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.