What is linear thermal bridging?

8 August 2018 Kingspan Insulation UK
Linear Thermal Bridging

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
Repeating thermal bridges, as the name suggests, repeat throughout the building envelope and tend to have a regular pattern. U-value calculations for planar elements take account of the effect of repeating thermal bridges, so for example a 15% timber bridging fraction might be taken for studs in a timber framed wall.

 
  1. Linear (non-repeating) thermal bridges
Linear thermal bridging describes the heat losses that occur at junctions between elements. This can include, for example, junctions at corners, where external walls join with the floor, jambs or sills where window or door openings are installed.
 
  1. Point thermal bridges

Point thermal bridging may 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 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 bridging? 

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.

Insulation continuity involves making sure that there is a continuous layer of insulation at junctions. As an example for sills, jambs and lintels, an insulated cavity closer e.g. Kingspan Kooltherm Cavity Closer or Kingspan Kooltherm Cavity Closer PLUS can be used to help reduce the heat-losses that can occur around windows and doors.  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.

We include various installation guidance in our product literature which can help minimise heat losses through junctions, take a look at our website for more information.