Hempcrete and hemp construction materials

Hempcrete is hemp shiv — the woody core of the stalk, left after the fibre is stripped — mixed with a lime binder and cast or sprayed around a structural frame. Almost every misunderstanding of it comes from the second half of its name.

It is not structural, and this is not a shortcoming

Hempcrete’s compressive strength is in the region of 0.2 to 1 MPa. Structural concrete is 20 to 40 MPa and upward. It is one to two orders of magnitude away, and no formulation adjustment closes that gap, because the low strength is a direct consequence of the low density that makes it insulating.

A hempcrete building therefore always has a separate structural frame — usually timber — and the hempcrete is cast around it as infill. Its job is envelope: insulation, air-tightness, moisture regulation and fire resistance. Compared against insulation, it performs well; compared against concrete, it is not competing.

Density runs roughly 250–500 kg/m³ and thermal conductivity roughly 0.06–0.12 W/m·K, depending on binder ratio and compaction. That conductivity is poorer than a dedicated foam insulant, and hempcrete’s argument is that it delivers several envelope functions in one monolithic, vapour-open, non-toxic material.

Two porosities doing two different jobs

The material’s behaviour comes from porosity at two scales.

Macroporosity between shiv particles traps air, and still air is the insulator — as in every fibrous or cellular insulation, the solid’s job is to prevent the air from convecting.

Microporosity within the shiv itself — the plant’s own vessel and cell structure — makes the material strongly hygroscopic. This is what produces the property hempcrete is genuinely distinctive for: moisture buffering. The wall adsorbs water vapour when indoor humidity rises and releases it when humidity falls, damping the swings.

The thermal side of this is often missed. Adsorption releases latent heat and desorption absorbs it, so a hygroscopic wall responds to a temperature change with a moisture change that opposes it. The effect adds to the material’s apparent thermal inertia — an effective thermal mass considerably larger than its density and specific heat alone would predict.

The lime binder, and what carbonation actually reclaims

Air lime is made by calcining limestone: CaCO₃ heated to around 900 °C gives CaO and releases CO₂. Slaking with water gives calcium hydroxide, Ca(OH)₂.

It sets by carbonation — the reverse reaction. Ca(OH)₂ absorbs CO₂ from the air and reverts to calcium carbonate, releasing water. So the binder reabsorbs carbon dioxide that its own manufacture emitted.

Two honest qualifications belong with that. The reaction reclaims the process CO₂ from calcination, not the fuel CO₂ burned to reach 900 °C, and in practice not all of it. And carbonation is slow and requires access to air and the right moisture level, proceeding inward from exposed surfaces, so a thick wall carbonates over months to years and the binder’s strength develops on the same timescale.

Hydraulic components are commonly blended in to give an initial set that does not depend on carbonation, which speeds construction and slightly changes the carbon arithmetic.

Lime’s alkalinity has a second function worth naming: the high pH inhibits fungal and insect attack on the plant material and mineralises the shiv surface. The organic component is protected by the binder chemistry rather than by an added biocide — the same principle as non-biocidal wood modification, reached by a different route.