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Tech Article: The Impact of Thermal Bridging on Building Elements

11 May 2020

Under the new NCC2019 Section J requirements, the introduction of thermal bridging allowance can result in a significant reduction in calculated building fabric performance. The following article explores this change and its impact on how Section J insulation can achieve compliance.

All Section J reports assessed under NCC2019 must contain thermal bridging calculations to demonstrate Section J Energy Efficiency compliance. 

Overview

Previously, the method to determine the Total R-value of a building element was to sum its individual component R-values. In other words, simply adding the R-value of each component together to get the Total R-value (Rt).

To comply with the new NCC2019 methodology, heat losses through alternative pathways must be accounted for when calculating building fabric performance. Therefore, the calculated Total R-value may be rated at far lower compared to NCC2016 for the same building element.

The Old Methodology

Using the previous NCC2016 method, the example below demonstrates a typical lightweight wall with FC cladding has a calculated Rt of 2.53.  This methodology incorrectly assumes the bulk insulation provided 100% coverage within the internal airspace and neglects heat transfer through the metal frame (thermal bridging).

NCC2016 Wall total R-value calculation:

The New Methodology

Using the same example, however, calculate using the NCC2019 methodology, the new Rt of 1.03 is determined based on the area proportion of the thermal bridged components below:

  • Glasswool – 87.6%
  • Steel frame – 12.6%

NCC2019 Wall total R-value calculation (Base case):       

It can be seen by switching from NCC2016 to NCC2019 provision, the same steel-stud wall system experiences a performance reduction of virtually 60% without a thermal break.

This means the conventional assumption of ‘if the insulation R-value exceeds the minimum required Total R-value then the wall system is compliant’ is no longer valid and can lead to NCC non-compliance.

For projects which missed the 1st of May transition period deadline and were designed under NCC2016 will now require an update to comply with NCC2019 provisions. The potential impact of the thermal bridging may require increases in wall thickness to allow for additional insulation.

Alternatively, a simpler approach is to directly minimise the thermal bridge pathway without altering the existing wall thickness. Options include:

  1. Provide a thermal break to the metal stud
  2. Replace metal stud with less conductive type, such as timber.

Option 1 – Thermal Break

The addition of a thermal break layer can be a cost-effective approach to achieve NCC compliance. Using the same example, an addition of a 6mm R0.15 thermal break between FC cladding and steel stud will achieve an Rt of 1.46, which meets the minimum Rt1.40 as required for most climate zones under NCC2019 (Refer to table J1.5a, Part J1).

NCC2019 Wall total R-value calculation (Base case + Thermal break):    

Option 2 – Timber Frame

Timber is less conductive than steel and therefore more thermally effective. Using the same example, except for a timber frame in place of metal, the wall system achieves an Rt of 2.17 without an additional thermal break layer.

NCC2019 Wall total R-value calculation (Base case with Timber stud wall):      

The above examples have shown that a metal frame wall can significantly reduce the calculated performance of the wall element under NCC2019. Therefore, early-phase design decisions and Section J assessment are crucial to ensure insulation and façade compliance. Similarly, the impacts of thermal bridging apply to other building fabric elements, including floors and roofs.

The additional thermal bridging allowance may seem daunting. However, a compliant design will mean reduced heat loss or gain and hence lower building energy consumption through heating and cooling.

Please feel free to get in touch with us if you would like to know more about thermal bridging or other Section J related matters. info@bcalogic.com.au