Mud, glorious mud: Homes made of earth

Warm in winter and cool in summer, earth is the perfect material for green construction. No wonder it's making a comeback, says Graham Norwood

Winnie Brimacombe-Nelissen may have a home dating back to 1598 but it is made from a building material that is enjoying a distinctly 21st-century revival: mud. Her six-bedroom farmhouse near Crediton in Devon is built from cob, a mud-based mix first used for construction in north Africa in the 11th century. Some 300 years later it had become the standard building material in the UK and remained so until industrialisation made manufacturing bricks cheap.

Those cob houses that remain today are mainly found in the Vale of Glamorgan and the Gower Peninsula, across Ireland and in south-west England. Winnie's home is a Devon longhouse. "The walls are thick, made of mud, dung and straw, and have the odd crack, which you would expect, given their age," she says. "They are extremely efficient. Because the property is listed, we can't fit double glazing but we have an evening fire and the walls retain heat. In the summer, even on very hot days, we can keep doors shut because the walls are cool."

Hundreds of years ago, when cob was first "discovered" as a building material, the mud and straw was trampled by oxen before being pitchforked into place and then trimmed after drying for up to nine months.

Today's builders use cement mixers, but otherwise apply the material in much the same way – and they insist it is the perfect material for green construction.

"There's nothing more sustainable than cob. It's natural, it hasn't been processed and it's produced on-site," says Adam Weissman, one half of the team behind Cob in Cornwall, a Helston-based building firm. He and his wife, Katy Bryce, have built cob homes and extensions, but more than half of his work is repairing and maintaining period houses in the South-west.

"Most are between 200 and 400 years old so that's highly sustainable in itself," says Adam. "If there are problems, they tend to date from the Sixties and Seventies, when owners put cement render on walls. Some haven't looked after roofs and that has allowed water to seep in."

The couple work with Cornish schools to explain the secrets of cob and also run three courses a year to teach people how to use it. He says: "Attendees are from 16 to 70. Most want to know how to look after their own cob home."

Britain's first brand-new cob home for three quarters of a century was built in Devon in 1997 by Kevin McCabe. In 2005 another cob new-build, this time in Worcestershire, won the Royal Institute of British Architects' sustainable building of the year award. At least six more cob homes are being built in different locations across the UK.

The material's chief drawback is its propensity to dampness. Most old cob houses, and all new ones, carry coats of flexible "breathing" lime plaster. But in the past, some owners have used a cement render that was inflexible and cracked as the walls "moved" slightly. As a result, water penetrated the surface.

"It's been a problem. People want an old home and get a survey of somewhere we find for them. But they then worry when they're told it's got cob walls, especially when they're told exactly what cob is," explains Nicola Oddy of Stacks, a property buying agency.

Now, however, a synthetic substitute for mud is being tested. It is called Tradical Hemcrete and is a "light" concrete made from hemp plants. It is only 50 per cent of the cost of cob, it takes a fraction of the time to make and is claimed to be even more thermally efficient. It is thought that this material might attract volume builders to use it in some mainstream housing schemes – until now they have spurned cob, mainly on cost and image grounds.

But for owners of period cob properties, there is nothing to beat the original material. The same goes for the visitors who have been learning about the cob at Winnie Brimacombe-Nelissen's home in Devon, which she uses as a bed and breakfast (details on www. warrensfarm.co.uk). "When I tell them what cob is, some visitors are a little alarmed, especially when we explain that cob walls still move a as the mud 'breathes'.

"But the walls are thick enough to use as seats and that intrigues many people who stay with us," says Winnie. "We like our mud, and we wouldn't change it for the world."