U-value of a timber frame wall with wood-fibre insulation

Wood-fibre batts between the studs and a rendered wood-fibre board outside — the vapour-open timber frame.

A rendered timber frame wall with 140mm of wood-fibre batts between the studs (λ 0.036) and a 100mm wood-fibre render board outside them (λ 0.040), with OSB, a service void and plasterboard inside, has a U-value of about 0.17 W/m²K — inside the 0.18 notional value. The common 60mm board gives about 0.20, and 80mm shows as 0.18 but misses it, at 0.1803. Wood fibre insulates less well per millimetre than PIR, so the walls are thicker, but the board runs unbroken over the studs, which bridge 15% of the wall.

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The build-up (outside → inside)

Wood-fibre products as in U-Monkey’s library: STEICOflex 036 batts (λ 0.036) and STEICOprotect dry type M render board (λ 0.040, made 60–200mm). Studs at BR 443’s default 15%; the service-void battens at 11.8%.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
40mm render board (type H, λ 0.043)≈ 0.23Type M starts at 60mm
60mm render board≈ 0.20A common thickness
80mm render board≈ 0.18Misses 0.18: 0.1803
100mm render board≈ 0.17As pre-filled: meets 0.18
120mm render board≈ 0.15
100mm board, 12.5% timber≈ 0.16BR 443 §4.4.1(ii) frame
100mm board, glass wool (λ 0.035) between the studs≈ 0.16Much the same
Timber-clad: 60mm sarking board (λ 0.043), 140mm studs≈ 0.20Cladding and its batten space not counted

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 studs and the board decide it. Studs bridge 15% of the wall by BR 443’s default (§4.4.1), or 12.5% with a single top plate, the sole plate below floor level, no mid-height noggings and junction studs no more than 38mm deep with insulation behind them; the board outside covers all of that timber, so each extra 20mm of it takes off about 0.013–0.023 W/m²K, less as it thickens. Wood-fibre batts and glass wool of similar λ give much the same figure, and the batts and the board sit together with no gap, which is BR 443’s air-gap level 0. What wood fibre changes is the wall’s behaviour rather than its U-value: it is vapour-open and holds moisture, so the wall is built to dry outwards, with the OSB inside as the vapour check and airtight layer and a render system made for wood-fibre boards outside.

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 timber frame wall with wood-fibre insulation?

A rendered timber frame wall with 140mm of wood-fibre batts between the studs (λ 0.036) and a 100mm wood-fibre render board outside them (λ 0.040), with OSB, a service void and plasterboard inside, has a U-value of about 0.17 W/m²K — inside the 0.18 notional value. The common 60mm board gives about 0.20, and 80mm shows as 0.18 but misses it, at 0.1803. Wood fibre insulates less well per millimetre than PIR, so the walls are thicker, but the board runs unbroken over the studs, which bridge 15% of the wall.

Is wood fibre as good as PIR in a timber frame?

Not per millimetre: wood-fibre boards run λ 0.036–0.043 against PIR’s 0.022, so it takes 100mm of wood-fibre board outside the frame to do what 60mm of PIR does — both walls come out at about 0.17 (see the timber frame wall with external PIR). Wood fibre is chosen for other reasons: it is vapour-open and hygroscopic, it can carry render directly, and it suits walls designed to let moisture out rather than keep it in.

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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