Frequently Asked Questions

Hempcrete is made by mixing hemp shiv (the woody stem of the industrial hemp plant} together with a binder. The binder is usually either pure natural cement (a strongly hydraulic lime), or a formulated hempcrete binder made from lime mixed with a smaller proportion of pozzolans, natural additives  or Portland cement.  

Hempcrete can be wet-mixed on site and cast around a structural frame, or pre-cast off site to form blocks or panels. Although very hard and self-supporting once set, hempcrete is not load bearing. This is due to the amount of air trapped within the material and the slightly flexible nature of the hemp shiv aggregate.

Hempcrete has a medium density, which means it provides both insulation and thermal mass within the same material. It is vapour-permeable, meaning that water vapour is allowed to pass freely through the wall assembly, rather than being trapped within it, which can cause damp and deterioration to the building’s fabric.

In new-build, hempcrete is most often used to make walls, but it can also be used to form insulating floor slabs, ceilings, and roof insulation. In contrast to conventional insulation materials (which tend to be installed in a cavity within the wall, or added to the wall as an extra layer in the build-up), hempcrete forms the wall and insulation in one solid piece of material; the only other integral material being the structural frame (usually untreated softwood). 

Wet finishes, a lime or clay plaster internally and lime render externally, are applied directly to the surface of the hempcrete wall, and these are the only other materials that need to be added to the basic hempcrete wall, although cladding (timber, stone, brick etc.) can be used in place of wet finishes, if desired. 

Hempcrete is a vapour-permeable material which is hygroscopic; it absorbs moisture from the air when humidity is high (releasing it again when humidity levels drop). These properties are very important, both for the health of the building’s occupants and in order to keep the fabric of the building in good condition.  

In traditionally constructed (pre-1919) buildings, hempcrete works in harmony with the original materials, allowing the building to  “breathe”; meaning that water vapour can pass in and out of the wall rather than being retained within it to cause damp, mould, and eventually damage to the building’s fabric.  

Hempcrete is made entirely from natural materials and is naturally fire-, rot- and pest-resistant, which means there is no need for potentially toxic chemical treatment of the material. This fact, together with their hygroscopicity, means that hempcrete buildings are extremely healthy living environments.  

Hempcrete is a sustainable material. The hemp plant used as the aggregate in hempcrete absorbs so much carbon during its rapid growth that, even after the energy used in production of the lime binder, transportation and during construction  is allowed for, more CO2 is  locked up in a hempcrete wall than is used to build it. In other words, hempcrete has negative net carbon emissions; it’s a “better-than-zero-carbon” material. 

The exceptional eco-credentials of this natural, sustainable material make hempcrete the obvious choice if you want to reduce your energy bills, your carbon footprint and the overall impact of your building on the environment.

Hempcrete is vapour-permeable and hygroscopic, which means it can absorb and release moisture in response to the internal environment. This means humidity levels inside the building are naturally regulated and the risk of damp and mould eliminated. 

It is important that the finishes applied to hempcrete are also vapour-open, so the wall is able to breathe, to ensure vapour is not trapped within the wall. Lime and clay plasters are the most common finishes because they can be applied directly to the hempcrete, allowing the material to ‘breathe’ and regulate moisture.

As a relatively new material, which is quite different from most conventional building materials, cast-on-site hempcrete can be tricky to work with until some key concepts and techniques are understood. The only downside of hempcrete’s recent rapid acceptance as a building material across the UK, is the fact that examples can be found where a lack of understanding of the material and correct construction techniques resulted in problems with the build process. 

This is especially common where inexperienced contractors expect hempcrete to behave in the same way as conventional building materials. However, the problems which arise are not usually long term or serious, and with a little basic knowledge and the proper training,  casting hempcrete on site is a straightforward and rewarding way to build.  

For large commercial projects, the on-site drying time of cast-in-situ hempcrete can cause problems with the schedule as the length of drying time varies depending on the weather conditions. For this reason we always recommend a precast hempcrete application (blocks or panels) for commercial scale projects.

Not only does hempcrete provide exceptional thermal performance, significantly reducing operational carbon emissions through the lifetime of the building, but in terms of embodied carbon it is actually a better-than-zero-carbon material. This means that more carbon is removed from the atmosphere and stored in the material than is emitted by its production and installation on-site. 

The amount of carbon sequestered in a hempcrete wall varies depending on the exact type and source of materials, as well as on the application technique. Depending on the material type and source, estimates range from 60-165kg of CO2 sequestered per cubic metre of hempcrete.

