U-value of a CLT (cross-laminated timber) wall

A solid CLT panel with wood-fibre insulation outside: what each thickness reaches, and how little the timber itself adds.

A 100mm cross-laminated timber wall with 200mm of wood-fibre insulation board outside it (STEICO Protect L, λ 0.037), a thin lime render and a plasterboard lining on battens has a U-value of about 0.16 W/m²K, with the fixings that hold the board on counted. 180mm of board gives about 0.18, inside the 0.18 notional value, and 160mm about 0.19. The panel itself does little: at λ 0.12, 100mm of CLT is worth about 0.83 m²K/W, against 5.4 for the wood fibre, so the board outside sets the figure.

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The build-up (outside → inside)

CLT at λ 0.12 from KLH’s Declaration of Performance (2025); Stora Enso declares the same (ETA-14/0349). The board is STEICO Protect L dry from U-Monkey’s library, with its fixings counted as stainless screws at 6 per m² (BS EN ISO 6946 Annex F). The service battens are counted at 11.8% (BR 443 §4.6.2).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
160mm wood-fibre board (STEICO Protect L)≈ 0.19Clears the 0.26 limit, misses the 0.18 notional
180mm≈ 0.18Inside 0.18: 0.175
200mm≈ 0.16As pre-filled
240mm≈ 0.14
160mm Gutex Thermowall (λ 0.040)≈ 0.20
160mm graphite EPS (λ 0.031), rendered≈ 0.17
200mm wood fibre, CLT left exposed inside (no lining)≈ 0.170.168
200mm on a 120mm panel≈ 0.160.158
200mm, fixings left out≈ 0.160.157: the fixings add about 0.005

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 insulation outside the panel does nearly all the work, and because CLT is solid there are no studs to bridge it — only the screws that hold the boards on, which add about 0.005 here at six a square metre (BS EN ISO 6946 Annex F). Wood fibre (λ 0.037–0.040) needs a fifth to nearly a third more thickness than graphite EPS (λ 0.031) for the same figure; what it gives in return is a vapour-open build-up that lets the timber dry outwards. KLH and Stora Enso both declare their panels at λ 0.12 (to EN ISO 10456); taking 0.13, the figure for softwood generally, adds only about 0.002. Leaving the panel exposed inside costs the lining and the air behind it, about 0.007. Stora Enso’s own building-physics tables give 0.17–0.18 for a 100mm panel with 200mm of wood fibre at λ 0.038–0.039; with that board U-Monkey gives about 0.17. Height matters for the materials: in England, on a relevant building — a building with a storey at least 18m above ground, containing dwellings, an institution or rooms for residential purposes — regulation 7(2) of the Building Regulations limits the materials in an external wall to Euroclass A1 or A2-s1,d0, which rules out timber panels, wood fibre and EPS there.

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 CLT (cross-laminated timber) wall?

A 100mm cross-laminated timber wall with 200mm of wood-fibre insulation board outside it (STEICO Protect L, λ 0.037), a thin lime render and a plasterboard lining on battens has a U-value of about 0.16 W/m²K, with the fixings that hold the board on counted. 180mm of board gives about 0.18, inside the 0.18 notional value, and 160mm about 0.19. The panel itself does little: at λ 0.12, 100mm of CLT is worth about 0.83 m²K/W, against 5.4 for the wood fibre, so the board outside sets the figure.

Does a CLT wall need a vapour control layer?

Not usually as a separate membrane. The panel is a vapour check in itself: KLH declares a vapour resistance factor of 46 (wet) to 300 (dry), so 100mm of panel resists vapour like 4.6–30m of still air, while the wood fibre outside is vapour-open and lets the wall dry outwards. U-Monkey’s free condensation check finds no condensation in any month in any of the walls on this page. The joints between panels and around windows and services still need sealing, because air leaking through them carries far more moisture than diffusion does.

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.

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