What is Linear Thermal Bridging?

25 April 2017 Kingspan Insulation Middle East
Understanding thermal bridge

Thermal bridges can occur wherever a poor conductivity material bridges the insulation layer of a construction, resulting in additional heat lost through that bridge.

There are three types of thermal bridging that can be considered for buildings:

1. Repeating thermal bridges

U-value calculations for planar elements should take account of the effect of repeating thermal bridges e.g. a 15% steel bridging fraction might be taken for studs in a steel framed wall. Our literature states where we have taken these into account.

2. Linear (non-repeating) thermal bridges

Linear thermal bridging describes the heat gains that occur at junctions between elements. This can include for example, junctions at corners, where external walls join with the floor, or where external walls are bridged by lintels, jambs or sills where window or door openings are installed.

3. Point thermal bridges

A third type of thermal bridge, point thermal bridging may also be calculated and is occasionally used as an adjustment to a U-value for an element, this might be used as an adjustment to a planar U-value to take account of fixings or fasteners, or possibly as an adjustment for isolated steel beams or columns.

psi-Values

Linear thermal bridging, represents the extra heat flow occurring at building junctions, which is over and above that through the adjoining planar elements. Linear thermal transmittance is measured in W/m.K, referred to as a ‘psi-value’ and expressed as a ‘ψ–value’. The lower the ψ–value, the better the performance of a junction detail.

ψ–values are not taken into account in U–value calculations, but, instead, they are taken into account separately in the calculation methodologies used to assess the operational CO2 emissions of buildings e.g. the Standard Assessment Procedure (SAP) or Simplified Building Energy Modelling (SBEM).

Should You Be Concerned about Thermal Bridges?

Obviously, it is best to reduce the impact of thermal bridges wherever possible when insulating a building. In order to do this, you need to work out where they will appear and the impact of them. By using good construction techniques, especially when insulating, the effects of thermal bridging can be reduced. The key factors to consider are insulation continuity and airtightness.

In simple terms, poor detailing increases the heat gained from a building through the bridge, which therefore increases the building’s cooling demand and therefore, its cooling costs and associated carbon emissions. A secondary effect of poor detailing can be a warmer internal temperature around and along the bridge, which can mean an increased risk of surface condensation or mould growth.

Insulation continuity involves making sure that there is a continuous layer of insulation at junctions. For example, on concrete floors we recommend using an insulation upstand. Good air tightness means making sure that the unintentional air leakage from the building is kept to a minimum and correctly sealing joints between construction elements is one method of helping to achieve this.

Good details should include process sequences alongside calculated psi values, providing guidance on how to build a junction, whilst achieving good thermal continuity and minimising air leakage. Good detailing can assist significantly in achieving Building Regulations compliance.