Of course there is nothing stopping you from using a different plant-based aggregate and there are plenty of people experimenting with new materials. However, hemp has been proven to be a highly-performing aggregate and there are a range of hempcrete binders already on the market that have been developed to work specifically with hemp shiv as an aggregate.

The strong cellulose of the hemp stalk makes it highly durable and when bound in lime, it is capable of going from wet to dry almost indefinitely without degrading. A hempcrete wall has a good ability to absorb and release moisture in response to the internal environment, and has a lot of air trapped within both the cell structure of the shiv and the matrix of the hempcrete itself.  Combined with the dense mass of the lime binder, this means the wall has both insulation and a good amount of thermal mass, in addition to being vapour-open and hygroscopic; meaning it offers excellent thermal and humidity regulation in the internal space.

In addition to its technical performance, storing hemp shiv in the wall has huge environmental benefits. The main environmental benefit is the very high level of atmospheric carbon that gets sequestered within the material. The reason the hemp plant absorbs so much carbon dioxide is because it is a tall, fast-growing plant which needs to create a hard woody stem to support itself at its full height. It grows up to 4 ½ metres in 4-5 months in the UK climate. The hemp plant is naturally pest-resistant and weed-suppressant, eliminating the need for chemical fertilisers and insecticides, and is useful as a break crop, naturally clearing the land of pests. It requires very little fertiliser, and is deep rooting; returning key nutrients to the soil and improving the condition of our over-compacted, depleted farmland by breaking up and aerating the soil to a significant depth.

It depends what you mean by ‘better’, as the two materials are good at different jobs. Whilst the name can be misleading, it is worth noting that hempcrete and concrete are very different materials and are not used in the same way. The term ‘hempcrete’ is often used because historically it has been mixed and cast on site into a shuttered framework, in a similar way to concrete. Concrete uses a binder of cement and an aggregate of sand mixed with other larger materials like gravel, stone or rubble. In comparison, hempcrete uses a lime-based binder and the hemp shiv is the aggregate.

Hempcrete (in its usual “hemp-lime” form) has been developed as a non-structural material. Therefore, it cannot replace concrete for structural applications. Also, due to the plant-based aggregate in the material, hempcrete does not perform well in locations where it is exposed to standing water, or constantly has water running across it. Therefore unlike concrete, it cannot be used in below ground or foundation applications. 

Whilst concrete is good at keeping water out, it can also trap moisture inside the building, meaning that it relies heavily on mechanical ventilation systems to purge moisture from the interior. Hempcrete is vapour-permeable and hygroscopic, which means it can absorb and release moisture in response to the internal environment. This means humidity levels are naturally regulated and ventilation systems, if required, can be a lower spec and allow cost savings.

Whilst hempcrete and concrete both act as heat stores due to their thermal mass, hempcrete also has far superior insulation properties. Whilst concrete has been receiving a lot of bad press recently due to its enormous carbon emissions, hempcrete actually sequesters more carbon than it emits (as outlined above) so is actually having a net positive impact on the levels of atmospheric carbon.

In comparison to conventional concrete cavity systems, hempcrete is generally a simpler construction with simply a structural frame, the hempcrete and the finishes. This makes it more accessible to self-builders and can save time and money in construction.

As with all construction, the best way to ensure cost-efficiency when building with cast on site hempcrete is to be efficient with the material. This means ensuring you have a designer who understands how hempcrete works, an engineer who understands its racking abilities, an installer who does not over tamp the material and a project manager who manages the construction schedule to allow sufficient drying. It may be tempting to accept a low fee for technical detailing, but this often ends up being far more expensive when the building is not designed correctly or efficiently.

Hempcrete is a low-tech material because, unlike conventional construction techniques that rely on layers of materials doing different things, hempcrete does the work of several materials. Installing cast-on-site hempcrete is labour-intensive, so much of the construction cost is in the labour. If you are able to install the hempcrete yourselves (ideally with family and friends), a large proportion of the cost can be saved, though it’s important to not underestimate how much work is required to install hempcrete to an entire house!

If you would like to discuss a particular project and whether hempcrete would be a suitable solution for you, please get in touch with us by phone at 01629 343143 or by email at: office@ukhempcrete.com.

For further reading, these books are all widely available:

The Hempcrete Book: Designing and building with hemp-lime | William Stanwix and Alex Sparrow

Hemp Lime Construction: A Guide to Building With Hemp Lime Composites | Rachel Bevan and Tom Woolley

Building with Hemp | Steve Allin  

Clay and Lime Renders, Plasters and Paints: A how‑to guide to using natural finishes  | Adam Weismann and Katy Bryce