Plant a hectare of hemp, wait 14 weeks and you've got enough to build a house. That's the nub of the environmental case for hemp and lime-based Tradical Hemcrete and one reason why it seems to be becoming a material of choice for an increasing number of eco-aware builders, especially allied to timber construction. You can't get greener than a building product that grows like Topsy.

“Only bamboo grows faster,” said Ian Pritchett, founder and managing director of Tradical Hemcrete supplier Lime Technology Ltd. “A hectare produces 10 tonnes in 14 weeks, six tonnes of which is the woody core we use in Tradical Hemcrete.”

Besides shooting up at a rate of knots and locking up copious CO2 in the process, hemp has other attractions as an environmentally-sound, commercial crop. “It doesn't need agrochemical spraying and it suppresses weeds and resists pests,” said Pritchett.

To find out how this high-powered plant became a key ingredient in Tradical Hemcrete you have to go back to Pritchett's original business, historic building restoration specialist, IJP Building Conservation. The company was having trouble finding traditional construction materials, so set up the Old House Store to source and sell them. This, in turn, spawned Lime Technology to make lime mortars, plasters and renders under its brand of Limetec.

Tradical Hemcrete was used in Adnams Brewery’s new green distribution warehouse

“You wonder why some traditional building products are not still mainstream and lime products are among them,” said Pritchett. “They're more flexible than cement-based equivalents, so you don't need expansion and movement joints. They also take less energy to produce, reabsorb CO2 as they set and don't bond so strongly to bricks as cement, so, when the time comes, you can separate and recycle them.”

Lime Technology launched in 2003, with manufacturing and distribution partnerships with Castle Cement and CPI Euromix (supplying bagged product and dry silo markets respectively). “As a lime-based building product manufacturer on our own, we suspected we'd be seen as a modern day cottage industry, which wouldn't win over any of the major contractors or developers,” said Pritchett. “The solution was to team up with people who could help us make it commercially viable.”

The approach paid off. Limetec products have been used in hundreds of projects, including the new National Trust headquarters, winner of the 2006 RIBA Sustainability Award, and the regeneration of St Pancras station.

It was in looking for ways to broaden the lime products market further that the seeds for Hemcrete were sown. “We considered blending lime with a range of materials to produce building materials, but hemp, which is similar to other materials that have been used with lime in building for centuries, offered the most exciting opportunities,” said Pritchett.

The product can be poured or sprayed

To tap into existing expertise in the field, Lime Technology opted to work with Belgium-based global lime producer Lhoist, which had developed a lime-hemp binder in Tradical HB and was promoting the use of lime and hemp in construction across Europe.

A partnership quickly established between Lhoist UK and Lime Technology. The former also brought UK hemp processor Hemcore into the team, plus Castle Cement as distributor. With all the pieces in place, Tradical Hemcrete was unveiled at the Ecobuild show in London last year.

The product can be cast like concrete and also used in spray form. “There is a learning curve, but it's not complicated to use,” said Pritchett. “Casting in a timber frame building is a doddle: you just put the lightweight shuttering up supported by the frame and place the mixed Hemcrete into the shutters, making sure that you don't over compress it but lightly place it. The finished, set wall has to be protected from the rain with a lime render, timber cladding, brick or other vapour permeable skin but, kept dry, should last indefinitely.”

The headline environmental plus of the product is its organic content. “Because the hemp absorbs more CO2 in growing than is generated producing the binder, Tradical Hemcrete is carbon negative,” said Pritchett. “It can save between 30-50 tonnes of CO2 in the average house at construction stage.”

The finishing building

One reason Tradical Hemcrete is billed as the perfect partner for timber building is because it's the marriage of two carbon-absorbing natural materials. But it's also due to its insulation properties. “It has high thermal inertia,” said Pritchett. “A concern with lightweight timber frame is lack of thermal mass and the risk of overheating. Tradical Hemcrete provides a passive solution.”

Tradical Hemcrete had a major promotional boost last year from its use in Adnams Brewery's new 'green' distribution warehouse (TB Winter 2006). Here it was poured into the cavity of a 500mm-thick diaphragm wall built of Hemcrete blocks. Examples of the use of Tradical Hemcrete in spray form include Lime Technology's own offices and the Wales Institute of Sustainable Education (TB spring 2007), while other projects are going for the poured shuttering approach.

Costing £100-160/m2, compared to a construction industry walling average of £100-150, Lime Technology also sees major potential for Tradical Hemcrete in general housing. It is working on applications with timber frame specialists Chignell Buildings and English Bros and on the construction of 26 affordable homes designed by architects RHM for Orwell Housing Association, Elmswell Parish Council and Suffolk Preservation Society.

In total there are around 100 Tradical Hemcrete projects 'on the drawing board' and the introduction of more variants is likely to boost demand further. A cast block version and prefabricated panel option are under consideration.

If there were a question mark over the product it might be whether hemp production can keep up with demand. But while farming the industrial form was only legalised in 1993, acreage is on the up and Lime Technology, Lhoist, Castle Cement and Hemcore are confident it will keep growing, with producers promised a rising market and a crop that will enable them to reap the rewards just 14 weeks after planting.