NatHERS 2022 Thermal Bridging - Summary

The Australian Building Codes Board (ABCB) is proposing changes to the requirements for thermal bridging in the 2022 National Construction Code (NCC). Thermal bridging is the movement of heat across an object that is more conductive than the materials around it. For example, the frame behind a gyprock interior wall could form a thermal bridge for heat to move through the wall cavity. Both timber and steel framing can be a thermal bridge. However, as steel is more conductive than timber, the effect is more pronounced with steel frames.

The proposed changes for NCC 2022 prescribe that metal framed construction should match the thermal performance of timber framed construction in energy rating calculations. This can be done by building in additional energy efficiency measures to offset the additional thermal bridging effect that steel has above that of timber framed construction. The proposed changes will apply to all NCC compliance pathways.

Currently, apart from window framing, the NatHERS software tools do not calculate the effect of thermal bridging that would need to be improved to align with this change.

CSIRO study  

In preparation for the proposed thermal bridging changes to the NCC, the NatHERS Administrator commissioned CSIRO to:

  • Develop a set of thermal bridging default parameters,
  • Provide draft modelling guidance on how to apply them, and
  • Determine the impact of applying defaults on residential building energy ratings.

The report details the specific research undertaken.

The largest star rating impacts were observed on a 4 bedroom, single-storey house with steel-framed floors, walls and ceilings. It had R 0.2 thermal breaks installed between the frame and the exterior metal cladding, as per the requirements of the NCC. The following table summarises the star rating changes observed for this house in capital cities across the country.

Capital city

Status quo star rating

New star rating

Star rating decrease

Adelaide

6.4

5.9

0.5

Brisbane

6.1

5.8

0.3

Canberra

6.7

6.3

0.4

Darwin

7.4

7.2

0.2

Hobart

7.0

6.6

0.4

Melbourne

6.6

6.2

0.4

Perth

6.4

6.0

0.4

Sydney

6.4

5.9

0.5

NatHERS thermal bridging function

Should the proposed NCC 2022 thermal bridging requirement be adopted, NatHERS will be modified to include a thermal bridging function with the following features, based on the CSIRO report:

  • Thermal bridging will not apply to timber framed construction so ratings of timber framed dwellings will be unaffected.
  • Thermal bridging calculations will only apply to repeating frame elements (e.g. joists and studs) for floors, walls and ceilings. Default values will be used when the precise framing specifications are not provided at the time of rating.
  • In steel framed construction, only the thermal bridging impact that is over and above the impact that timber would have, will be shown.
  • Thermal bridging impacts will be based on the calculation method outlined in standard NZS 4214:2006 Methods of determining the total thermal resistance of parts of buildings.

To learn more about the proposed changes, please see frequently asked questions.

For further information please refer to the Thermal Bridging Report.