A partial-fill cavity wall with 90mm PIR and a 100mm aircrete inner leaf achieves a U-value of about 0.18 W/m²K — the Part L 2021 and Future Homes Standard notional value. The older 60mm-PIR spec manages only about 0.25. Open it pre-filled below and adjust the board and blocks to your spec.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- 102.5mm brick outer leaf
- ~40mm residual (clear) cavity
- 60–90mm PIR partial-fill board
- 100mm aircrete block inner leaf
- Plasterboard on dabs
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 60mm PIR + 40mm clear cavity | ≈ 0.25 | Common 2013-era spec |
| 90mm PIR + 35–40mm clear cavity | ≈ 0.18 | Current new-build notional spec |
Figures computed with the BS EN ISO 6946 combined method (bridging included) using BR 443 conventional lambda values — indicative for the generic build-up shown. Your products, thicknesses and (for floors) footprint will move the number: that's what the calculator is for.
What changes the number
Board thickness and lambda dominate; keeping the residual cavity clean matters for both thermal performance and moisture. Swapping generic PIR (0.022) for phenolic (0.020) buys a little more, and the inner-leaf block grade still matters.
Next: check the junctions
A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT):
Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.
Frequently asked questions
What is the U-value of a cavity wall with PIR partial-fill insulation?
A partial-fill cavity wall with 90mm PIR and a 100mm aircrete inner leaf achieves a U-value of about 0.18 W/m²K — the Part L 2021 and Future Homes Standard notional value. The older 60mm-PIR spec manages only about 0.25. Open it pre-filled below and adjust the board and blocks to your spec.
Partial-fill PIR or full-fill mineral wool — which is better?
PIR gets more performance per millimetre (λ≈0.022 vs 0.038), so partial-fill hits 0.18 in a slimmer wall. Full-fill mineral wool is cheaper, more forgiving to install and better acoustically. Both are mainstream — model each in U-Monkey and compare like-for-like.
Is this calculator free?
Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard) and the condensation-risk screening check are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.
Does it follow the official calculation method?
Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging and air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- U-value of a cavity wall with full-fill insulation
- U-value of a timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of 270mm loft insulation
- U-value of insulation between rafters (loft conversion)
- U-value of a warm flat roof
- How much insulation does a flat roof need?
- U-value of an insulated concrete ground floor
- U-value of an insulated suspended timber floor
- U-value of a beam and block floor with PIR
Different build-up? See the full wall U-value calculator, or start from scratch in the main U-value calculator.
Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are.