U-value of brick-slip external wall insulation

External insulation finished in brick slips on a solid brick wall: the thickness that meets 0.30, fixings included.

External wall insulation finished with brick slips on a solid 9-inch brick wall needs about 110mm of grey EPS (Wetherby Epsitherm 70E, λ 0.032) to meet the 0.30 W/m²K Approved Document L asks of an upgraded wall: U-Monkey gives 0.28 with the system’s certified fixings counted (7 per m² at χ 0.004 W/K), and 100mm misses, at 0.31. The BBA certificate’s table puts 100mm at 0.30. Like U-Monkey, it takes the brick behind as protected (λ 0.56), as BR 443 does once a wall has external insulation, and its figures are close to a calculation with no air-gap correction: U-Monkey gives 0.299 on that basis, and adds BR 443’s default correction otherwise. The slips themselves add almost nothing: what sets them apart from render is the finish, the weight and the fixings.

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The build-up (outside → inside)

The slips, the insulation and the fixings follow BBA 14/5137 Product Sheet 1 (Wetherby Epsibrick EWI system; a 2014 issue, read in a council planning file): 15mm brick slips at λ 0.77, Epsitherm 70E λ 0.032 (90E 0.030), seven galvanised fixings per m² at χ 0.004 W/K, entered as certified point thermal transmittances. The brick behind the insulation is taken as protected, λ 0.56 (BR 443 §7), as the certificate takes it, with its mortar joints counted (§4.2); one row shows the exposed 0.77. Mineral wool rows: K Systems’ dual-density slab (λ 0.036) with the same fixings. The pre-filled link opens as an upgraded solid wall.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
80mm Epsitherm 70E (λ 0.032)≈ 0.36
90mm≈ 0.33The maker’s web page says 90mm meets refurbishment standards “in the majority of cases”
100mm≈ 0.31Misses 0.30 (0.306); the BBA table’s 100mm has no air-gap correction
110mm≈ 0.28As pre-filled: meets 0.30
120mm≈ 0.27
130mm≈ 0.25BBA: 130mm for 0.25
90mm Epsitherm 90E (λ 0.030)≈ 0.32
100mm Epsitherm 90E≈ 0.29Meets 0.30 (0.292)
100mm mineral wool (λ 0.036)≈ 0.33Non-combustible insulation, for taller buildings
120mm mineral wool≈ 0.29Meets 0.30 (0.292)
140mm mineral wool≈ 0.26
100mm EPS, fixings left out≈ 0.28The fixings add 0.028
100mm EPS, no air-gap correction (the BBA table’s basis)≈ 0.30Meets 0.30 (0.299); BBA: 0.30
100mm EPS with the brick at the exposed λ 0.77≈ 0.31Misses (0.312): 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

Brick slips are a finish, not insulation: 15mm of slip on adhesive is worth about R 0.02 m²K/W, the same as a thin render, so the insulation and its fixings set the U-value. The BBA certificate for Wetherby’s Epsibrick system (14/5137 Product Sheet 1) counts seven galvanised fixings per m² at χ 0.004 W/K each, 0.028 on every row, and U-Monkey takes them as certified point thermal transmittances; left out, 100mm reads 0.28. The conventions explain the rest. BR 443 takes solid masonry as protected once it has external insulation, and U-Monkey and the certificate’s table both use protected brick (λ 0.56): the table gives 100mm for 0.30, 130mm for 0.25 and 190mm for 0.18. With the brick protected, BR 443 §4.2 counts its mortar joints (17.3% at λ 0.88, about +0.002), and U-Monkey also adds BR 443’s default air-gap correction (level 1) to the boards; without the correction it gives 0.299, 0.244 and 0.182 for the table’s 100, 130 and 190mm, close to the certificate. Taking the brick at its exposed value (0.77) instead would add about 0.006 at 100mm. 110mm meets 0.30 on any of these bases. Mineral wool (λ 0.036) needs about 120mm, but in England it is the insulation for a relevant building — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes: the EPS system is classed B-s1,d0, which regulation 7(2) rules out there, and the mineral wool versions are A2-s1,d0 or A1. The fixing pattern is the system’s own, and “for application to second storey floors and above” the certificate recommends that the designer considers at least one stainless-steel fixing per m² and fire barriers, in line with BRE’s fire guidance for external insulation (BR 135). Planning can be the reason for slips at all: householder permitted development asks for external materials “of a similar appearance” to the existing house (Schedule 2, Part 1, Class A, condition A.3(a)), and in a conservation area permitted development excludes cladding in stone, render, timber, plastic or tiles (A.2(a)), so check with the council; Historic England considers external insulation unlikely to be acceptable on a listed building.

Condensation risk with brick-slip external insulation

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with the slips, the EPS and the brick each tested across BS 5250’s published range of vapour resistance, one at a time and all together: insulation outside keeps the brick warm. The slip-and-adhesive skin is cementitious and less vapour-open than a lime render, which is why Historic England favours lime renders and rainscreens on traditional walls.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 brick-slip external wall insulation?

External wall insulation finished with brick slips on a solid 9-inch brick wall needs about 110mm of grey EPS (Wetherby Epsitherm 70E, λ 0.032) to meet the 0.30 W/m²K Approved Document L asks of an upgraded wall: U-Monkey gives 0.28 with the system’s certified fixings counted (7 per m² at χ 0.004 W/K), and 100mm misses, at 0.31. The BBA certificate’s table puts 100mm at 0.30. Like U-Monkey, it takes the brick behind as protected (λ 0.56), as BR 443 does once a wall has external insulation, and its figures are close to a calculation with no air-gap correction: U-Monkey gives 0.299 on that basis, and adds BR 443’s default correction otherwise. The slips themselves add almost nothing: what sets them apart from render is the finish, the weight and the fixings.

How thick does brick-slip external insulation need to be to meet Building Regulations?

About 110mm of grey EPS (λ 0.032) on a 9-inch solid brick wall, counting the system’s fixings: U-Monkey gives 0.28 W/m²K against the 0.30 Approved Document L sets for an upgraded wall (Table 4.3). The system’s BBA certificate puts 100mm at 0.30; U-Monkey reads 0.31 there (0.306), because it adds BR 443’s default air-gap correction, which the certificate’s figures appear to leave out: without it, 0.299. Both take the brick behind as protected (λ 0.56). With higher-grade EPS (λ 0.030) 100mm reaches 0.29; with mineral wool (λ 0.036) it takes about 120mm.

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

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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.

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