U-value of a cavity wall with external wall insulation

Render on EPS over a filled cavity wall — the next step after cavity fill.

A typical 1930s–70s cavity wall — brick, a 50mm cavity, dense block — is about 0.58 W/m²K once the cavity has been filled. 90mm of grey EPS external wall insulation over it brings the wall to about 0.22, well inside the 0.30 that Approved Document L sets for a wall upgraded with external insulation; 60mm clears that at 0.28, while 50mm only just misses it, at 0.301. The cavity fill still counts: the same 90mm over an empty cavity gives about 0.29.

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The build-up (outside → inside)

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Filled cavity wall, no EWI≈ 0.58The starting point after cavity fill
+ 50mm grey EPS + render≈ 0.30Misses the 0.30 renovation target (0.301)
+ 60mm grey EPS + render≈ 0.28Clears the 0.30 renovation target
+ 90mm grey EPS + render≈ 0.22Comfortable pass
90mm grey EPS over an empty cavity≈ 0.29The cavity fill is still doing work

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

Board thickness and lambda set the number: grey EPS (λ 0.031) does the job of white (λ 0.038) in less thickness — 90mm of white gives about 0.25 rather than 0.22. Behind the EPS the brick outer leaf is taken as protected (λ 0.56) rather than exposed (0.77, as it was before the work): BR 443 §7 takes brick behind an external insulation system as protected on a solid wall, and says that with one “all the masonry is protected” on a diaphragm wall. U-Monkey suggests the switch when you add the insulation. With most of the insulation outside the masonry the wall stays warm, and U-Monkey’s free condensation check finds no condensation in this build-up in any month. The pre-filled link opens it as an upgraded wall judged against that 0.30 (Upgrading existing, then External / internal insulation), not the 0.55 for cavity fill. Mechanical fixings through the boards add a small correction U-Monkey can include, and the detailing at eaves, sills, reveals and the DPC decides how much of the paper figure survives on site.

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 external wall insulation?

A typical 1930s–70s cavity wall — brick, a 50mm cavity, dense block — is about 0.58 W/m²K once the cavity has been filled. 90mm of grey EPS external wall insulation over it brings the wall to about 0.22, well inside the 0.30 that Approved Document L sets for a wall upgraded with external insulation; 60mm clears that at 0.28, while 50mm only just misses it, at 0.301. The cavity fill still counts: the same 90mm over an empty cavity gives about 0.29.

Is external insulation worth it on a cavity wall that is already filled?

Thermally, yes: a filled 50mm cavity leaves the wall at about 0.58 W/m²K, and 60–90mm of grey EPS outside takes it to 0.28–0.22. It is a much bigger job than cavity fill — scaffolding, new render and reworked eaves, sills and pipework — so it makes most sense where the fill alone is not enough or the outside needs work anyway. Model your own wall in U-Monkey and compare it with and without the EWI.

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