U-value of a log cabin wall

A 44mm log cabin wall has a U-value of about 1.97 W/m²K, and a 70mm one about 1.41 — roughly eight times and five times the heat loss of a new house wall at the 0.26 limiting value. Logs alone can’t be made efficient by going thicker: even 120mm only reaches about 0.91. A cabin for use in winter needs insulation behind a lining: 50mm of PIR between battens inside 70mm logs takes it to about 0.42, 100mm to about 0.26, and 100mm with 25mm more across the battens to about 0.19. Retailers’ tables agree with these figures (one gives 1.93 for 44mm and 1.39 for 70mm).

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The build-up (outside → inside)

44mm solid softwood logs (λ 0.13), interlocking

Logs modelled as solid softwood at λ 0.13 (pine; 0.12 for spruce, BSW timber DoP) with surface resistances only; the air leakage through the log joints is not part of a U-value. Retail figures for comparison: Summerhouse24 gives 2.53, 1.93 and 1.39 for 28, 44 and 70mm walls. Battens in the lined rows at 11.8% (BR 443 §4.6.2).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
28mm logs	≈ 2.59
34mm logs	≈ 2.32
44mm logs	≈ 1.97	As pre-filled
58mm logs	≈ 1.62
70mm logs	≈ 1.41
92mm logs	≈ 1.14
120mm logs	≈ 0.91	Passes the surface-mould check (from about 93mm)
44mm logs at λ 0.12 (spruce)	≈ 1.86
70mm logs + 50mm PIR between battens + plasterboard	≈ 0.42
70mm logs + 100mm PIR between battens + plasterboard	≈ 0.26	Just misses 0.26: 0.263
70mm logs + 100mm PIR between battens + 25mm PIR over them	≈ 0.19

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

A log is just timber, and timber is a fair insulator but no more: at λ 0.13, every 10mm of log is worth about 0.08 m²K/W, against 0.45 for 10mm of PIR. So thickness helps slowly — going from 44mm logs to 92mm only takes 1.97 to 1.14 — and the real gains come from insulation behind a lining, where the battens bridge it (11.8% here) unless a continuous layer covers them. Spruce, the usual species for imported cabins, is a little better than generic softwood (λ 0.12 in the BSW timber Declaration of Performance, against 0.13 for pine). The joints between logs matter as much as the logs: they shrink and settle, so draughts through gaps in the walls can lose more heat than conduction does. And thin logs are cold to the touch on the inside: U-Monkey’s surface-mould check fails every unlined wall up to about 92mm (fRsi under 0.75), so expect condensation and mould in corners if a thin cabin is heated and lived in through the winter.

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 log cabin wall?

Does a log cabin need building regulations approval?

Often not, in England and Wales (Scotland and Northern Ireland have their own rules). A detached single-storey building of up to 30m², with no sleeping accommodation, is exempt from the Building Regulations if it is at least a metre from the boundary or built substantially of non-combustible material — which a log cabin isn’t — and so is any detached building of up to 15m² with no sleeping accommodation (Schedule 2, Class 6). Part P still applies to electrics fed from the house (regulation 9(3)). The energy efficiency requirements don’t apply to a stand-alone building, other than a dwelling, of under 50m² (regulation 21(3)(d)), even where other parts of the Regulations do. Add sleeping accommodation and the Class 6 exemption no longer applies: ask building control how it will assess the cabin for Part L before you build — as a new dwelling, it would need a SAP calculation and, in England, walls of 0.26 W/m²K or better.

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.