NatHERS 2022 Thermal Bridging

The Australian Building Codes Board (ABCB) has brought in changes to the requirements for thermal bridging in the 2022 National Construction Code (NCC). Thermal bridging is the movement of heat across an object with a higher thermal conductivity than the materials around it. For example, an insulated steel wall frame, directly connected to the exterior cladding could form a thermal bridge, allowing the heat to travel through the steel frame more readily than the surrounding insulation. Both timber and steel framing can act as a thermal bridge, however the effect is more pronounced with steel because it is has a higher thermal conductivity.

The proposed changes for NCC2022, for all NCC compliance pathways, prescribes that the thermal performance of thermally bridged steel framed constructions aligns with the thermal performance of thermally bridged timber framed constructions. In reality, the thermal performance of these steel constructions would be worse than their timber counterparts. In NatHERS, to offset the negative effects of thermal bridging on the thermal performance, any other element of the homes’ design can be improved.

CSIRO study  

In preparation for the 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 Thermal Bridging 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

NatHERS has been modified to include a thermal bridging function, based on CSIRO's Thermal Bridging Report with the following features:

  • Thermal bridging will not apply to timber framed constructions, therefore ratings of timber framed dwellings will be unaffected.
  • Thermal bridging calculations will only apply to repeating steel 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.
  • The performance of thermally bridged steel framed constructions will automatically align with the performance of thermally bridged timber framed constructions i.e. the thermal performance of these steel frames constructions will not reflect their performance in reality, but will align with the performance of its timber counterpart.
  • 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 changes, please see frequently asked questions.