U-value of a straw bale wall

Load-bearing or infill straw bales with lime plaster: why the way the bales are laid matters more than their size.

A straw bale wall of 450mm bales laid flat, with 25mm of lime plaster on each face, has a U-value of about 0.18 W/m²K (0.1797), just inside the 0.18 notional value, taking the λ of 0.080 that the German building approval for construction straw bales (2014, since lapsed) gave for heat flowing along the stalks. Which way the stalks run matters more than the size of the bale: set on edge, with the heat crossing the stalks (λ 0.052), a 350mm bale gives about 0.15 — better than the thicker bale laid flat. U-Monkey adds BR 443’s correction for air gaps in a single layer of insulation, about 0.009; most published figures leave it out.

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The build-up (outside → inside)

λ 0.080 / 0.052 from the German building approval for construction straw bales (DIBt Z-23.11-1595, 2014, Table 2: design values along and across the stalks; it lapsed in 2019), which Passipedia repeats; U-Monkey’s library carries both bale orientations. The air-gap correction is BR 443’s level 1, which U-Monkey applies to a single layer of insulation.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
450mm bales laid flat (λ 0.080, heat along the stalks)≈ 0.18As pre-filled: 0.1797
480mm bales laid flat≈ 0.17
350mm bales on edge (λ 0.052, heat across the stalks)≈ 0.15Thinner, but better
360mm bales on edge≈ 0.150.149
450mm at λ 0.065 (big bales, Passipedia)≈ 0.15
450mm at λ 0.09 (a 2001 UK guide)≈ 0.20
400mm EcoCocon straw panel (λ 0.0645)≈ 0.16EcoCocon quotes 0.150–0.152 without finishes or the air-gap correction
400mm EcoCocon panel + 60mm wood-fibre render board≈ 0.13EcoCocon: 0.127–0.129 with 60mm of wood-fibre board

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 bale’s orientation and density set the λ, and the published values differ. In a two-string bale the stalks run along the bale’s width, across the strings, so a bale laid flat carries heat along the stalks and one on edge across them: the German approval for construction straw (DIBt Z-23.11-1595, 2014, lapsed in 2019) gave design values of 0.080 and 0.052 for the two, and Passipedia quotes the same pair, with 0.065 for big bales; a newer European assessment for one maker’s bales (ETA-17/0247) declares 0.048 across the stalks, and Bath University’s tests for the ModCell panel proposed 0.064. A 2001 UK guide quotes 0.09 and, in the same breath, 0.13 for walls over 450mm thick — which would need λ of about 0.06; at 0.09 a 450mm wall is about 0.20. The plaster adds little to the U-value but makes the wall airtight, which straw needs. Prefabricated straw panels such as EcoCocon’s, straw packed into a timber frame, work the same way. U-Monkey applies BR 443’s air-gap correction for a single layer of insulation, about 0.009 on these walls; makers’ figures usually leave it out, which accounts for most of the gap between EcoCocon’s 0.150–0.152 for its 400mm panel without finishes and U-Monkey’s 0.164 with lime plaster both sides. With a second layer, the wood-fibre board, the correction drops away and the figures agree: 0.128 here with 60mm of STEICO Protect H, against EcoCocon’s 0.127–0.129 with 60mm of wood-fibre board.

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 straw bale wall?

A straw bale wall of 450mm bales laid flat, with 25mm of lime plaster on each face, has a U-value of about 0.18 W/m²K (0.1797), just inside the 0.18 notional value, taking the λ of 0.080 that the German building approval for construction straw bales (2014, since lapsed) gave for heat flowing along the stalks. Which way the stalks run matters more than the size of the bale: set on edge, with the heat crossing the stalks (λ 0.052), a 350mm bale gives about 0.15 — better than the thicker bale laid flat. U-Monkey adds BR 443’s correction for air gaps in a single layer of insulation, about 0.009; most published figures leave it out.

Is there a condensation risk in a straw bale wall?

A small, seasonal one if it is plastered with lime. U-Monkey’s free condensation check finds condensation forming at the back of the outer render from December, peaking at about 100 g/m² in February and drying out fully in summer, and it stays acceptable across the range of vapour values it tests for the straw. A 25mm cement render outside, which resists vapour about twice as much as lime (μ 20 against 10 in U-Monkey’s defaults), raises the peak to about 590 g/m² in March, and straw that stays damp decays, so keep both coats vapour-open and the walls protected from driving rain with a good overhang and a raised plinth.

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.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

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