U-value of a cob wall

Solid earth walls 450–700mm thick: what calculation gives, what measurement finds, and how to insulate one safely.

A 600mm cob wall, lime-rendered outside and lime-plastered inside, has a calculated U-value of about 0.82 W/m²K, taking λ 0.6 for the cob — the middle of a published range that runs from 0.45, the mean Goodhew & Griffiths give for Devon cob at 1450 kg/m³, to 0.73, a Devon Earth Building Association figure the SPAB used in its U-value research, which puts the same wall anywhere between 0.64 and 0.96. Measurement agrees: most of the SPAB’s in-situ readings on cob walls came out between 0.76 and 1.05, and RdSAP assumes 0.80 for a cob wall as built. Insulating one is about moisture as much as heat: 60mm of wood-fibre board inside gives about 0.36, and 250mm of hempcrete outside about 0.21.

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The build-up (outside → inside)

Cob at λ 0.6 from U-Monkey’s library, inside the published range: Goodhew & Griffiths (2005) give 0.45 for Devon cob at 1450 kg/m³, and a Devon Earth Building Association figure of 0.73 that the SPAB used in its U-value research. The SPAB’s measured walls (Rye & Scott, Research Report 1, 2012) were about 400–620mm of cob. RdSAP figures are from RdSAP 10, Table 6 (England, age bands A–F). The pre-filled link opens as an existing element.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
450mm cob≈ 1.03
500mm cob≈ 0.95
600mm cob≈ 0.82As pre-filled
700mm cob≈ 0.72
600mm at λ 0.45 (Devon cob mean, Goodhew & Griffiths 2005)≈ 0.64
600mm at λ 0.73 (Devon Earth Building Association, as the SPAB used)≈ 0.96
600mm + 60mm wood-fibre board inside, lime plaster≈ 0.36U-Monkey’s condensation check: screening only
600mm + 100mm wood-fibre board inside≈ 0.26Meets the 0.30 for an upgraded wall
600mm + 250mm hempcrete outside (λ 0.07)≈ 0.21A Plymouth study of this wall gives about 0.3 with its hempcrete
600mm + 40mm cork-lime render outside (λ 0.09)≈ 0.61λ from Secil’s data sheet FT 12004.04 (April 2024); an older distributor sheet gave 0.10

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 cob’s conductivity is the big unknown: it depends on the earth, the straw and above all the density and the moisture in the wall, and calculation is often pessimistic for traditional walls — the SPAB’s research found it overestimated the U-value in 77% of the walls it measured (Research Report 1, 2012). For an existing wall, a measured U-value (BS ISO 9869) is worth having before deciding how much insulation it needs. Whatever goes on has to let the wall keep drying: Historic England notes that materials highly resistant to the passage of water vapour or liquid water are not usually appropriate for a traditionally constructed building, and risk trapping moisture in the fabric. So the usual choices are vapour-open ones — wood fibre or cork inside with lime plaster, hemp-lime or cork-lime outside — on a wall kept dry at its base, which in historic earth buildings means a masonry plinth. Inside insulation needs the most care: U-Monkey’s check predicts seasonal condensation where the wood-fibre board meets the old plaster — about 1.1 kg/m² by March with 60mm, 1.7 kg/m² by April with 100mm — that dries out in summer, and calls it a screening result, because wood fibre and earth move water by capillary action, which a Glaser calculation can’t model; a BS EN 15026 assessment by a specialist is the next step. RdSAP assumes a 540mm cob wall and 0.80 W/m²K for one built before 1983, and gives 0.40 with 50mm of insulation added and 0.26 with 100mm.

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 cob wall?

A 600mm cob wall, lime-rendered outside and lime-plastered inside, has a calculated U-value of about 0.82 W/m²K, taking λ 0.6 for the cob — the middle of a published range that runs from 0.45, the mean Goodhew & Griffiths give for Devon cob at 1450 kg/m³, to 0.73, a Devon Earth Building Association figure the SPAB used in its U-value research, which puts the same wall anywhere between 0.64 and 0.96. Measurement agrees: most of the SPAB’s in-situ readings on cob walls came out between 0.76 and 1.05, and RdSAP assumes 0.80 for a cob wall as built. Insulating one is about moisture as much as heat: 60mm of wood-fibre board inside gives about 0.36, and 250mm of hempcrete outside about 0.21.

Do I have to insulate a cob wall when I renovate it?

Not necessarily. In England, renovating more than half of a wall — re-rendering it, say — normally brings in Approved Document L’s improved value for an existing wall, 0.30 W/m²K (Table 4.3), where that is technically, functionally and economically feasible (paragraph 4.13). But paragraph 0.10 says the energy efficiency of a historic or traditional dwelling should be improved only where that will not cause long-term deterioration of its fabric or fittings, and names cob among the vapour-permeable constructions this applies to; for a listed building or one in a conservation area, the standards apply only so far as they would not unacceptably alter its character or appearance (paragraphs 0.8–0.9). Agree what is reasonable with building control, and with the conservation officer where the building is listed.

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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