By Alex Sparrow
Disclaimer: The author provides this overview of hempcrete construction in the spirit of advancing the knowledge and skills of people wanting to use the material. This article does not constitute a full training programme in the use of hempcrete and it remains the responsibility of designers and builders to detail and specify materials for their own buildings, and to ensure that contractors are fully trained. The author can accept no liability for the actions of his readers.
Following on from Tom Woolley’s article in the last issue of TLS, I thought some practical tips on building with hemp-lime would be a natural progression (if you’ll forgive the pun). My business partner William Stanwix and I run a company called Hemp-LimeConstruct, which has been building commercially with hempcrete in the UK for 6 years now, both in new-build (from small extensions, of domestic houses to large community buildings) and in the restoration and retro-fit of traditional and historic buildings. We have learned a great deal along the way, and now provide consultancy and training services to others wishing to use hempcrete; self-builders, construction industry contractors and architects alike. The desire to disseminate information to others has culminated in our forthcoming book The Hempcrete Book: Designing and building with hemp-lime which is to be published later this year.
In this two-part article I’d like to share some key tips and important “dos and don’ts” when using the material in construction. In this, the first part, after discussing terminology and the basics of using hempcrete in construction, we will go on to look at the first steps in constructing a hempcrete wall taking us as far as the construction of the structural timber frame. In the next issue of The Last Straw the concluding part will cover key techniques for mixing and placing hempcrete together with essential tips for the drying stage.
To start with, let’s look at the terminology; there are numerous ways of using hemp in construction and the different methods can sound very similar:
“Hempcrete” or “Hemp-lime”: A hemp-lime composite, non-load bearing construction material providing both insulation and thermal mass. The hemp shiv (chopped hemp stalk) forms the majority of the composite, and creates an open matrix structure with a lime (or lime-based) binder coating each particle of shiv and binding them together.
“Lime-hemp plasters”: A lime plaster (either based on fat lime putty- or hydraulic lime-sand mix) with the addition of a small proportion of hemp fibres (usually the strong bast fibres – the long fibres which wrap around the outside of the stem – but the woody hemp shiv can also be used). The addition of the hemp provides a little insulation to the finished plaster, improves its hygroscopicity, and also adds strength in tension (in the same way that hair does in traditional lime plastering), allowing the application of thicker layers of plaster, and preventing shrinkage cracking. Hygroscopicity refers to a material’s ability to absorb moisture from the air during periods of high humidity, thus maintaining healthy indoor air within the building.