Hempcrete: A Bio-Based Building Material Explained

Hempcrete is a lightweight building material made from the woody core of the hemp plant mixed with a lime-based binder and water. Despite the name, it is not a concrete in the structural sense. It is an insulating, vapour-open infill that is cast around a frame, sprayed, or made into blocks, and it has become a reference point in conversations about low-carbon construction.

This article sets out what hempcrete is, how it behaves structurally, thermally and in fire, what its carbon claims rest on, and the practical questions an engineer would ask before using it, particularly in a hot, humid climate like Singapore’s.

What hempcrete is made of

The aggregate is hemp shiv (also called hurd): the chopped, woody inner core of the hemp stalk, left after the outer fibres are removed for textiles and rope. Shiv is very porous and light, which is where most of the material’s insulating value comes from.

The binder is usually based on hydrated lime, often with small additions of hydraulic lime, pozzolans or cement to speed up the initial set. Lime is chosen because it is vapour-permeable, alkaline and compatible with the plant material. Water is added to activate the binder and make the mix workable.

The result is a material with a density of only a few hundred kilograms per cubic metre, far lighter than normal concrete. Density is controlled by the mix proportions and by how hard the mix is compacted, and it is the single biggest factor in the final properties: denser mixes are stronger but insulate less well.

How hempcrete is built

There are three common methods:

Cast in place. The mix is placed by hand into temporary shuttering around a structural frame, usually timber, and lightly tamped. The shuttering is moved up as the wall rises.
Sprayed. A machine projects the mix against a permanent or temporary board, which is faster on larger walls and gives a lower, more even density.
Precast blocks or panels. Blocks are made and dried in a factory, then laid with a lime mortar or glued. Prefabricated panels with an integral frame are also available in some markets.

Whichever method is used, the structure that carries the building is normally separate. A timber frame is most common, as described in our guide to timber frame construction, though steel and concrete frames have also been infilled. The hempcrete wraps the frame, protecting it and providing insulation, airtightness and a substrate for lime render or plaster.

Structural behaviour: why it is not load-bearing

Research on hemp-lime consistently reports low compressive strength, typically well below that of masonry; a review of studies gives values from around 0.05 to just over 1 MPa for common mixes, depending mainly on density. For comparison, a normal structural concrete is specified in tens of megapascals. Hempcrete also deforms a great deal before failure, which is a useful ductile trait but not a basis for carrying floors and roofs.

In practice, hempcrete is treated as non-load-bearing. It carries its own weight and some racking resistance can be gained where it encases a frame, but the structural engineer designs the frame to take all vertical and lateral loads without relying on the infill. Fixings for cabinets, handrails and services also need to go back to the frame or to dedicated battens, because the material has little pull-out capacity.

Shrinkage and early-age movement are small, but the walls are thick, often 300 mm or more in cold climates, which affects floor area and foundation widths. The frame and the hempcrete must be detailed so that differential movement and drying do not open cracks at junctions with windows and floors.

Thermal and moisture performance of hempcrete

Hempcrete is valued in temperate climates for combining moderate insulation with some thermal mass and very good moisture buffering. The porous shiv and lime absorb and release water vapour as indoor humidity changes, which helps even out peaks in humidity and makes the wall forgiving of small amounts of moisture.

Two cautions follow. First, its insulating value per millimetre is lower than that of mineral wool or rigid foam boards, which is why walls are thick. Second, the material only works as intended if it can dry. Walls should be finished with vapour-open lime renders, plasters or limewash, not with cement render, impermeable paint or vinyl wallpaper, which trap moisture inside the wall.

Drying time is also a construction issue. A freshly cast wall contains a lot of water and can take weeks to months to dry, depending on thickness, weather and ventilation. Finishes cannot go on until it has dried sufficiently.

Fire, pests and durability

Hemp shiv is a plant material, but when fully coated in lime binder it is difficult to ignite, and published fire tests, including a reported one-hour fire-resistance test on a lime-plastered sprayed wall in the United States, have been encouraging. However, fire performance depends on the system, the thickness and the finishes, so a project must rely on test evidence for the specific product and build-up rather than on the general reputation of the material.

The alkalinity of lime discourages mould and insects within the material itself. The timber frame it encases is a different matter, particularly in termite-prone regions, and should be treated and detailed in the same way as any other timber structure. Our article on termites in Singapore sets out the risk locally.

Durability depends heavily on keeping liquid water out. A good roof overhang, a raised plinth to keep the base of the wall above splash height, and a well-maintained lime render are the traditional protections. Hempcrete should never sit in standing water or be in direct contact with the ground.

Carbon claims and what they rest on

Hempcrete’s appeal is largely about carbon. Hemp grows quickly and absorbs carbon dioxide as it grows; that biogenic carbon is stored in the shiv for as long as the wall stands. Lime binders also reabsorb some carbon dioxide over time as they carbonate. Against this sits the energy used to burn lime, process the hemp and transport both.

Several life-cycle assessments report a low or even net-negative embodied carbon balance for the wall material alone, but results depend on assumptions about binder content, transport distances and how biogenic carbon is accounted for. A whole-building comparison must also include the frame, foundations, finishes and any extra floor area lost to thick walls. Our explainer on embodied carbon describes how these assessments are structured.

Is hempcrete suitable for Singapore?

Several factors make hempcrete harder to justify in Singapore than in Europe. The climate is consistently hot and humid, which slows drying on site and removes much of the benefit of thick insulation and thermal mass in a climate where cooling, shading and ventilation matter more. Hemp is not grown locally, so shiv would have to be imported, and import controls on hemp-derived materials should be checked early.

There is also no established local standard or track record. A designer would need to demonstrate compliance with structural, fire and building envelope requirements through test evidence and engineering justification, and a Qualified Person would carry responsibility for that case. For most Singapore projects aiming to reduce carbon, more established routes such as mass timber, low-carbon concrete and passive design for sustainable architecture are likely to deliver more with less risk. Hempcrete is better suited to interior, non-loadbearing, sheltered applications or demonstration projects where its behaviour can be monitored.

If you are considering a bio-based or experimental envelope on a project, involving a structural engineering consultancy early helps set out the testing and approvals path before design is fixed.

Frequently asked questions

Is hempcrete as strong as concrete?

No. Hempcrete has a very low compressive strength, typically around one megapascal or less, compared with tens of megapascals for structural concrete. It is used as an insulating infill around a separate structural frame and should not be relied on to carry floors or roofs.

How long does hempcrete take to dry?

Drying depends on wall thickness, mix, method and weather, and can range from several weeks to a few months. Thin sprayed walls in warm, ventilated conditions dry faster than thick cast walls in humid or cold conditions. Finishes should only be applied once the wall has dried sufficiently.

Is hempcrete fireproof?

No building material is fireproof, but hemp-lime walls have performed well in fire tests because the lime coats the plant particles. Performance varies with the system and finishes, so certified test evidence for the specific product is needed for regulatory purposes.

Can hempcrete be used with cement render?

It is not recommended. Cement render and impermeable paints trap moisture in the wall, which can lead to decay of the shiv and the frame. Vapour-open lime renders, lime plasters and limewash are the usual finishes.

Is hempcrete carbon negative?

Some life-cycle studies report a net-negative carbon balance for the wall material, because the plant absorbs carbon as it grows. The result depends on binder content, transport and accounting methods, so claims should be checked against a project-specific assessment that includes the whole building.

Exploring an unconventional material or low-carbon envelope? Talk to our consultancy or get in touch.