Linear thermal bridging
The building fabric should be constructed so that 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.
Reasonable provision would be to:
a. Adopt approved design details as set out in DCLG Approved Construction Details or those that are formally recognised by DCLG. The calculated linear thermal transmittance values can be used directly in the DER and DFEE rate calculations; or
b. Use construction joint details that have been calculated by a person with suitable expertise and experience following the guidance set out in the BRE Report BR 497 Conventions for calculating linear thermal transmittance and temperature factors. The linear thermal transmittance values can be used directly in the DER and DFEE rate calculations. Reasonable provision for the temperature factors is that they should achieve a performance no worse than that set out in BRE Information Paper IP1/06 Assessing the effects of thermal bridging at junctions and around openings in the external elements of buildings; or
Evidence of suitable expertise and experience for calculating linear thermal transmittance would be to demonstrate that the person has been trained in the software used to carry out the calculation, has applied that model to the example calculations set out in BR 497 and has achieved results that are within the stated tolerances.
c. Use linear thermal transmittance values in the 'default' column of Table K1 in SAP 2012 directly in the DER and DFEE rate calculations.
d. Use a conservative default y-value of 0.15 W/m2K, rather than linear transmittance values for each construction joint, in the DER and DFEE rate calculation.