from areas adjoining external wall, ducts will be needed to remove the exhaust air from the building. 

This is most problematic where affected rooms are located on the ground floors or intermediate floors within multistorey Class 2 buildings. In such cases, a false ceiling below the upper level floor will likely be needed to enable ducting to be installed to aid air removal, especially for bounding construction. 

Where the length of the ductwork is excessive, in-line fans may be required to assist with air exhausting. 

Where exhaust systems are installed in bathrooms and sanitary compartments without access to natural ventilation (i.e. an openable window or fixed ventilation openings), they must be interconnected to the room’s light switch and have a run-on timer connected to the exhaust system that keeps it running for a minimum period of 10 minutes once the light switch is switched off. 

Rooms without natural ventilation must be provided with make up air is either through a mechanical ventilation system or an opening with an area of 14,000mm² (i.e via a door undercut) at the base of the door of approximately 20 mm or a fixed vent in the door or wall. 

Venting clothes dryers must discharge directly to the outside air or into a shaft or duct to the outside air. This is not required for condensing type clothes dryers. 

Vented clothes dryers expel hot air through an external vent and if not ducted expel the hot air into the room or space causing condensation and mould build up.

A further change is that recirculating rangehoods, commonly used above kitchen benchtop stoves, are no longer permitted under the Deemed-to-Satisfy Provisions unless that rangehood is able to be ducted to outside air and meet prescribed flow rates. 

When do these changes apply?

Although the NCC 2022 becomes live on 1 May 2023, additional condensation provisions won’t become law until 1 October 2023. This date may be different in some states in particular in Western Australia where there will be an extended transition period for NCC 2022 and apply slightly differently for houses and apartments.

The ABCB is publishing an updated “Condensation in Buildings” handbook in the coming months. This provides further support to practitioners to use the new condensation management provisions in NCC 2022.

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

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