Thermal bridging in commercial buildings

The NCC Performance Requirements can be met using either a Performance Solution, Deemed-to-Satisfy (DTS) Solution, or a combination of both. The following is a general representation of the DTS Provisions for thermal bridging.

The thermal bridging requirements are contained in the energy efficiency DTS Provisions in NCC Volumes One and Two. This is an introduction to thermal bridging and how the NCC deals with it for commercial buildings in NCC Volume One (Clause J4D3(5)). 

This document explains how to account for thermal bridges when calculating the Total R-Value of certain construction types. It provides a national perspective of the NCC and does not contain any state or territory variations.

Thermal bridging, in practical terms for the NCC, is an unintended path of heat flow between the outside and inside of the building envelope. 

Thermal bridges may occur where there is an interruption in the insulation or where highly conductive materials (e.g. metal) are used.

Thermal bridges can significantly reduce the effectiveness of the insulation (thermal resistance) of the façade by essentially bypassing the insulation in favour of a more conductive material (e.g. metal). This results in either losing heat from inside the building to the outside on a cold day, or adding warmth to the inside the building on a hot day. This may cause unwanted comfort issues in a building, and a likely increase in energy use by a building’s heating and cooling systems.

Additionally, unaddressed thermal bridges may lead to condensation where warm, moist air contacts a colder surface and condenses into water droplets. Condensation can result in mould growth, causing indoor air quality issues, negative health impacts for occupants, and potentially affects the durability of the structure.

The NCC requires thermal bridging to be considered when calculating the Total R-Value/Total System 
U-Value in the following construction types: 

• steel and timber frames in the building envelope
• windows
• spandrel panels.

While the NCC does not prescribe methods for the following construction types, thermal bridging may also occur in:

• junctions between the floor, wall and roof
• penetrations in the building envelope for pipes and cables
• brackets or connection points for external shades or balconies
• slab projections
• steel wall ties used in masonry construction.

Total R-Value and Total System U-Value describe thermal resistance/transmittance (i.e. the ability of heat to transfer through a system or material). These values relate to one another, with the R-Value being the inverse of the U-Value.

Typically, R-Value is used to refer to a material’s ability to prevent heat flow from a cold environment to a warm environment.

Whereas U-Value is used to describe a material's ability to transfer heat from a warm environment to a cold environment.

When using the NCC, R-Value (m².K/W) means the thermal resistance of a component, such as a layer of insulation. It is calculated by dividing its thickness by its thermal conductivity. The Total R-Value (m².K/W) means the sum of the R-Values of the individual component layers in a composite element, such as an external wall. It includes any building material, insulating material, airspace, thermal bridging and associated surface resistances.

When using the NCC, Total System U-Value (W/m².K) means the thermal transmittance of the composite element allowing for the effect of any airspaces, thermal bridging and associated surface resistances. 

To understand what a 'good' or 'bad' value is, the higher the Total R-Value, the better insulator it is, whilst the opposite is true for the Total System U-Value.

While adding more insulation can help to account for thermal bridges, if they are to be truly fixed a thermal break or continuous insulation layer is needed.

A thermal break is an element with low thermal transmittance placed strategically to interrupt the heat flow path through elements with high thermal transmittance. 

Figure 1 provides an example of a thermal break in a spandrel panel. The thermal break is created by using a non-metal structural sealant, weather sealant, gasket and trim, as these have low thermal transmittance. The thermal break interrupts the connection between the inside and outside air through the metal mullion (with high thermal transmittance).

Total R-Values can be calculated with allowances for thermal bridging in accordance with AS/NZS 4859.2:2018 Thermal insulation materials for buildings – Design. 

This Standard comprises of a calculation method (NZS 4214 Methods of determining the total thermal resistance of parts of buildings) that accounts for the impact of thermal bridges on thermal performance.

Examples showing how to calculate Total R-Values with thermal bridges are available separately from the ABCB website.