When carrying out SAP calculations one of the most overlooked areas is Thermal Bridging. In this article we explain the different approaches that can be taken to improve the SAP rating.

Thermal Bridges occur where building elements join. They cause increased heat transfer & create cold spots which can lead to condensation & mould growth.

The value of Thermal Bridges are represented by the symbol Ψ. A smaller figure represents a better result.

To comply with the building regulations, in the SAP calculation the Dwelling Emission Rate (DER) must be less than the Target Emission Rate (TER) The way we deal with thermal bridges makes a big difference to the finished DER figure thereby improving the result. There are four approaches.

Default approach (don’t bother)

The SAP default value for this is  Ψ = 0.15. In most cases this won’t work and we need to apply at least the following.

Accredited Construction Details—ACD’s (spend a bit of time on it)

Accredited Construction Details (from the Planning Portal) reduce the Ψ value to 0.08. They are easily applied to most constructions.

Enhanced Construction Details - ECD’s (spend a bit more time on it)

An even better method is to combine Accredited details with at least three complimentary Enhanced Construction Details (from the Energy Saving Trust) using this method we can claim a Ψ value of 0.04.

Calculate Each Bridge Individually (spend loads of time on it)

At the moment this is only necessary where there is an unusual or complicated construction method. Most bridges are pretty standard and either the ACD or ECD approach is more appropriate.

When doing your calculation we can apply whichever details are required to make your building comply and then supply you with the necessary sheets or PDF files. We will always provide a bound set of these sheets , that incorporate check lists, for  the site manager to fill in and return to us when the job is finished.

If claiming ACD’s or ECD’s we need these completed sheets before we can issue the EPC.

Thermal Bridging

Effect of the different approaches on the DER

http://www.energysavingtrust.org.uk/business/Business/Housing-professionals/Interactive-tools/Enhanced-Construction-Details

 http://www.planningportal.gov.uk/buildingregulations/approveddocuments/partl/bcassociateddocuments9/acd 

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