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Thermal bridging

With walls and floors becoming much more highly insulated, the proportion of heat lost through thermal bridging, if not addressed, is significantly increased, reaching around 20-30% in a typical new-build home. Consequently, it is now especially important for designers and builders to reduce the impact of thermal bridging on the overall fabric performance.

To make this task easier, the masonry sector provides an extensive range of high-performance thermal bridging details covering all the key junctions. These are freely available and have recently been updated so that they are fully compatible with the lower U-value walls required by the 2021 edition of Part L and the anticipated 2025 Future Homes Standard.

The revised details are now referred to as 'Recognised Construction Details' and cover all the usual methods of construction and materials involved. Use of these details goes a long way to helping optimise fabric performance and is highly recommended. 

As further recognised construction details are received, information will be published here.

Thermal Bridging Details

Thermal bridging details are available from:






14 junctions: walls/walls (corners, party), walls/floors (ground, separating, non-separating) wall/roof (eave, gable), sill, jamb, lintel.

6 inner leaf block types: ( 3 aircrete, 3 aggregate blocks)

3 wall types: cavity full fill insulation, cavity partial fill, solid wall external insulation

Range of Insulations


Many of these also include fire detailing info. and notes on compliance with acoustic robust details


Concrete Block

14 junctions: walls/walls (corners, party), walls/floors (ground, separating, non-separating) wall/roof (eave, gable), sill, jamb, lintel.

3 inner leaf block types: (3 aggregate blocks)

2 wall types: cavity full fill insulation, cavity partial fill

Range of Insulations


Data is split by different wall types. Then for each wall type each junction has a detail/certificate for the three generic aggregate block types and all insulation types and thicknesses. Data is a subset of source 1. Onsite checklist provided


Constructive Details

14 junctions: walls/walls (corners, party), walls/floors (ground, separating, non-separating) wall/roof (eave, gable), sill, jamb, lintel.

3 inner leaf block types: (3 aircrete)

3 wall types: cavity full fill insulation, cavity partial fill, solid wall

Range of Insulations

Data presented in downloadable pdf books and arranged as source 2. Details include fire stop details. Details provided can accommodate RDL acoustic requirements.  Books include guidance checklist for construction


Sources 2 and 3 are arranged in a manner that enables simple comparisons as parameters are changed.  LABC details (source 1) are particularly aimed at the SME market, use products that should be readily available from the Builders Merchants and cavity thicknesses that are commonly used. 

For aircrete inner leaf, the psi values in source 1 are generally better than those given by Constructive Details (source 3) as the latter are more generic.  However Constructive Details (source 3) includes more cavity thickness options and different types of insulation.

The psi values from sources tabled above are more accurate than those available in SAP 2012 Appendix K. and take account of the high standard of thermal performance. They enable SAP assessors to fully optimise fabric performance calculations and demonstrate compliance without recourse to more expensive solutions. Their use can also help avoid the need for renewables.

In Building Regulation terms, Part L1A 2013 (section 3.9) requires that buildings are constructed so that there are “no reasonably avoidable thermal bridges” at junctions. It provides guidance on acceptable sources of psi-values for inclusion in SAP calculations, these being:

1. To use construction joint details and their attendant psi-values included in DCLG Approved Construction Details or those formally recognised by DCLG.  (DCLG approved Construction Details are available but are considered to be out of date and lack details of the psi values, so are of limited use with SAP, which required this information. It is understood that these will not be updated)

2. To use details and psi-values modelled by a person with suitable expertise and experience, who can demonstrate competence in using the software and in correctly interpreting BR497 guidance. (Signposts to these are provided above)

3. To use in the absence of modelled details the default psi-values included in SAP 2012 Appendix K.

4. To avoid calculating the energy loss from individual junctions completely by using a conservative energy loss value for the dwelling (a y-value of 0.15) in the SAP calculation. (It should be noted that this option is highly punitive in terms of its impact on heat loss, which is significantly increased and should therefore be avoided).