What ACDs were for
Accredited Construction Details are a government-published set of junction specifications — wall-to-floor, eaves, openings and so on — each with a pre-approved Ψ-value. Build exactly to the published detail, and your SAP assessor could apply that improved value instead of the punitive default. They were the easiest way to soften thermal bridging losses without commissioning calculations.
Why SAP 10 removed them
The ACD set aged badly: the details no longer reflect modern constructions, and there were long-standing concerns about the accuracy of the published values. SAP 10 removed the option to use them entirely.
Your options now
- Manufacturer or industry schemes — some insulation manufacturers publish assessed junction details with Ψ-values, valid if your construction matches exactly.
- Calculated Ψ-values to BS EN ISO 10211 — bespoke values for your actual details. Almost always gives the best result, and it’s what our free ΨMonkey psi calculator produces, with Building Control-ready reports at £25 a junction.
- Default values — always available, always punitive. Avoid if the SAP is anywhere near marginal.
As-built EPCs need the junction evidence documented, so decide your thermal bridging strategy at design stage, not at sign-off.