— reviewed against the regs as they stand.
ΨMonkey — free in your browser
Calculate the Ψ-value for the junction you have actually drawn
Start from one of 38 ready-made SAP junctions, change the layers to match your detail, and watch Ψ and the fᵣₛᵢ temperature factor update as you type — modelled to BS EN ISO 10211, validated against the ISO test reference case.
Free to model on screen, no sign-up. Building Control-ready PDF £25 +VAT a junction, or unlimited on Monkey Pro.
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.
The junction set — E-numbers and what each one is
Both the old ACD set and the schemes that replaced it are organised around the same reference junctions, and SAP carries a default Ψ-value for each. Most specs name them by E-number and assume everyone knows which is which. Here is the set people actually model, what each one is, and where it tends to cost you — or see the full SAP junction reference for all 44 references with their Table K1 default and Table R2 notional Ψ-values:
| Ref | Junction | What it is |
|---|---|---|
| E1 / E2 | Lintel | Head of an opening. Steel lintels bridge hard; the default is punitive and a calculated value usually beats it comfortably. |
| E3 | Window sill | Below the opening, where the cavity closer meets the frame. |
| E4 | Window jamb (reveal) | The vertical return at each side of an opening — the longest opening junction by total length on most dwellings. |
| E5 / E19 / E22 | Wall-to-floor | Ground floor perimeter. Depends heavily on whether the floor insulation meets the cavity insulation. |
| E6 / E7 | Intermediate floor | Where a first floor is built into or hung off the wall. |
| E8 / E9 / E23 | Balcony | Anything penetrating the insulation line to carry a load outside it. The worst bridges on most apartment schemes. |
| E10 / E11 / E24 | Eaves | Wall to roof at the eaves, insulated at ceiling or at rafter. |
| E12 / E13 | Gable | Wall to roof at the gable, and the ceiling junction behind it. |
| E16 | External corner | A normal corner. Small Ψ, long total length. |
| E17 | Inverted corner | A corner turning the other way — routinely gives a negative Ψ-value, which is correct and which SAP accepts. |
| E18 / E25 / P1–P8 | Party wall | Separating wall junctions, including the party wall itself where it is not fully filled and sealed. |
| E20 / E21 | Exposed floor | A floor over an unheated space or open air — over a passage, a car port, an overhang. |
Neither the retired ACD set nor the current schemes cover every construction, and none of them cover yours exactly unless you drew it that way on purpose. If you want to see what these junctions actually come out at across different wall build-ups — rather than a single generic figure — we publish our own solved Ψ-value library: every standard junction template solved against every wall type in the calculator, with the figures regenerated whenever the app changes.
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 value calculator produces, with Building Control-ready reports at £25 +VAT 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.