Ed Note: More in-depth information on hemp lime will be in Issue #64 due out in July.
When Rachel Bevan and I did the research that led to the publication of Hemp Lime Construction in 2006-7 (IHS BRE Press ISBN 978-1-84806-033-3) we had a fairly good idea of the location of every building using hempcrete in the UK and Ireland. Seven years or so later it is impossible to keep track of the use of this remarkable composite building material as it has become commonplace in the UK. This is good news because it is evidence of the widespread acceptance of this excellent sustainable way of building. However once such an innovative form of construction becomes so widely used there is also a risk of careless and poorly supervised construction, detailing and specification if it is used by people who expect it to behave like ‘conventional’ materials. Fortunately a new book : The Hempcrete Book: Designing and building with hemp lime will soon be available . By Alex Sparrow and Will Stanwix it will be published by Green Books (ISBN: 978 0 85784 120 9 )*** and will set out guidance for best practice in building construction for hemp lime
Hemp Lime construction is a method for creating a natural “concrete” which provides a solid wall system either cast around or within a timber frame structure. The composite uses small pieces of hemp shiv or hurd, which is the chopped up woody core of the plant and then mixed with water and a special lime binder mix. It is incredibly strong almost as soon as it is cast into formwork or sprayed onto permanent shuttering. The formwork can be removed almost straight away or left on for 12 hours before casting the next lift. It then takes a couple of weeks to dry out and longer to gain its full strength. Sceptics often ask, “why use hemp, and why not use wood chips or straw?” people often say (there is an inbuilt prejudice because of its relationship to Marijuana). The best way to convince such people is for a practical demonstration and it is possible to see straight away the strength of the composite. Hemp is much tougher and can cope with moisture better than other cellulose materials.
The resulting composite provides a solid wall with superb air tightness capabilities and very good insulation. Its density of 300-400 kg/m3 is strong but light and contains air pockets in the tubular hemp plant structure giving a “u”value of about 0.2 for a 300mm thick wall. (Lambda 0.06/mK). In practice the thermal performance of hemp lime, or hempcrete as it is often called, is enhanced by its thermal mass and thus the actual performance of a building is often much better than predicted by the abstract thermal resistance figures. Hemp Lime also has the benefit of being full breathable and hygroscopic so that humidity is controlled. Because of this it has been adopted by major commercial food and wine storage companies* to insulate storage warehouses as the walls provide a stable temperature and indoor climate without the need for heating, cooling or air conditioning. Hemp lime insulating walls have also been used by the British Musuem for storing special artifacts.
Hundreds of social housing schemes and one-off private houses have been built using hemp lime and it has also been used in major public and educational buildings, 5 or 6 storeys high.
Hemp lime is versatile so it can be used as an infill in multi-storey construction, in floors and roofs, as a renovating or insulating plaster and as an external render for straw bale buildings and other eco forms of construction.
Supply of materials has not been fully sorted out yet. Hemp shiv or hurd is readily available but not always in the right place so it has to be transported from processing factories where the hemp fibre is stripped off the plant. The hemp fibre is a valuable crop with a thousand uses, so the shiv used for building is almost just a by-product. Making or sourcing the lime binder is also tricky. There are a range of proprietary products available such as Tradical, Batichanvre and recently Ciment Prompt [French]. These are not always available from local suppliers of building materials. It is possible to mix up your own binder but it is essential to use the right materials with careful quality control. The binder is largely lime based, mainly hydraulic lime but some hydrated lime and or cement is also added. There have been a few “cowboys” who have been supplying hemp and lime materials that are not fit for purpose and this has led to a few building failures. Their main mistake has been to use cheap hydrated lime, often too much water and hemp fibre as well as shiv. One company even says it is more ecological to use the whole of the hemp plant even though this invariably leads to a soggy mess. We are working hard to establish proper standards. Sadly the internet gives people partial information about how to build with hemp lime and makes them into overnight experts.
In some ways hempcrete is easy to use and is even tolerant of misuse, within limits, but this means that there are many dangers and possible pitfalls. On the other hand, once you become aware of its advantages it is hard to find another way of building walls, (and possibly floors and roofs), that can meet so many of today’s demands of sustainable, healthy and energy efficient construction so successfully. As pressure builds to meet ever more strenuous energy efficiency targets, many weird and wonderful building techniques and materials have appeared in the market. While some mainstream architects and clients have embraced hemp lime quite quickly the construction industry is still largely wedded to synthetic petrochemical based methods of construction that contain many risks both to the health of building occupants and the planet. Valuable and non-renewable fossil fuel resources produce significant CO2 emissions even though ironically they are being used to reduce such emissions! Recent research shows that many so-called low or zero energy buildings consume more energy in producing the materials and construction (embodied energy) than is saved in the lifetime of the building. These synthetic quick fix approaches to building also present serious fire hazards, emit toxic chemicals. Leading to poor indoor air quality and pollute the planet when disposed of in landfill. Despite this the devotees of “Passiv Hause” in the UK tend to use synthetic materials, though there are a handful of Passiv Haus projects in Ireland that have been built with hemp.
Hempcrete is not only a low embodied energy material, it locks up CO2 in the building fabric. While land is required to grow it, hemp is also a valuable food crop and is used as an intercrop between wheat and other cereals. Those who are fixated on the ‘techy’ quick fix synthetic solutions, disparage hempcrete as being too slow to construct and dry out and not giving good enough thermal performance. However even the most deeply prejudiced, once they actually experience hempcrete, are soon won over. Despite the obstacles to using hempcrete, its rise has been rapid as it almost sells itself as a solution to producing environmentally friendly buildings. Hemp lime is widely used in France and recent workshops in Holland, Denmark, Sweden, Poland etc. have led to projects in many of these countries.
I would just like to point out that the museum in the UK which has trialled hemp-lime construction for museum storage is the Science Museum, NOT the British Museum. The Hemcrete Store is on the Science Museum’s storage site at Wroughton, in Wiltshire, and has won several awards for innovation in the museum sector. Further work continues at the Science Museum into sustainable storage methods encompassing passive buffering of interior environment to museum guidelines.