U-value of a hempcrete wall (hemp-lime)

Cast hemp-lime round a timber frame: why “300mm = 0.20” depends on the λ you use.

A 300mm cast hempcrete wall round a timber frame, lime-rendered outside and lime-plastered inside, has a U-value of about 0.23 W/m²K at λ 0.07 — inside the 0.26 limiting value for a new wall but short of the 0.18 notional value. The often-quoted 0.20 for 300mm needs λ 0.06, the figure given for Tradical Hemcrete at 275 kg/m³; denser mixes declared at 0.076–0.085 give 0.25–0.27, and the 0.27 misses the limit. 400mm at λ 0.07 gives about 0.17.

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The build-up (outside → inside)

Hempcrete at λ 0.07 (U-Monkey’s library; typical cast hemp-lime at 275–330 kg/m³). Sources for the variants: Lime Technology’s figure for Tradical Hemcrete as published by the Limecrete Company (0.06 at 275 kg/m³) and BCB-Tradical’s 2016 brochure (0.076 / 0.085 at 280 / 320 kg/m³). Studs at BR 443’s default 15%, in the 100mm of wall they occupy.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
300mm at λ 0.06≈ 0.20Tradical Hemcrete, 275 kg/m³
300mm at λ 0.07≈ 0.23As pre-filled
300mm at λ 0.076≈ 0.25Tradical (France), 280 kg/m³
300mm at λ 0.085≈ 0.27Tradical (France), 320 kg/m³; misses the 0.26 limiting value
200mm at λ 0.07≈ 0.34Misses the 0.26 limiting value
400mm at λ 0.07≈ 0.17Meets 0.18
300mm at λ 0.07, studs not counted≈ 0.22

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

λ is the whole question. Hempcrete’s conductivity rises with density — how hard it is tamped or sprayed, and the binder — and with moisture: declared values run from about 0.06 at 275 kg/m³ to 0.085 at 320 kg/m³, and Vicat’s data sheet advises adding 20–30% to dry values for the humidity of the air. IsoHemp’s certified design values for its blocks (λUi, UBAtc approval ATG 3169) are 0.071 and 0.080, so U-Monkey’s 0.07 sits at the good end. The timber frame barely matters: the studs (λ 0.13) are only about twice as conductive as the hempcrete round them, so counting them adds about 0.007. What hempcrete brings beyond its U-value is moisture buffering and a vapour-open wall, so keep the finishes lime-based.

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 hempcrete wall (hemp-lime)?

A 300mm cast hempcrete wall round a timber frame, lime-rendered outside and lime-plastered inside, has a U-value of about 0.23 W/m²K at λ 0.07 — inside the 0.26 limiting value for a new wall but short of the 0.18 notional value. The often-quoted 0.20 for 300mm needs λ 0.06, the figure given for Tradical Hemcrete at 275 kg/m³; denser mixes declared at 0.076–0.085 give 0.25–0.27, and the 0.27 misses the limit. 400mm at λ 0.07 gives about 0.17.

Is 300mm of hempcrete enough for building regulations?

It depends on the λ the mix can show in service. At λ 0.07, 300mm gives about 0.23 W/m²K, inside the 0.26 limiting value for a new wall (Approved Document L, Table 4.1); at 0.085, a denser mix, it gives about 0.27 and misses, and some makers advise adding 20–30% to dry λ values for the humidity of the air. So use the declared design value for your mix, not a dry laboratory figure — or build thicker: 400mm gives about 0.17 at λ 0.07. Either way 300mm is short of the 0.18 the notional dwelling uses, so the rest of the house has to make that up in the SAP calculation; for an extension, where each new wall should meet 0.18 (Table 4.2), 300mm misses even at λ 0.06 (about 0.20): it takes about 385mm at λ 0.07, or about 335mm at 0.06.

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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