Property: The best weed in the house

HOW DOES that old joke go? Smoking dope results in short-term memory loss, euphoria, increased sensitivity ... and short-term memory loss.

Now, what was I saying? Oh yes, the thousands of people who marched through London last weekend in support of the decriminalisation of cannabis may not have realised it, but they were also marching to further the use of one of the best building materials known to man.

Hemp fibres and cellulose can be used to make a variety of building materials with ideal characteristics: high strength-to-weight ratios, good thermal insulation properties, good sound absorption, natural fire resistance and resistance to attack by vermin, insects and fungi. Add to this the facts that hemp grows quickly without the need for pesticides or fertilisers, can be processed without recourse to toxic chemicals and, when grown as a break between cereal crops, actually conditions the soil. Sounds fantastic - why aren't builders falling over themselves to use it?

Clearly the association between industrial hemp and the drug-crazed savages of "reefer madness" fame may have something to do with it, because hemp was widely grown and used in the West until the 1950s, when drug- related bad publicity and the development of synthetic polymer fibres combined to bring about a rapid decline.

Now, though, there are signs that the paranoia is receding and hemp may once more be accepted for what it is: a valuable material with a host of industrial applications and, most importantly, an environmentally sustainable renewable resource.

Ralph Carpenter, a Suffolk architect, is in no doubt about the benefits of hemp for building. "Hemp products are light but sturdy," he says, "and they have the ability to breathe and to stabilise the moisture content within a house."

He is especially impressed with the performance of materials made by combining hemp with lime, whether as infill panels in timber-framed buildings, as solid floor slabs, or as plastered internal finishes. The combination of lime and hemp results in a material which always has a dry surface and always feels warm to the touch. "In northern Europe we use a lot of heating simply to keep our homes dry," says Mr Carpenter. "But a house built with lime and hemp is always dry and has a high thermal inertia, so the heating requirement is always lower."

Hemp as an environmentally sustainable building material is not an entirely new cause. La Chanvriere de L'Aube is an agricultural co-operative in France which has been growing hemp for industrial use for 23 years. It claims that the variety grown, Cannabis sativa, which reaches a height of three metres between sowing in April and harvesting in September, contains only 0.3 per cent THC (the active ingredient in dope) and therefore has no use as a drug. (It is not clear whether more potent varieties such as Cannabis indica can also be used for industrial purposes.)

After cutting, the stalks are dried in the sun and then passed through a simple machine which strips off the outer fibres. These are the highest- quality part of the plant, traditionally used for rope-making but also to make paper and textiles. The woody core of the plant is chopped up and the resulting cellulose used for a number of purposes, including loose- fill insulation and as an aggregate for lightweight concrete.

The French co-operative produces three main building products: Mehabit is an insulating screed used for flooring, made by mixing the hemp cellulose with bitumen; Canobiote is the loose-fill insulation, made by fire-proofing hemp cellulose with mineral salts; and Canosmose is the concrete product, made by mixing the hemp cellulose with sand, lime and water and pouring it into timber shuttering. Canosmose is used widely in France and Germany for walls, floors and roofs. When set, the material is surprisingly hard and strong but also has high thermal- insulating and sound-absorbing qualities - in many respects the ideal building material.

In the US, development of hemp-based building products has centred on producing medium-density fibre board (MDF). Hemp is better for this purpose than wood, producing boards with more than twice the strength and three times the elasticity. David Seber, an American building materials specialist, explains: "The first law of composite science is that the strength of the product is proportional to the length of the fibre, and hemp is the King Kong of plant fibres." Interest in hemp for board manufacture is also driven by the fact that, although MDF is one of the fastest-growing building products, its primary raw material, wood, is becoming scarcer and more expensive.

Mr Seber predicts that long-fibre hemp composite materials, as strong as steel I-beams, will revolutionise the construction industry. The main problem for researchers and manufacturers is the current shortage of hemp fibre on the world market.

q Information on hemp building products can be obtained from Aubiose UK, tel: 01452 780499, and Hemcore, tel: 01371 820066.