What is Linear Thermal Bridging?

Thermabate on site

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 take account of the effect of repeating thermal bridges e.g. a 15% timber bridging fraction might be taken for studs in a timber framed wall. Our literature states where we have taken these into account.

  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, or where external walls are bridged by lintels, jambs or sills where window or door openings are installed.

  1. 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).

Minimising or preventing thermal bridging should be considered when designing 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.  Details of which can be found at www.kingspaninsulation.co.uk/kkcc.

 

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 lost from a building through the bridge, which therefore increases the building’s heating demand and therefore its heating costs and associated carbon emissions. A secondary effect of poor detailing can be a colder 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.

The Accredited Construction Details (ACDs) include process sequences providing guidance on how to achieve good thermal continuity and air tightness; however the ACDs are fairly generic and not always representative of modern constructions, so in some cases the performance of these junctions can be improved upon through better continuity of insulation and use of higher performance materials.

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 with achieving Building Regulations compliance.

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

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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