Hemp: The Construction Plant

January 23, 2010 – Bath,U.K. – The director of the UK’S Building Research Establishment Centre for Innovative Construction Materials at the University of Bath recently started a £740,000 project, funded by the UK government and the construction industry, to study and develop the material’s use in building. The research builds on foundations laid more than a thousand years ago. Archaeologists in France have discovered a sixth-century bridge where the stones are held together with hemp mortar.
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Cultivated for thousands of years for its fibres, which are used to make ropes and textiles, hemp, otherwise known as Cannabis sativa, was so important to the economy during King Henry VIII’s reign that farmers had to grow at least a quarter of an acre or risk a fine. In the latter decades of the 20th century production slumped as cotton cloth and man-made fabrics became prominent but in the 21st century hemp’s reputation is being rebuilt, partly thanks to properties that make it an ideal building fabric for homes.

From bedding to building

The inner woody core of the hemp plant, the “waste” by-product of fibre extraction, is used in construction. Until recently the most common use for these stalks, also known as “shiv”, was as horse bedding.

In effect, the hemp replaces the aggregate – stones and pebbles – that are usually mixed with cement to form concrete. (One of the leading companies involved in hemp building, Lime Technology, calls its product “hemcrete”.)

By varying the quantities of shiv to lime, different preparations can be made that are either cast or sprayed into a frame or formed into structural blocks. Curved walls can be formed as, being a fairly dry material, the hemp/lime mix can be shaped in lightweight, flexible shuttering.

The hardened, finished material, which looks a little like baked mud full of flecks of straw, can be rendered with a layer of lime plaster or left untreated for a rustic look.

Today, industrial hemp (a strain of cannabis that has almost no narcotic quality) is being processed into an environmentally friendly construction material that is then mixed with a lime binder to make a material that can be poured and cast like concrete or formed into bricks.

Picture 15Hemp’s environmental credentials are many: it needs little or no pesticide or herbicide to grow. Its impact on food production is mitigated by the fact that it is the second-fastest growing agricultural crop in the world (after bamboo), maturing from seed to harvest in four months. Therefore, food crops can still be cultivated for two-thirds of the year and they will have the advantage of growing in soil that has been improved by hemp’s nutrient-enhancing actions.

Hemp does not take up much surface area – one hectare provides enough material to build one house. “All the 180,000 new-build homes the UK government ambitiously estimates are needed each year could be built with hemp grown on just 1 per cent of Britain’s agricultural land,” says Walker. (Last year, 5,000 hectares were grown in the UK – mostly for the fibre, which is used in the car industry.) Of course, hemp could also be used in the renovation of the millions of empty houses in the world. In terms of carbon emissions, hemp certainly trumps ubiquitous cement and concrete, the production of which causes between 5 and 10 per cent of global carbon emissions, says Walker.

Indeed, hemp construction can, theoretically, result in a building with zero carbon emissions or one that contributes to the eco-crusade by storing carbon dioxide. The plant – like all plants – absorbs the gas during its growth. Walker estimates that a square metre of a 300mm-thick hemp/lime wall stores about 33kg of carbon dioxide. By contrast, the manufacture of the materials that go into a square metre of standard cavity masonry wall is responsible for 100kg of emissions.

As global concern about carbon emissions leads governments to legislate for low carbon housing (the UK aims for all new-builds to be zero carbon by 2016), hemp’s use in construction is growing.

In France, where its revival began a few decades ago, there are several thousand hemp houses. In the UK, over the past two years a few hundred properties have been built, including a 4,400 sq metre warehouse with a living roof for the Adnams brewery in Suffolk, eastern England. Constructed from 90,000 hemp/lime blocks with hemp/lime cavity insulation, all made from locally sourced crops, it is the largest hemp building in the world.

The company estimates that there is the equivalent of 100-150 tonnes of carbon dioxide locked up within its walls while a conventional brick building of the same size would have been responsible for about 300 to 600 tonnes of CO2 emissions.

As well as the possibility of being carbon negative in construction, hemp has many characteristics that mean buildings will have low – or zero – emissions during use. Unlike most lightweight building materials, such as wood, it has a high thermal mass, meaning that it stores heat from the sun and releases it slowly during the cool of the night.

It is also breathable while being airtight, is good for soundproofing and is hygroscopic, meaning it moderates humidity. No wonder that Walker boasts that hemp provides “one of the most credible and novel uses of renewable crop materials in construction. It offers benefits for the agriculture and construction industries, occupant health and for wider society through the delivery of lower carbon buildings.”

Its proponents are keen to point out that hemp/lime is ideally suitable for domestic uses too. “You can build a very conventional house with it,” says Ian Pritchett, chairman of building products company Lime Technology. “It doesn’t have to look wacky.” He shows me round a very pleasant but ordinary-looking demonstration house the company has built in Watford, north of London, at the Building Research Establishment. Significantly, the company has changed its name from The Hemp House to The Renewable House.

Pritchett denies that this is to avoid some of the negative connotations people still associate with the cannabis plant. “The name-change reflects the nature of some of the other materials used in construction – mostly sustainably harvested natural products such as Scandinavian pine and sheep’s wool.” The three-bedroom house, which took just 15 weeks to build and cost £75,000, has exterior walls that have been smoothly rendered and painted. You would never guess that they are constructed from a plant most commonly associated in the public mind with a kind of joint not found in a building skills manual.

The house has so impressed the UK government that in November last year the Department of Energy and Climate Change, in partnership with the Homes and Communities Agency, announced that £5m of grant funding would be available for developers that build affordable homes from renewable materials such as hemp. After years of being associated with a hippy ethos and despite the potential for jokes, it appears that there is now good reason to take Cannabis sativa seriously. By Paul Miles. Source.

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