U-value of a wide cavity wall (200mm, 250mm, 300mm)

Brick and block with a 200–300mm cavity: low-energy masonry from 0.13 down to 0.09.

A brick-and-block wall with a 200mm cavity filled with two 100mm layers of λ 0.032 batts, a thermal-grade aircrete inner leaf and plasterboard on dabs has a U-value of about 0.13 W/m²K — comfortably inside the 0.18 notional value. A 250mm cavity gives about 0.11 and 300mm about 0.09; blown graphite EPS bead in a 300mm cavity gives about 0.10. Wall ties across a wide cavity add a little: stainless wire ties at 2.5 per m² take the 200mm wall from 0.134 to 0.135.

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The build-up (outside → inside)

Batts at λ 0.032 (Knauf DriTherm Cavity Slab 32, made up to 150mm thick, so a wider cavity takes two layers); bead at λ 0.033. Ties are not in the pre-filled build-up: U-Monkey offers to add them.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
150mm cavity, one layer of batts≈ 0.18Meets 0.18: 0.176
200mm (2 × 100mm batts)≈ 0.13As pre-filled
250mm (2 × 125mm)≈ 0.11
300mm (2 × 150mm)≈ 0.09
300mm graphite EPS bead, blown≈ 0.10λ 0.033; a blown fill is air-gap level 0
200mm batts, dense block inner leaf≈ 0.15
200mm batts + stainless wire ties (2.5/m²)≈ 0.140.135: under BS EN ISO 6946’s 3% rule
300mm batts + stainless wire ties≈ 0.100.095

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

Each 50mm of λ 0.032 insulation takes off less than the last — 0.04 W/m²K from 150 to 200mm, under 0.02 from 250 to 300mm — so the wall’s thickness and its foundations, not the insulation, become the cost. Batts in two layers with staggered joints are BR 443’s air-gap level 0, and so is bead blown in once the walls are up (§7). Wide cavities need longer ties, and BR 443 asks for a correction where ties cross the insulation unless it is under 3% of the U-value: stainless wire ties stay under that here, and basalt-fibre or other ties below 1 W/mK need none at all. The inner leaf still counts: dense blocks instead of thermal aircrete cost about 0.013 at 200mm.

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 wide cavity wall (200mm, 250mm, 300mm)?

A brick-and-block wall with a 200mm cavity filled with two 100mm layers of λ 0.032 batts, a thermal-grade aircrete inner leaf and plasterboard on dabs has a U-value of about 0.13 W/m²K — comfortably inside the 0.18 notional value. A 250mm cavity gives about 0.11 and 300mm about 0.09; blown graphite EPS bead in a 300mm cavity gives about 0.10. Wall ties across a wide cavity add a little: stainless wire ties at 2.5 per m² take the 200mm wall from 0.134 to 0.135.

How wide does a cavity need to be for a U-value of 0.15?

About 175mm of λ 0.032 batts with a thermal-grade aircrete inner leaf: 150mm gives 0.176, 175mm (100 + 75mm layers) 0.150 and 200mm 0.134. Getting down to about 0.10 takes nearly 300mm — 0.094 with two 150mm layers of batts or 0.097 with blown graphite bead, before ties.

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