U-value of wood-fibre internal wall insulation on a solid brick wall

Vapour-open wood-fibre boards inside a 9-inch wall: what each thickness gives, and why most stop short of 0.30.

A 215mm solid brick wall lined inside with 60mm of wood-fibre insulation board (λ 0.038) and lime plaster has a U-value of about 0.49 W/m²K, down from about 2.1. 80mm gives about 0.39 and 100mm about 0.33; it takes about 112mm to reach the 0.30 Approved Document L asks of an internally insulated wall. Most wood-fibre systems on solid walls stop well short of that on purpose: the thicker the lining, the colder and wetter the brick behind it, and the DEEP study, as Historic England reports it, found that targeting around 0.8 rather than 0.30 cut the moisture risk while keeping about two-thirds of the energy saving.

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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 (2021 edition, Table 4.3). Board at λ 0.038 (STEICOinternal, made 40–100mm, in U-Monkey’s library). U-Monkey’s ISO 13788 condensation check is a screening test here; a BS EN 15026 hygrothermal assessment is the fuller one.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Bare wall: 215mm brick, 15mm lime plaster≈ 2.12
40mm wood fibre≈ 0.66
60mm wood fibre≈ 0.49As pre-filled
80mm wood fibre≈ 0.39
100mm wood fibre≈ 0.33Misses 0.30
120mm wood fibre (two boards)≈ 0.28Meets 0.30
60mm board at λ 0.040≈ 0.51

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 is bonded to the wall and finished in lime, so the wall can still dry to the inside: wood fibre is vapour-open and capillary-active, moving moisture back towards the room, which is the reason to choose it over a foil-faced board on an old wall. Thickness is the trade-off. The first 40–60mm does most of the good — at 60mm the wall loses under a quarter of the heat it did bare — while each extra 20mm saves less and leaves the brick colder. DEEP, the government-funded study of solid-wall insulation, found that targeting a U-value around 0.8 rather than 0.30 reduced the moisture risk while keeping about two-thirds of the energy saving, as Historic England reports it. The wall has to be in good order first: sound pointing, working gutters and no cement render or plastic paint holding water in.

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 internal wall insulation on a solid brick wall?

A 215mm solid brick wall lined inside with 60mm of wood-fibre insulation board (λ 0.038) and lime plaster has a U-value of about 0.49 W/m²K, down from about 2.1. 80mm gives about 0.39 and 100mm about 0.33; it takes about 112mm to reach the 0.30 Approved Document L asks of an internally insulated wall. Most wood-fibre systems on solid walls stop well short of that on purpose: the thicker the lining, the colder and wetter the brick behind it, and the DEEP study, as Historic England reports it, found that targeting around 0.8 rather than 0.30 cut the moisture risk while keeping about two-thirds of the energy saving.

Does wood-fibre insulation have to meet 0.30 under Part L?

Approved Document L’s figure for a solid wall insulated internally is 0.30 W/m²K (Table 4.3), but it is not absolute. Where 0.30 is not technically or functionally feasible, the wall should reach the lowest U-value that is, and generally no worse than 0.7 (paragraph 4.13); a lesser standard may also be appropriate where 0.30 would cut a room’s floor area by more than 5% (note 6), or where the work complies with Part C on condensation. Paragraph 0.10 adds that historic and traditional dwellings should be improved only if it will not cause long-term deterioration of the fabric. On a solid wall, moisture risk is the usual reason a thinner lining is the right answer, and PAS 2035 notes that a BS EN ISO 13788 check alone may not be enough to judge it.

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.

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