U-value of an I-joist timber frame wall (Passivhaus-level)

Deep engineered I-studs dense-filled with cellulose — low-energy timber frame, and how BR 443 handles the I-shape.

A timber frame wall of 300mm I-joists dense-filled with blown cellulose (λ 0.038), with 60mm of wood-fibre sheathing outside and OSB, a service void and plasterboard inside, has a U-value of about 0.11 W/m²K. 250mm I-joists give about 0.12 and 400mm about 0.08. The thin web is the point: BR 443 counts the flanges as 8.8% timber but the web as only 1.7%, so most of the depth is barely bridged — 300mm I-joists with no sheathing at all (0.124) do as well as solid studs at BR 443’s 15% default with the 60mm sheathing.

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The build-up (outside → inside)

BR 443 (2019) §4.4.2: an I-beam stud whose flange and web conductivities are within 20% of each other (softwood and OSB, both λ 0.13) is calculated as three bridged layers, with the fractions from its table for 47mm flanges and an 8–10mm web at 600mm centres. Cellulose at λ 0.038 (Warmcel, as in U-Monkey’s library).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
200mm I-joists≈ 0.14
250mm I-joists≈ 0.12
300mm I-joists≈ 0.11As pre-filled
350mm I-joists≈ 0.09
400mm I-joists≈ 0.08
300mm I-joists, no wood-fibre sheathing≈ 0.120.124
300mm solid studs instead (BR 443’s 15% default), with the sheathing≈ 0.120.124 — the same as the I-joists without sheathing

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

BR 443 (2019) §4.4.2 treats an I-beam stud as three layers — flange, web, flange — each with its own timber fraction: 47mm flanges at 600mm centres are 8.8% and an 8–10mm web 1.7%. Because the web is so thin, deepening the joist adds insulation that is hardly bridged, which is why each extra 50mm still takes off about 0.01–0.02 W/m²K. Dense-blown cellulose fills the I-shape completely and runs continuously through the flange and web zones, so the air-gap correction is level 0. The OSB inside is usually the airtight layer, with the services kept in the void in front of it so the airtight line is not punctured.

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 an I-joist timber frame wall (Passivhaus-level)?

A timber frame wall of 300mm I-joists dense-filled with blown cellulose (λ 0.038), with 60mm of wood-fibre sheathing outside and OSB, a service void and plasterboard inside, has a U-value of about 0.11 W/m²K. 250mm I-joists give about 0.12 and 400mm about 0.08. The thin web is the point: BR 443 counts the flanges as 8.8% timber but the web as only 1.7%, so most of the depth is barely bridged — 300mm I-joists with no sheathing at all (0.124) do as well as solid studs at BR 443’s 15% default with the 60mm sheathing.

Do I need a wall this good for the Future Homes Standard or Passivhaus?

Not for the Future Homes Standard: its notional dwelling uses 0.18 W/m²K for walls (SAP 10.3 Appendix R) and the limiting value is 0.26, so 0.11 is far beyond both. Passivhaus certification sets limits on space heating demand, airtightness and primary energy rather than on U-values, so the wall is sized in the PHPP energy model together with the windows and the airtightness; deep, barely bridged timber walls like this one are one way of getting there.

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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