- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 24 Sep 2020
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.
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.
 Find out more
 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.
Featured articles and news
Can the profession use its skills to save the world from climate change?
How faulty science resulted in sanitation reform.
Improving facilities, accessibility and overall appearance.
Free download of TG 12/2021 available.
TESP works with The Youth Group to form skill sharing network.
Big tech collaborates on platform for the built environment.
Letter signed by 21 organisations sent to MHCLG.
A look at the Government's strategic approach.
Steps to help reduce the spread of infection inside buildings.
This social media-centred hobby can be both dangerous and illegal.
Millwork wall treatment with a long and illustrious history.
HSE introduces cumulative exposure calculator.
The Edwardians and their houses.
Cut off from civilian life for over 900 years.
Click the button to subscribe.