— reviewed against the regs as they stand.
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.
Recognised Construction Details (RCDs) — the current scheme
Recognised Construction Details are what replaced the ACD shortcut for masonry. They are a free database of independently assessed junction details — aggregate block and aircrete, at standard cavity widths — each published with a psi value and an f-value (the condensation-risk factor), and certified for use in demonstrating Part L compliance. The portal is run by Building Alliance CIC; the junctions were commissioned by the Concrete Block Association and the Aircrete Products Association and modelled independently.
The deal is the same as it was with ACDs: build the junction exactly as drawn and the assessor can enter the RCD psi value in SAP instead of the default. Deviate — a different cavity closer, insulation stopped short, a lintel the detail didn’t assume — and the value no longer applies. They cover the standard junction set for masonry housing; they do not cover timber frame, SIPs, steel frame or anything non-standard, which is where a calculated value comes in.
LABC’s Registered Construction Details are the other scheme you will meet, aimed at smaller builders. Both are valid sources where the match is exact.
Your options now
- Scheme details — RCDs, LABC Registered Details, manufacturer schemes — independently assessed junctions with published Ψ-values, free or near-free, valid only if your construction matches the drawing exactly. Masonry is well covered; other systems mostly are not.
- 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.
Common questions
What does ACD stand for in construction?
Accredited Construction Detail. ACDs were a government-published set of standard junction drawings — wall-to-floor, eaves, window reveals and so on — each with a pre-approved psi (Ψ) value. If you built exactly to the published detail, the SAP assessor could use that value instead of a punitive default. SAP 10 withdrew them, so on a Part L 2021 project an ACD is no longer an accepted source for a junction psi value.
What are Recognised Construction Details (RCDs)?
A free database of independently assessed thermal junction details for masonry construction — aggregate block and aircrete — certified for use in demonstrating Part L compliance. The portal is run by Building Alliance CIC using junctions commissioned by the Concrete Block Association and the Aircrete Products Association. Each detail comes with a psi value and an f-value for condensation risk. Build to the detail exactly and the assessor can use its psi value in SAP.
Can I still use Accredited Construction Details in SAP 10?
No. SAP 10 removed the ACD scheme, so a Part L 2021 assessment cannot take its junction values from an ACD. Your options are a Recognised Construction Detail or another independently assessed scheme detail where your construction matches it exactly, a psi value calculated for your actual junction to BS EN ISO 10211, or the SAP default value for that junction — which is deliberately conservative.
Which is better — an RCD or a calculated psi value?
An RCD costs nothing and is quick, but it only exists for standard masonry junctions and you have to build precisely to the drawing. A calculated psi value covers any construction — timber frame, SIPs, steel, non-standard masonry — and almost always comes out lower than a scheme or default value, because it reflects the actual detail rather than a conservative generic. If the SAP is marginal, calculate.
What happened to Enhanced Construction Details (ECDs)?
Enhanced Construction Details were the Energy Saving Trust's better-performing companion set to the ACDs. Like ACDs they belong to the pre-SAP-10 era and are not a current source for junction psi values. Treat any spec that still cites ECDs or ACDs as needing a thermal-bridging strategy update.