For new dwellings, thermal bridging is dealt with in Approved Document Part L1A 2006. Part L1A 2006 states that:
"The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements, and at the edges of elements such as those around window and door openings.” (ODPM, 2006, p.22)
Guidance on minimising thermal bridging is contained within Accredited Construction Details. Accredited Construction Details contains a range of domestic details that are designed to have ψ values that do not exceed the values given in BRE IP 1/06 (Ward, 2006). The default ψ values for a selection of various different junctions are detailed in the table below.
Default ψ values for various different junctions [Adapted from Ward, 2006]
Accredited Construction Details are available as pdf files on the internet and can be accessed from the following website:
In addition to ensuring that there are no significant thermal bridges within the dwelling, the heat losses attributable to thermal bridges must be included in the calculation of the Dwelling Emissions Rate (DER). The DER is calculated using the Government’s Standard Assessment Procedure for the Energy Rating of Dwellings (SAP).
The methodological approach adopted by SAP uses internal envelope dimensions to calculate heat losses. The effects of repeating thermal bridges are taken into account when calculating the U-values of the various elements within the dwelling that contain the thermal bridge, such as walls, roofs, floors, etc. These calculations are undertaken in accordance with BS EN ISO 6946: 2007 (British Standards Institution, 2007). The additional heat losses attributable to non-repeating and geometrical thermal bridges are calculated separately and then added to the elemental heat losses.
Appendix K of SAP lists four options for determining the heat losses attributable to non-repeating and geometrical thermal bridges (BRE, 2008). These are as follows:
1. If the ψ values have not been calculated explicitly, but all of the dwelling details are in accordance with or are equivalent to Accredited Construction Details, then an approximation factor of y = 0.08 can be inserted into the following equation:
where is the total exposed surface area of the dwelling in m2
2. If the ψ values have not been calculated explicitly, and all of the dwelling details are not in accordance with or equivalent to Accredited Construction Details, then an approximation factor of y = 0.15 can be inserted into the following equation:
3. If the ψ values have been calculated explicitly for each junction type, then the individual values can be inserted into the following equation to determine the heat loss for each junction:
where L is the length of the thermal bridge over which applies.
4. If the ψ values have been calculated explicitly and used to calculate a y value for the dwelling, then this value can be inserted into the following equation:
By using the approximation factor y , it is assumed that the thermal bridging is spread uniformly over the whole of thermal envelope of the building. In reality, this is unlikely to be the case, and the thermal bridges are likely to occur at specific points in the thermal envelope.