A brick-and-block cavity wall with 100mm full-fill mineral wool achieves a U-value of about 0.27 W/m²K — short of the 0.18 W/m²K new-build notional target. That is why modern specifications use 150mm cavities (≈0.20) with thermal-grade blocks or enhanced-lambda batts to reach 0.18. Open the build-up below pre-filled and tune it to your own 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
- 100–150mm full-fill mineral wool cavity batt
- 100mm aircrete block inner leaf
- Plasterboard on dabs
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 100mm full-fill, standard aircrete | ≈ 0.27 | Pre-2021 volume spec — misses the current notional |
| 150mm full-fill, standard aircrete | ≈ 0.20 | Passes the 0.26 limit; close to notional |
| 150mm full-fill, thermal aircrete (λ0.11) | ≈ 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
The batt lambda (0.038 generic vs 0.032–0.035 enhanced), the block grade (dense 1.13 → aircrete 0.11 is a huge swing), and the cavity width do almost all the work. Mortar joints and wall ties nibble a little more — U-Monkey applies the corrections for you.
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 full-fill insulation?
A brick-and-block cavity wall with 100mm full-fill mineral wool achieves a U-value of about 0.27 W/m²K — short of the 0.18 W/m²K new-build notional target. That is why modern specifications use 150mm cavities (≈0.20) with thermal-grade blocks or enhanced-lambda batts to reach 0.18. Open the build-up below pre-filled and tune it to your own spec.
Do I need a 150mm cavity to pass Part L?
Broadly, yes for full-fill: 100mm of mineral wool tops out around 0.27 W/m²K, while the notional dwelling assumes 0.18. A 150mm full-fill cavity with thermal-grade blocks — or partial-fill PIR — is how current specifications get there.
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 PIR partial-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.