U-value of wood-fibre external wall insulation (EWI) on a solid wall

A 9-inch solid brick wall (about 2.1 W/m²K) with 140mm of render-carrying wood-fibre board (STEICO Protect M, λ 0.040) and a lime render system has a U-value of about 0.26 W/m²K with the board’s fixings counted, meeting the 0.30 Approved Document L sets for an upgraded wall. 120mm meets it too, at 0.297 (shown as 0.30), and 120mm of STEICO’s Protect L (λ 0.037) gives 0.28; 100mm of either falls short. Reaching 0.18 — the value for a new wall in an existing home — takes about 220mm of Protect L. Wood fibre is thicker than EPS or phenolic for the same figure, and vapour-open, which suits solid masonry.

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The build-up (outside → inside)

10mm lime render system
140mm wood-fibre render board (STEICO Protect M, λ 0.040), mechanically fixed (6 fixings/m²)
215mm solid brick (protected by the insulation: λ 0.56)
13mm plaster

Boards from U-Monkey’s library: STEICO Protect dry M (λ 0.040) and L (λ 0.037), Gutex Thermowall (λ 0.040). Fixings counted as stainless screws at 6 per m² through the full board depth (BS EN ISO 6946 Annex F). The brick behind the boards is taken as protected, λ 0.56 (BR 443 §7), with its mortar joints counted (§4.2). The pre-filled link opens as an upgraded solid wall, judged against Approved Document L’s 0.30 (2021 edition, Table 4.3).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Bare wall: 215mm brick, 13mm plaster	≈ 2.10
100mm STEICO Protect M (λ 0.040)	≈ 0.34	Misses 0.30
120mm Protect M	≈ 0.30	Meets 0.30: 0.297
140mm Protect M	≈ 0.26	As pre-filled
160mm Protect M	≈ 0.23
120mm STEICO Protect L (λ 0.037)	≈ 0.28	Meets 0.30
220mm Protect L	≈ 0.17	Meets 0.18, the value for a new wall (200mm: 0.182)
140mm Gutex Thermowall (λ 0.040)	≈ 0.26
140mm, fixings left out	≈ 0.25	The fixings add about 0.008

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 board’s thickness and grade decide the figure: STEICO’s render-carrying boards come as Protect M (λ 0.040, 60–200mm) and Protect L (λ 0.037, 80–240mm), and Gutex Thermowall matches Protect M. The fixings cost more here than through the thicker board on the CLT wall page: BS EN ISO 6946’s correction for a fixing that crosses the whole board grows as the board gets thinner, so at six a square metre they add about 0.008 here, against about 0.005 there — on a real job the number per square metre depends on the wind load and the system’s certificate, so check it. Wood fibre needs up to about 30% more thickness than EPS (λ 0.031–0.038) for the same figure, and nearly twice as much as phenolic board (λ 0.021), which matters at reveals, eaves and verges; in return it is vapour-open, so the brick can still dry outwards, and U-Monkey’s condensation check finds no condensation in any month for any thickness here. External insulation also puts the brick on the warm side: the surface-mould factor goes from 0.58 for the bare wall, a fail, to 0.94.

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 wood-fibre external wall insulation (EWI) on a solid wall?

What U-value does external wall insulation need to meet?

In England, Approved Document L asks for 0.30 W/m²K when an existing solid wall is insulated (Table 4.3, improved value, under “wall – internal or external insulation”), so far as that is technically, functionally and economically feasible with a simple payback of 15 years or less (paragraph 4.13). Insulating more than half of a wall counts as renovating it, which brings this in (regulation 23). With wood fibre on a 9-inch brick wall that means about 120mm of Protect M or Protect L. A new wall in an existing home — an extension — is held to 0.18 instead (Table 4.2), about 220mm of Protect L. In a conservation area, external insulation may also need planning permission, so check before you start.

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.