According to The Hempcrete Book by William Stanwix and Alex Sparrow, hempcrete is a “hemp-lime composite construction material … comprising the chopped stalk of the industrial hemp plant mixed with a lime-based binder”.
The authors go on to say that “hempcrete provides a natural, healthy, sustainable, local, low-embodied-energy building material that can truly claim to be better than zero carbon”.
In practical terms, the chopped hemp stalk is something like a cross between straw and wood chips. It is a dry, fibrous material and is typically packed into plastic bales. In the image to the left, some hemp has been put into buckets ready for mixing.
The “lime based binder” comes in a bag, just like a bag of cement and indeed the lime powder looks much like cement.
In my case, I used a lime binder that was specially formulated for hempcrete. The trade name is Batichanvre and the half opened bag can be seen at the top right of the photo.
The hemp I used is sold under the name Isocanna and is again meant specifically for use in hempcrete.
When mixed together with water, the lime and hemp make a kind of lightweight concrete. By varying the composition of the mix a variety of densities can be achieved, depending on the intended purpose. For instance, roof insulation requires a less dense mix than that used in wall construction.
The wet hempcrete can be used in a variety of ways within the building process. When used in walls, for example, it is often hand placed into shuttering that has been fixed over a timber frame. When it has set, the shuttering is removed, leaving the rigid hempcrete in place, as shown below.
Once dry, the hempcrete is surprisingly tough, rigid and fire resistant.
The advantages and disadvantages of hempcrete will be discussed in detail elsewhere on this site but I chose it mainly because of its thermal performance, simplicity, versatility and environmental credentials.