U-value of a solid wall with external insulation

Render on EPS outside 215mm brick — the ECO-scheme staple.

External wall insulation takes a 215mm solid brick wall from roughly 2.1 W/m²K to the 0.30 renovation target with 90mm of grey EPS, at 0.296 (shown as 0.30); 100mm gives about 0.27 and 110mm about 0.25, while 80mm, at 0.33, falls short. Behind the insulation the brick is taken at its protected λ, 0.56, as BR 443 directs once a wall has external insulation, rather than the 0.77 of an exposed wall — at 0.77 the same 90mm would just miss, at 0.303. EWI keeps the brick warm (kind to the wall and the dew point) at the cost of external appearance and detailing at eaves and openings.

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The build-up (outside → inside)

The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 for external insulation (2021 edition, Table 4.3). The brick behind the insulation is taken as protected, λ 0.56 (BR 443 §7), with its mortar joints counted (§4.2).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
80mm grey EPS + render≈ 0.33Misses the 0.30 renovation target
90mm grey EPS + render≈ 0.30Typical ECO-scheme spec — meets the 0.30 target (0.296)
100mm grey EPS + render≈ 0.27Clears the 0.30 renovation target
110mm grey EPS + render≈ 0.25Comfortable pass
90mm, with the brick at the exposed λ 0.77≈ 0.30Just misses (0.303): the exposed value is for brick with nothing outside it

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 (grey EPS 0.031 vs white 0.038) set the number, with one convention worth knowing: BR 443 §7 takes brick or block as exposed (λ 0.77 for brick) “except with an external insulation or cladding system when the protected value applies” (0.56). With the brick protected, BR 443 §4.2 also counts its mortar joints (17.3% of the wall at λ 0.88), which U-Monkey adds for you; net of them, the protected value is worth 0.005–0.008 here. U-Monkey suggests it when its generic brick (λ 0.77) has insulation, cladding or another leaf outside it — for a maker’s brick, use the maker’s own protected value. Unlike IWI, EWI keeps the masonry warm, so condensation risk is generally lower — but junctions, DPC level and fire classification of the system need real attention.

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 solid wall with external insulation?

External wall insulation takes a 215mm solid brick wall from roughly 2.1 W/m²K to the 0.30 renovation target with 90mm of grey EPS, at 0.296 (shown as 0.30); 100mm gives about 0.27 and 110mm about 0.25, while 80mm, at 0.33, falls short. Behind the insulation the brick is taken at its protected λ, 0.56, as BR 443 directs once a wall has external insulation, rather than the 0.77 of an exposed wall — at 0.77 the same 90mm would just miss, at 0.303. EWI keeps the brick warm (kind to the wall and the dew point) at the cost of external appearance and detailing at eaves and openings.

EWI or IWI for a solid wall?

Thermally they can reach similar U-values. EWI keeps the wall warm and the rooms full-size but changes the outside and costs more; IWI is room-by-room and cheaper but eats space and needs careful moisture design. Model both in U-Monkey and run the condensation check on the IWI option.

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.

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