A thermal bridge, also called cold bridge or heat bridge is an area of an object (usually within a building) which has a significantly higher heat transfer than the surrounding materials, resulting in an overall reduction of its thermal performance.
As a result, there is a reduced surface temperature, increased risk of condensation, mould growth and significant heat loss through the thermal bridge.
There are two types of thermal bridges:
To avoid excessive heat losses and local condensation problems, reasonable care should be taken to ensure continuity of insulation and to limit local thermal bridging, e.g. around windows, doors and other wall openings, at junctions between elements and other locations. Any thermal bridge should not pose a risk of surface or interstitial condensation.
A bespoke thermal bridge calculation reviews if a particular junction or building element has excessive heat loss or a risk of surface and interstitial condensation.
A bespoke thermal bridge calculation can also be part of a SAP or SBEM assessment, providing the overall Y value, which measures the heat loss from all the junctions around a building.
For example, heat loss through thermal bridging is taken account of in the Dwelling Energy Assessment Procedure (DEAP) or Standard Assessment Procedure (SAP) calculation by means of two alternative methods of accounting for heat loss:
To perform a Thermal Bridge Analysis, the following is required:
The main reference standars are:
If the SAP or SBEM calculations fail compliance (e.g CO2 emissions or fabric efficiency), then bespoke thermal bridge calculations may be required. This could be because the Y default value of 0.15 W/m2 K is 3 times worse than the value used by the reference dwelling which is 0.05 W/m2 K.
It is also quite common that the proposed construction does not match accredited construction details, therefore the value of 0.08 W/m2 K may not be applicable to the assessment. Bespoke calculations can provide a job specific Y value by assessing the longest and critical non- repeating thermal bridges. If a bespoke thermal bridge calculation is used to assess the risk of interstitial condensation the FRsi Temperature factor, should be lower than the value stated in IP/06 Assessing the effects of thermal bridging at junctions and around openings.
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