Hempcrete is a building material with many interesting properties for those interested in sustainability and construction or renovation. It combines a hemp shiv with a lime matrix to give it high elasticity and vapour permeability to help create a healthy building that actually locks up atmospheric carbon.

Of course, hemp is the English name for the cannabis plant and the shiv, the inner woody stem, is the part of the plant that is used for this process. The plant is one of the earliest recorded domestically grown plants with a long history of widespread use for a range of important products.

To a certain extent it can be used structurally, although its typical compressive strength is around 1MPa, over 20 times lower than low grade concrete. Hempcrete's density is 15% that of traditional concrete.

Tomorrows Garden City a 60 home scheme completed in 2012 at Letchworth Garden City

Hempcrete was used in Tomorrow’s Garden City, a 60 home scheme completed in 2012 at Letchworth Garden City, built by SDC and designed by architect Cole Thompson Anders, for housing association North Hertfordshire Homes. Many of the units were built using hempcrete from Lime Technology, and the rest were built using Pavatex wood fibre materials from Natural Building Technologies.

It is long lasting, does not shrink and is pest, water and fire resistant, with a low embodied energy. More than that, it actually locks up atmospheric carbon in the structure itself (just as timber does) in the hemp component; and, although the process of making lime reduces carbon dioxide, as it dries it also absorbs the gas back from the atmosphere.

This means that, in great contrast to Portland cement and other building materials, it acts as a carbon sink. The authors say: "timber products can also claim to be sequestering carbon; however, hemp is superior to wood in this respect since it absorbs CO2 much more quickly, creating a very hard woody stem (2-4 m in height) in only 4-5 months."

The Hempcrete BookA new book, The Hempcrete Book, by William Stanwix and Alex Sparrow, who between them have much experience of constructing with hemp-lime, tells those who wish to work with it everything they would need to know.

As the authors say, "Hempcrete is especially attractive to self-builders and community groups because of the relatively low-tech nature of the construction method. Also, owing to the fact that it's relatively labour-intensive, big savings can be made by providing your own labour".

It's a relatively new building material and there have been some mistakes made by subcontractors which have made those who have heard these stories wary of using the material, most famously in a social housing project begun by Grand Designs presenter Kevin McCloud in Swindon. But the authors explain how these mistakes came about and how to avoid them, mainly by using suitably qualified and experienced people to mix and apply the material.

With a k-value of between 0.12 and 0.13 W/mK, the material offers some insulation value but is primarily of interest because of its breathability, which lends it to use with other natural building materials to create buildings which have a pleasant internal atmosphere that does not suffer from damp or condensation.

It is easy to install (possible with dry-spray machinery) and for vertical services or walls it needs to be cast onto temporary shuttering as permanent impermeable shuttering would prevent the material from performing.

The pioneers in the use of hempcrete are Tom Woolley and Rachel Bevan, who assisted with the construction of the WISE building 10 years ago at the Centre for Alternative Technology and did the research for BRE that led to the publication of Hemp Lime Construction in 2006-7. (See the video below.)

Since then there have been a great many new buildings constructed. The material has become almost commonplace in the UK and The Hempcrete Book brings up-to-date all the knowledge and experience gained in recent years.

 building with HempcreteThe book is full of many of these examples, such as a social housing project at Callowlands in Watford which is built using the timber frame and cost-in-situ hempcrete. Some designs are thoroughly modern and, to the untrained eye, would look no different from any other rendered construction. Others are built in the style of vernacular cottages, since there is a lineage going back to mediaeval times of using wattle-and-daub, of which hempcrete is the proud, contemporary descendant.

For this reason it is an ideal material for renovating old houses in a sustainable manner. For instance the material can be used for external insulation. A range of rendered finishes are possible, which must be vapour permeable and the authors explain the choices: lime, clay and earth finishes in many colours are available. Timber cladding also provides an attractive finish.

The book even gives examples of detailing for construction methods and a full directory of resources. Every building method has its disadvantages and if there is one for hempcrete, besides it being labour-intensive, then it is the time required to let the material dry out before further layers or rendering can be applied. But with good planning this can easily be built into a schedule.

This book serves as an excellent introduction and manual to the subject for anyone wishing to explore it further.