A partner with Modece Architects, Carpenter is on a mission to promote hemp because of its green credentials. It grows to maturity in three months on marginal land without fertiliser, is easily processed and an excellent building material, he says.
Carpenter does not fit the slightly unkempt image of a green activist. Casually dressed in jeans with short grey hair, he looks just like your friendly local architect. He has, however, spent years persuading and cajoling the locals to try hemp – with some success: there are 15 local projects in progress or already complete. "How can they resist?" he smiles. The former hostage Terry Waite has a hemp retreat in his garden and the Bury St Edmunds Town Trust has built a clutch of hemp properties in the town centre.
He first saw hemp used as a building material in France in 1994. He says: "We were looking for ways of building sustainably. Hemp is the most amazingly sustainable material we have come across." Mixed with lime, it makes a building material and, initially, he found that it was ideal for repairing wattle-and-daub buildings.
Much of his hemp know-how comes from practical experience of building a hemp extension to his house. Carpenter put in standard green features, such as a rainwater collection and storage system and a home-made solar collector. But it was the bedroom that he decided to install that really impressed him. He reckoned that it would need two radiators to heat. A last-minute redesign of the roof, however, meant that the room was so enlarged that it would probably require four. In fact, Carpenter maintains, he can heat it with one.
The reason for this is that hemp is a good insulator and possesses high thermal mass, which means that it gradually takes up a lot of heat, then releases it slowly. This creates a building that is cool in the summer and warm in the winter. Other insulators, such as fibreglass quilt or sheep's wool, respond rapidly to temperature changes, so are less effective as temperature regulators.
Hemp also "breathes", allowing damp air from the house to pass through the outside wall. This eliminates condensation and so creates a drier environment that requires less heating. Another benefit is the material's water resistance – "Hemp just shrugs off the rain," says Carpenter – and it has good acoustic resistance to airborne and impact sound.
A sympathetic local council has allowed a number of hemp-built projects to go ahead in Suffolk, but if Carpenter is to realise his dream of hemp houses going up all over the country, the material's performance will need to be convincingly tested.
To this end, four identical two-bedroom houses, two built from hemp and two built traditionally, are to be constructed and monitored. This should go a long way to rebuff sceptics, or so Carpenter hopes.
BRE is carrying out tests on the four houses that will demonstrate how long each takes to construct and how much waste is produced. Whole-life performance will be assessed, and, funds permitting, one house of each construction type will be left empty and monitored for three months for energy usage. The houses are part of a larger development by the Suffolk Housing Society at Haverhill, Suffolk.
Carpenter says the tests are "based on a practical, common-sense approach, rather than scientific tests of actual materials in isolation". He hopes that a positive outcome will convince other local authorities that hemp works and will enable building control officers to approve its use nationally.
Carpenter is optimistic that housing associations will help him fulfil his vision of homes built using locally grown and processed hemp and locally sourced lime, with minimal need for travel and waste of materials.
As for the market, Carpenter is expecting a good take-up among housing associations, who are not unduly suspicious of new materials because they rent homes rather than sell them. However, he is not confident that private-sector housebuilders will be so open-minded: "My suspicion is that the housebuilders will be the last to take it on," he sighs.