The basis of hempcrete is hemp, the balsa wood-like core (or ‘Shiv’) of a cannabis sativa plant. Hemp can be combined with lime and water to form hempcrete. The hemp has a high silica content, a unique property among natural fibres, which allows it to bind well with the lime. As a lightweight cementitious insulating material, it weighs only one-seventh to one-eighth that of concrete.
Hemp as a building material has been used across Europe for centuries, and as a modern building material, industrial hemp is now grown by certified commercial growers. The strain of plant grown for hempcrete contains 0.3% of THC, the ingredient in cannabis that provides its psychoactive nature. This is compared to the THC content found in hallucinogenic and medicinal varieties of between 5-10%.
While it is legally grown in Europe and Canada, growing hemp in America has been illegal for several decades. However, with the country’s gradual liberalisation of laws regarding the substance for medicinal and personal use, this may change.
The material is mixed for 1-2 minutes before being applied, either into the wall cavities, or slip-forming with temporary timber or plastic shuttering. A hard render coating is generally applied as a finish on exterior surfaces with a thickness of around 20 mm. The interior can be left 'natural' or, for a traditional aesthetic, finished with lime plaster.
Hempcrete is very durable and has a number of other beneficial properties.
Similar to other natural plant products, carbon dioxide is absorbed from the atmosphere by hemp as it grows. During the curing process, as lime turns to limestone, the carbonation of the lime adds to this effect. As a result, hempcrete has a negative carbon footprint, with considerable potential for sustainable building.
It is able to naturally regulate a building’s humidity and temperature, which can reduce condensation and energy consumption, and improve thermal comfort for occupants. It provides natural insulation that is airtight, breathable and flexible. It is also toxin-free, impervious to mould and pests, and highly fire-resistant.
It is very suited to areas at risk of seismic activity since it is a low density material that is resistant to cracking under movement. The outer portion of the plant’s stalk can also provide fibres for building textiles.
However, hempcrete has a typical compressive strength of around 1 MPa, which is around 1/20 that of residential grade concrete, and has a density 15% that of concrete. This means that hempcrete walls must be used together with a load-bearing frame of another material.
 Related articles on Designing Buildings Wiki
- Cavity wall insulation.
- Cellular concrete.
- Could the buildings of the future be made with bones and eggshells?
- Earth building.
- Floor insulation.
- Green building.
- Hemp lime construction: A guide to building with hemp lime composites.
- High alumina cement.
- Straw bale construction.
- Thermal insulation.
 External resources
- American Lime Technology - What is hempcrete
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