Flat House by Practice Architecture. Photo by Oskar Proctor.
Hempcrete, a highly sustainable building material, has been gaining popularity in the UK for both new builds and heritage restoration projects. This innovative material was originally developed in France over 40 years ago to restore old timber-framed buildings.
Composed of hemp shiv (the woody stem of the industrial hemp plant) and a binder (typically hydraulic lime or a formulated mix), hempcrete is a lightweight yet dense material. Renowned for its excellent insulation and thermal mass properties, hempcrete offers a unique combination of benefits for construction.
In this article, we explore hempcrete's eco credentials, types, advantages and disadvantages, uses, costs, and installation methods to help you determine its suitability for your building project.
Exposed hempcrete interior by Practice Architecture. Photo by Oskar Proctor.
There are few building materials more sustainable than Hempcrete and it outperforms timber in carbon sequestration and renewability.
Research from the University of Cambridge shows industrial hemp absorbs 8–15 tonnes of CO₂ per hectare annually, compared to 2–6 tonnes captured by forests. Hemp grows to maturity in just four months, enabling multiple harvests within the lifespan of a single forest. Additionally, hempcrete is carbon-negative, locking in the CO₂ absorbed during growth and biodegrading without harmful emissions.
With its rapid renewability and closed-loop lifecycle, hempcrete is an eco-friendly alternative to carbon intensive building materials like plastic foam insulation and infill concrete blockwork.
There are two main types of Hempcrete: cast and precast blocks.
Cast-on-site hempcrete is installed in a similar way to in-situ concrete. Shuttering is erected and a wet hempcrete mix is either hand poured and lightly tampered down or sprayed in layers. Cast hempcrete panels can also be prefabricated off site to speed up construction.
Precast hempcrete blocks are available in different sizes and are assembled in a similar way to concrete blocks and bonded together using a special adhesive mortar.
Additionally, cast hempcrete can also be made into prefabricated panels and assembled on site.
Images clockwise from the top: Hand-cast hempcrete; sprayed hempcrete (photos by Hempcrete UK); hempcrete blocks (photo by That Hempcrete Guy).
Beyond its impressive environmental credentials, hempcrete is an ideal complement to timber frame construction.
Traditional timber frames have low thermal mass, making them inefficient at absorbing and releasing heat, and poor at regulating indoor temperatures. Hempcrete's density can significantly increase the thermal mass of timber construction, providing thermal comfort in both winter and summer whilst remaining lightweight enough to avoid excessive structural loads.
Hempcrete is also vapour permeable and hygroscopic, meaning it freely absorbs and releases water vapour. This prevents moisture from becoming trapped within the structure, reducing the risk of damp and rot.
Also, hempcrete is naturally fire and pest resistant, making it a well-rounded material choice for building projects.
House extension with hand-cast hempcrete walls by Cairn Architects. Photo by James Retief.
Unfortunately, no. While both concrete and hempcrete can be cast in-situ or laid using precast blocks, hempcrete is non loadbearing (its compressive strength is approximately 17% of concrete) and must be installed between a structural frame.
Think of concrete as a primary, structural material that supports the building and hempcrete is a secondary, infill material used for insulation and thermal mass.
Aside from the fact that it is non-loadbearing, its classification as "non-standard construction" can make securing mortgages and insurance more challenging.
Hempcrete is also less widely available, which can make it more expensive and create logistical challenges for larger projects.
In the UK, strict licensing laws for hemp cultivation mean much of the hemp used is imported from mainland Europe. This reliance on imports increases costs and reduces the environmental benefits of local sourcing.
From a practical perspective, cast-on-site hempcrete has long drying times, typically 6–12 weeks depending on wall thickness and weather. This can extend the overall construction programme.
Hemp house by Nimtim Architects incorporates exposed hempcrete walls set within a structural timber frame. Photo by Megan Taylor.
Hempcrete has proven to be an excellent material for insulating many types of historic buildings, including masonry and timber frame structures. Its vapour-permeable and hygroscopic properties, along with its ability to mould to irregular shapes and surfaces, make it ideal for this purpose.
In new-build construction, hempcrete is most commonly used to infill and insulate timber framed walls, providing excellent thermal and acoustic performance. It can also be installed within suspended timber floors and pitched roofs, offering a dense yet lightweight and breathable insulation solution.
As hempcrete technology evolves, it is increasingly being used for other applications, such as insulating screeds in solid floor construction.
A new-build pitched roof insulated with hempcrete. Photo by UK Hempcrete.
Hempcrete is sometimes described as being ‘monolithic’, meaning it can perform multiple functions in a single layer of construction. While it can reduce build-up complexity, this isn’t always entirely accurate.
Firstly, it requires a structural frame for support.
Secondly, the term can only be used to loosely describe certain wall applications. When used in other parts of a building more component parts are needed.
