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News & opinion

31 JAN 2018

Natural building materials: hemp

Hemp is not just for an illicit high – it can be used for medicine, in textiles and as a biofuel.

A weight-saving hemp polymer is in the door panels of BMW’s i3 electric car. Its use as a construction material stretches back at least 1,500 years. It can be found in wooden-framed buildings of central Europe. Hemp plaster from the 6th century still lines the walls of the Ellora Caves in India – a Unesco World Heritage Site. And hemp mortar has been discovered in ancient Merovingian bridge abutments in France.

Modern building with hempcrete, a mixture of hemp fibre, water and lime, originated in France in the 1980s. Although it is a niche product at present, it has steadily gained an international following. Fans enthuse about its zero-carbon properties, its breathability, airtightness and excellent thermal performance.

“It’s a material that ticks all the boxes,” says Ireland-based Steve Allin, founder of the International Hemp Building Association and author of Building with Hemp. “It’s gaining momentum as [the planet] enters its resource depletion phase.” 

It’s a material that ticks all the boxes. It’s gaining momentum as [the planet] enters its resource depletion phase.

Steve Allin
International Hemp Building Association

Traditional methods involve casting wet hempcrete on site. But curing can be time consuming, taking between four and six weeks in fair conditions. A cold, damp winter could slow the process to at least six months as Ian Pritchett, managing director of Oxfordshire-based Greencore Construction, has learned from bitter experience. “When you’re trying to scale things up, it becomes a limiting factor if the weather is not on your side,” he comments.

Neil Smith MRICS, head of research and innovation at the National House Building Council, adds that there are other challenges that have stopped hempcrete entering the mainstream. “Hempcrete has some issues in terms of shrinkage as it dries. That’s why it’s generally chosen by enthusiastic self-builders as opposed to major housebuilders.”

It was for this reason that Pritchett, who has been working with hempcrete since 2002, developed the world’s first modular system of prefabricated hempcrete panels. Drying under factory conditions, they do not suffer from shrinkage after installation. And unlike on-site cast hempcrete, they have the added advantage of being load bearing.

Pritchett’s previous company, Lime Technology, first used modular panels on a Marks & Spencer store at Cheshire Oaks in north-west England. Completed in 2012, the store achieved a BREEAM Excellent rating. On this project, the panels took more than a month to dry out indoors. His clients are often enlightened self-builders or ethical corporations, attracted by the embodied carbon that the plants have stored from the atmosphere. 

“Typically, our walls lock up 30-35kg of CO2 per square metre,” he explains. Pritchett adds that hempcrete walls also create a stable internal temperature: airtight, yet breathable, allowing water vapour to pass in and out “a bit like a Gore-Tex jacket”. As a result, minimal heating is required.

Businesses that require temperature-controlled facilities have also benefited from the use of hempcrete. Pritchett cites Suffolk-based brewery Adnams’ distribution centre as an example. Completed in 2006, the warehouse is passively heated and cooled, “saving around £100,000 a year in energy costs”.

For Allin, hempcrete has huge potential in developing countries. He has been working in Nepal, Morocco, Haiti and Fiji, teaching communities to build houses with hemp, using crops they have grown themselves. “This is working at a low-tech level, training people in modern approaches with traditional materials.”

In developed countries, hempcrete is often used for bespoke applications. But Allin argues that modular systems could also be an economic option for housebuilders. Achieving this depends on many factors, including consumer understanding and the push of carbon-reducing legislation.

“We are some way off any legislation around low-embodied carbon. It’s still quite niche and hard for the general public to understand. And the industry is still focused on operational carbon emissions,” says James Hepburn, a principal engineer at BDP in London.

Supply is also an issue, according to Allin, who suggests that regional or national processing facilities need to be established across Europe. “It’s starting to happen, but it’s [a] chicken and egg [situation]. You’ve got to convince the farmers that it’s worthwhile growing the crops. And you’ve got to prove to industry that you can produce large amounts of material needed to win the kind of contracts that could roll things out at scale,” he adds.