U-value of a cavity wall with a dense concrete block inner leaf

Brick + full-fill mineral wool + dense block — what a heavyweight inner leaf costs.

A brick and dense-block cavity wall with 150mm of full-fill mineral wool achieves a U-value of about 0.23 W/m²K: inside the 0.26 limiting value, but well short of the 0.18 notional value. The same wall with an aircrete inner leaf is about 0.21, because a dense block (λ 1.13) adds almost nothing to the wall’s resistance. With dense blocks the insulation has to do all the work: a 200mm cavity of full fill just reaches 0.18, and 100mm of PIR partial fill comes out at 0.19. Open the build-up below pre-filled and tune it to your spec.

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The build-up (outside → inside)

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm full-fill, dense block≈ 0.32Misses the 0.26 limiting value
150mm full-fill, dense block≈ 0.23Passes the 0.26 limit; well short of the 0.18 notional
150mm λ0.032 batts, dense block≈ 0.20Better batts help, but not enough
200mm full-fill (wide cavity), dense block≈ 0.18Just inside the 0.18 notional (0.1796)
100mm PIR partial fill + 50mm clear cavity, dense block≈ 0.19Misses the 0.18 notional (0.187)

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) 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 inner leaf is the difference. A dense block is there for strength, mass and sound, not insulation, so the U-value has to come from the cavity. Its mortar joints are negligible: BR 443 lets them be ignored where block and mortar differ in resistance by less than 0.1 m²K/W, so U-Monkey adds none here (in aircrete, where the difference is larger, it counts them). Medium-dense blocks (λ 0.51) put the same wall at about 0.22, and aircrete at 0.21. The heavier leaf does store more heat: U-Monkey’s free thermal-mass figures give this wall an areal heat capacity of about 55 kJ/m²K, against 33 for the aircrete version.

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 a dense concrete block inner leaf?

A brick and dense-block cavity wall with 150mm of full-fill mineral wool achieves a U-value of about 0.23 W/m²K: inside the 0.26 limiting value, but well short of the 0.18 notional value. The same wall with an aircrete inner leaf is about 0.21, because a dense block (λ 1.13) adds almost nothing to the wall’s resistance. With dense blocks the insulation has to do all the work: a 200mm cavity of full fill just reaches 0.18, and 100mm of PIR partial fill comes out at 0.19. Open the build-up below pre-filled and tune it to your spec.

Dense block or aircrete for the inner leaf?

Thermally, aircrete: with 150mm of full fill the wall is about 0.21 with aircrete and 0.23 with dense block, because aircrete adds real resistance and dense block almost none. Dense block is stronger and heavier, better for sound and holds more heat. If the spec calls for dense blocks, put the extra thickness into the cavity or the board — model both inner leaves 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), 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 in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Walls

Roofs

Floors

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.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

Open this build-up pre-filled or call 01202 623236