A brick-and-block cavity wall with 100mm full-fill mineral wool achieves a U-value of about 0.27 W/m²K — short of even the 0.26 limiting value, let alone the 0.18 W/m²K new-build notional target. That is why modern specifications use 150mm cavities (≈0.20), and reaching 0.18 takes enhanced batts (around λ 0.032) with thermal-grade blocks. 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 (mortar joints counted)
- 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 even the 0.26 limit |
| 150mm full-fill, standard aircrete | ≈ 0.20 | Passes the 0.26 limit; short of the 0.18 notional |
| 150mm full-fill, thermal aircrete (λ0.11) | ≈ 0.20 | Thermal blocks alone barely move it |
| 150mm λ0.032 batts, thermal aircrete (λ0.11) | ≈ 0.17 | Current new-build notional spec |
Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and 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 in the blockwork and wall ties nibble a little more — U-Monkey counts the mortar joints for you, as BR 443 requires for aircrete, and offers to add the ties.
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 a junction):
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 even the 0.26 limiting value, let alone the 0.18 W/m²K new-build notional target. That is why modern specifications use 150mm cavities (≈0.20), and reaching 0.18 takes enhanced batts (around λ 0.032) with thermal-grade blocks. 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 comes out around 0.27 W/m²K, which misses even the 0.26 limiting value, while the notional dwelling assumes 0.18. A 150mm cavity clears the limit; reaching 0.18 takes enhanced batts (around λ 0.032) with thermal-grade blocks — or partial-fill PIR.
Is this calculator free?
Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) 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, mortar joints in blockwork 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 one-and-a-half-brick (317.5mm) solid wall
- 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
- Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16
- U-value of a cold flat roof (cold deck)
- U-value of an inverted flat roof
- 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, and why two calculators can give different answers for the same wall.