This article sets out what Hempcrete is, how it can be used as part of a low carbon construction strategy and some key things to consider when specifying and using it in a building.

Hempcrete is a relatively new material that uses the ancient materials of lime and hemp to create an external wall build up that has both insulative properties and medium thermal mass, which is a lovely combination. It is also a hydroscopic and breathable construction which means it creates extremely healthy internal conditions for a building. Hempcrete can be both cast in situ or cast off-site as prefabricated blocks. To date at Tate+Co we have only created buildings using in-situ casting so I will concentrate on that form of construction in this article.

Hempcrete mix

In-situ cast Hempcrete is mixed on site using a mechanical mixer to create a porridge-like substance. The mix is hemp and a lime binder and the exact proportion of the mix can vary depending on the requirements of your proposed building, so this is something you should check with your Hempcrete supplier (see at the end of this article for useful links). Generally, you put the hemp, lime binder and some water into a mixer (see below photo), and when properly mixed pour this into a bucket to transport to your wall / site for casting / construction.

The casting process

The casting process is within a timber frame on one side and timber shuttering on the other. The timber frame we have always used for Hempcrete is a prefabricated, panelised construction consisting of 140mm deep treated timber studwork with an internal lining of 9mm thick magnesium silicate board. This should include some additional horizontal timber battens fixed to the outside of the timber frame so that when the Hempcrete is cast up against it, there is more effective ‘keying’ to ensure it stays in place. On the other site of the wall we have used normal ply shuttering which is removed after the casting process.

To cast the Hempcrete your pour the mix into this cavity, generally moving up about 1 metre each time and allowing at least one day between each cast. The Hempcrete mix is tamped down with a stick from above. This tamping needs to be reasonably light in terms of pressure as, whilst you need to ensure a solid wall, you also want the density of the wall to be quite lightweight and retain lots of air for insulative purposes.

When you remove the ply shuttering you will be left with exposed Hempcrete on the outer face so it is important you have good weather protection in place as this face can be damaged by rain before it has been rendered or clad.

The finishing process

Hempcrete is a hydroscopic wall construction, which means that it expels water without the need for a cavity. However, to achieve this you need to ensure that you use the correct lime-based finishing products both internally and externally.

For the external face we have always specified a 10mm thick ‘Baumit’ self-coloured KRP lime render, with glass fibre mesh reinforcement around areas sensitive to movement (such as windows / openings or corners). We quite like a slightly stippled, rendered finish as this is more forgiving in terms of aesthetics. As an alternative, it is also possible to construct a 50mm ventilated cavity with external cladding to provide the outside face of the wall build-up; the only complications with this are the additional space needed for the cavity and the challenge of fixing the cladding back to the structural timber frame with a thermally ‘broken’ tie to avoid cold-bridges.

For the internal face we have either specified a 3mm thick Baumit K30 lime plaster on a priming coat of Baumit MC55, directly applied to the Magnesium Silicate board. As an alternative, we have left the Magnesium Silicate board exposed and install a ventilated 50mm internal service void.

Things to consider

There are a few things to think about when using Hempcrete, as follows:

Drying times – As we have only used in-situ casting for Hempcrete the on-site drying times have been critical. I would recommend casting the walls during the warmer months (in the UK this would be from May to September). We have been told that the walls should dry out within 3 to 6 weeks but have found that in winter months this can be up to 3 months. You cannot render the walls before the Hempcrete has dried properly.

Services integration – It is much better, if you have the space, to include an internal 50mm service cavity so that you can run the electrics and plumbing in the walls without affecting the Hempcrete wall build up. It is possible to cast conduits and plumbing into the Hempcrete wall, which is something we have done, but we have found this compromises the build quality and airtightness.

Air tightness – I suspect I might get pushback when this article gets published online, but we have struggled to achieve good airtightness with a Hempcrete wall construction. The breathable wall construction does not seem to lend itself to Passivhaus levels of airtightness. Typically we have achieved a q50 value of only around 8 to 10 m³/hm²@50pa so far, but of course this is something we are still working on.

Very thick wall construction – To achieve decent u-values of around 0.15 W/m²k you need very thick wall build-ups for Hempcrete of around 400mm (excluding any internal service voids or external ventilated cavities). Apart from the space required for these wall thicknesses, this also means you need to consider design strategies such as chamfered corners around window openings to maximise internal daylight.

Thermal mass – A Hempcrete wall has a higher thermal mass than a typical insulated timber frame, which means that it will be more effective in terms of preventing summer overheating, especially as global temperatures increase. In the UK this should be something which is included in your building’s SAP calculation as a Thermal Mass Parameter (TMP).

Hempcrete is not structural – There is a common misconception about Hempcrete that I think perhaps comes from the ‘crete’ part of the name. Whilst it is a cast product, it is not a structural material. In a Hempcrete wall the timber frame is providing the structure.

Structural Warranty – Some structural warranty providers in the UK will not insure an external wall build up with no ventilated cavity. If you are planning on using Hempcrete and getting a structural warranty, it is worth checking with the warranty suppliers early that they are comfortable with the construction methodology if you are planning on applying render direct to the Hempcrete.

The benefits of Hempcrete

Hempcrete is a great material if you are looking for a bio-based material with some thermal mass and breathable properties. It can be grown in the UK climate and therefore potentially locally sourced; it is not a complicated construction method; it does not require large or expensive machinery; it sequesters carbon; and it provides a really great internal environment for a home (based on our experience). Some countries, such as France, are starting to go mainstream with the material and whilst it is currently relatively unconventional in the UK, emerging developments such as prefabricated Hempcrete blocks mean it is bound to become more widespread here as well.

Useful links

UK Hempcrete – www.ukhempcrete.com

The Natural Building Store – www.naturalbuildingstore.com

The Hemp Block Company – www.hempblock.co.uk