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Hempcrete: thermal conductivity, R-value and fire behaviour

What the measured data says about how hemp-lime walls insulate and how they behave in a fire — and where the common claims go further than the evidence.

2026-09-21

What hempcrete is

Hempcrete is a mix of hemp shiv — the woody core of the stalk — with a lime-based binder and water. Cast around a timber frame or pressed into blocks, it forms a wall that insulates, stores moisture and lets water vapour pass through.

It is an infill, not a structural material. The frame carries the load; the hempcrete fills and insulates it. Every figure below should be read with that in mind: these are the properties of an insulating wall material, not of a concrete that holds a building up.

Thermal conductivity

Thermal conductivity, written λ and measured in watts per metre-kelvin, describes how readily heat passes through a material. Lower is better for insulation.

For hempcrete mixes with dry densities between 200 and 400 kg/m³, published measurements fall in the range of roughly 0.06 to 0.11 W/(m·K). Denser mixes, with more binder relative to shiv, conduct more heat: mixes in the 450 to 800 kg/m³ range have been measured at 0.12 to 0.18 W/(m·K).

Density is by far the dominant factor. In one systematic study a 67 per cent increase in density raised thermal conductivity by 54 per cent, while taking the material from completely dry to 90 per cent relative humidity raised it by only 15 to 20 per cent. For a designer this means the mix and the compaction matter more than the climate.

Because results depend so heavily on how a sample is prepared and measured, the RILEM technical committee on bio-aggregate building materials ran round-robin tests across several laboratories and published a recommended protocol for characterising hemp shiv, thermal conductivity included. Figures quoted without saying how they were measured should be treated with caution.

From conductivity to R-value

Thermal resistance, R, is what a wall actually delivers. For a single layer it is simply the thickness divided by the conductivity: R = d / λ, in m²·K/W. Higher is better.

The table below applies that formula to the measured range. It is arithmetic, not a test result, and it ignores the frame, renders and junctions, which a real wall calculation has to include.

Moisture

Hempcrete is vapour-open: water vapour passes through it rather than condensing inside the wall. It also absorbs moisture from the air when humidity rises and gives it back when the air dries, which evens out indoor humidity without mechanical help.

This is measured as the moisture buffer value, in grams per square metre per percentage point of humidity change. Published values for hemp-lime run from about 2.1 to 3.5: one study found 3.47 for a lighter mix at 290 kg/m³ with a binder-to-hemp ratio of 1.2 to 1, another 2.14 for a denser mix at 430 kg/m³ with a ratio of 2 to 1. Values above 2 are conventionally described as excellent.

The pattern repeats the one seen for heat: the lighter the mix, the better it performs. A dense, binder-rich hempcrete is stronger, but it insulates and buffers less.

Fire behaviour under EN 13501-1

Across Europe, reaction to fire is classified under EN 13501-1. The classes run from A1 to F. A1 and A2 are the non-combustible classes — stone, concrete, glass, mineral wool, ceramics. Classes B to F are combustible, in increasing order.

Hempcrete is not an A1 material, and claims that it is should be read with care. Two-thirds of it by volume is plant matter, and plant matter burns. The honest position is more interesting than the exaggeration.

In fire testing, unrendered hemp concrete has been classified in class B, while hemp-lime render has reached class A2 — so a mineral render improves the result. Reviews of bio-based concretes report that the additional ratings for smoke and flaming droplets are consistently at the best level, s1 and d0.

The difference between the raw ingredient and the finished material is large. In a 2023 study, loose hemp shiv ignited with sustained flaming and a relatively high heat release rate. Hemp blocks made from the same shiv showed no flaming ignition at all — only smouldering — with a heat release rate an order of magnitude lower, and very little smoke. The lime is what makes the difference.

One caveat belongs in any honest account. Because the material is porous, smouldering rather than flaming is its dominant way of burning, and standard fire tests were not designed around smouldering. It is a behaviour worth planning for, not a reason to avoid the material.

Carbon

Two things store carbon in a hempcrete wall. The hemp itself drew carbon dioxide from the air as it grew. The lime binder then reabsorbs carbon dioxide as it cures, turning back into calcium carbonate over months and years.

Research has examined the theoretical limits of both effects. The word theoretical matters: how much carbon a real wall stores depends on the binder, the mix, the transport and the life of the building, and published figures vary widely with those assumptions. A single headline number for “carbon-negative” walls is best treated as the start of a calculation, not its result.

What this means in practice

Hempcrete is a good insulator when it is made light, and a worse one when it is made dense; the mix decides. Its fire performance is genuinely good for a bio-based material — class B unrendered, A2 with a mineral render, minimal smoke and no flaming droplets — but it is not non-combustible, and it tends to smoulder. And it stores carbon, in amounts that depend on choices a designer makes.

None of that needs overstating. The measured record is strong enough on its own.

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