Can you use other plant-based aggregates, instead of hemp?

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.

So, why use hemp?

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.

Is hempcrete better than concrete?

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.

How can I save money when building with hempcrete?

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!

Where can I learn more about hempcrete?

If you would like to discuss a particular project and whether hempcrete would be a suitable solution for you, please use our contact form and we’ll be in touch.

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