Using ACD's for Thermal Bridging in SAP 2012
What is thermal bridging?
Thermal bridging is the term used to define the heat loss through junctions in a building fabric, which is expressed as a liner thermal transmittance value (Ψ-value [Psi]). “Junctions” could include wall-roof and wall-floor junctions, party walls, corners, sills, jambs, lintels, and any other significant junctions of the building fabric. Ψ-values can then be used to arrive at a Y-value: a factor, which allows the calculation of the building’s overall heat loss through thermal bridges.
Heat loss through thermal bridging are very significant. Currently you have the option to use accredited construction details (ACD), or have the thermal junctions of a property modelled by an expert or use a worst case default figure provided by SAP appendix R.
What's does this mean to me?
The current Building Regulations Part L 2013 with 2016 amendments requires the SAP calculation to include heat losses through thermal bridges. It is recommended to help ensure the DER/TER pass for the SAP calculation you should use one of the following options:
Accredited Construction Details (ACD's)
Enhanced Construction Details (ECD) from energy saving trusts
Thermally Model the junction (we can provide these click here)
Unfortunately not using any of the above options will results in default figures being used within the SAP calculation and will most likely result in a compliance failure. Below is a visual illustration of the effects of the each option.
How to evidence for As Built Stage & EPC?
If you are using ACD's, you will be required to evidence this to the energy/SAP assessor upon completion. How best to achieve this is to sign, date and complete each applicable sheet as the works proceed for the relevant junctions on your project. The following links will can be used to download all the junctions types.
Getting ready for SAP 10 and 2022 Part L changes?
The removal and use of ACD's under the 2022 Building Regulations changes means ACD's won't be available under the new rules. You will either need to use default figures or most likely need to thermally model the junctions using an independent expert such as ourselves. To learn more about the impacts of the changes visit our Psi Values Explained page or visit our Psi Value Modelling page to see how we can help.
Our advice is to plan early and ensure your specification will be compliant, ready for the 2022 changes.