Finally, it needs to be finished externally with another material. If lime render is used the build-up is very simple as the render is applied directly on to the hempcrete. However, when clad with other materials, say brick or timber, the overall build up is similar to timber frame construction requiring multiple layers of materials.
Simple hempcrete wall construction: 1. Cast hempcrete; 2. Timber structure; 3. Internal lime plaster; 4. External lime render.
Hempcrete wall with timber cladding: 1. Cast hempcrete; 2. Timber structure; 3. Exposed hempcrete finish or lime plaster or additional insulation and lime plaster or hempcrete blocks with interior finish; 4. Vapour permeable sheathing board; 5. Rigid wood fibre insulation boards; 6. Breather membrane; 7. Battens for cladding as required; 8. Timber cladding.
The properties of cast hempcrete vary depending on the mix and installer skill, with a thermal conductivity of 0.06–0.07 W/mK and a density of 200–400 kg/m³. Pre-manufactured hempcrete blocks are more consistent, offering a thermal conductivity of around 0.07 W/mK and a density of 330 kg/m³.
Rigid wood fibre board, a common alternative, has a density of 160–250 kg/m³ and a thermal conductivity of 0.038–0.050 W/mK. Higher-density boards provide more thermal mass, while lower-density boards offer better insulation.
To achieve a wall with a U-value of 0.17 W/m²K, you would need a 350mm thick hempcrete compared to 140mm of flexible wood fibre (0.036 W/mK) paired with 60mm of rigid wood fibre board (0.038 W/mK).
Wood fibre is therefore more space efficient while hempcrete offers superior thermal mass.
As a very rough estimate, using the 0.17 W/m²K new-build wall example mentioned above, here is an approximate breakdown of costs.
Cast Hempcrete
Precast Hempcrete Blocks
Compared to other sustainable insulation products, hempcrete is more expensive. For example, wood fibre insulation costs approximately £30/m² supply and £20/m² installation. However, overall savings can be achieved if you are building a rendered structure (as hempcrete reduces the number of components in the wall build-up) or self-building (since the material is well-suited to DIY construction).
Please note that build costs will vary widely depending on the specifics of your project, so it’s essential to contact installers for detailed quotes when budgeting.
Self-build garden office by Commonbond Architects, constructed with hempcrete walls finished externally with rough lime render and internally with clay paint.
Once the main structural frame is constructed, hempcrete can be installed using cast hempcrete, spray-applied hempcrete, or hempcrete blocks.
Hand Cast Hempcrete
Hemp shiv, lime binder, and water are mixed and poured into shuttering in 150 millimetre deep layers. Each layer is lightly tamped to ensure even distribution without over-compacting, which reduces breathability. Shuttering is removed after 24–48 hours, while full curing takes around 4–6 weeks depending on the weather.
Spray-Applied Hempcrete
Spray-applied hempcrete involves mixing hemp shiv and lime binder using specialised equipment. The material is sprayed directly into temporary or permanent formwork around the structural frame. This method is faster than hand casting and eliminates the need for tamping. However, drying and curing times are similar.
Hempcrete Blocks
Blocks are laid with staggered joints using a thin lime adhesive mortar on top of a damp proof course supported by a loadbearing base. Galvanised ties secure the blocks to the structural frame, and gaps between the top row of blocks and the ceiling are filled with flexible insulation.
All methods require finishing with breathable renders or cladding (e.g., lime render or timber) to maintain vapour permeability and protect the structure.
Hempcrete block installation. Photo by That Hempcrete Guy.
Hempcrete has been successfully used in the UK for many years in both renovation and new build projects. It boasts exceptional environmental credentials along with excellent breathability, thermal mass and moisture-regulating properties, making it particularly suitable for older buildings and new build timber frame structures. It can also simplify wall construction when finished with render.
However, hempcrete can be expensive, and alternative materials like wood fibre offer similar benefits at lower cost. The long installation times (when cast) can make construction more challenging and you’ll need to check your finance and insurance options before using the material to get the best loan rates and coverage.
Consider hempcrete if you:
Hempcrete is not load-bearing and requires a structural frame for support. It is primarily used as a non-load-bearing insulating infill material.
For a typical residential project, hempcrete walls are usually 300-400mm thick. The exact thickness depends on factors such as thermal performance requirements and structural design. A 350mm thick hempcrete wall typically achieves a U-value of 0.17 W/m2K.
Hempcrete generally needs to be finished with an external material. Lime render is commonly used for simplicity, but it can be clad in almost any material. Internally, hempcrete can be left exposed if desired.
If hempcrete completely envelops the structural frame, additional insulation is not necessary. The thermal properties of hempcrete are sufficient. However, if it is installed only between the structural elements, additional insulating material, such as rigid wood fibre, may be needed on the outside of the frame.
Hempcrete is highly fire-resistant. Its composition of hemp shiv and lime contributes to its naturally high burning temperature and fire-resistant properties.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
