To pass Building Control requirements in England and Wales, it is necessary to demonstrate compliance with Building Regulations. The latest version of the Building Regulations Part L (2013) and associated guidance document for residential construction Approved Document L1A (ADL1A) require that thermal bridging be included in the fabric heat loss calculations.
The Government Standard Assessment Procedure (SAP 2012) is the simple energy use and carbon emissions model used to provide evidence that the carbon emissions target has been achieved. The SAP calculation includes the term HTB (heat loss due to thermal bridging) which is calculated or estimated as below:
a) The sum of all linear thermal transmittances (Ψ) x length of detail (L)
or, if no linear thermal transmittances are known:
b) Using the factor y = 0.15 in the equation below:
(where Aexp = total exposed fabric area)
Linear thermal transmittance (Ψ) values used in (a) can be a combination of:
- Approved Design Details if used (in 'Approved' column of SAP Appendix K Table K1)
- Uncalculated details (in 'Default' column of Appendix K Table K1)
- Modelled details, in which numerical modelling has been carried out by a person of suitable experience and expertise
Method (a) is always preferable as it avoids the penalty imposed by (b) which can double the overall calculated heat loss in a wellinsulated construction. A similar approach is taken for non-residential buildings in Part L2A, in which the Simplified Building Energy Model (SBEM) is used in place of SAP.
One off calculations of Ψ can be carried out on request for all details using Schoeck thermal breaks to obtain the optimal solution.
Condensation Control and temperature factor
Building Regulations Part L includes the requirement that minimum internal surface temperatures be such that condensation risk is minimized and mould growth avoided. A measure of condensation risk is the temperature factor fRsi:
Approved Document L1A (L2A for non-residential buildings) cites the BRE Information Paper IP1/06 (Assessing the effects of thermal bridging at junctions and around openings) which includes some limiting values for fRsi:
Details using Schoeck thermal breaks show temperature factors far in excess of Part L requirements in all cases. Temperature factors can be calculated by Schoeck for bespoke details on request to verify code compliance.