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Cutting CO2 with hemp and lime

Walls made of hemp can capture carbon

Hemp-based products have always proven to be a hit with those seeking an alternative lifestyle, but if one Oxfordshire-based specialist has anything to do with it its use will become a little more mainstream.

Lime Technology has developed a spray-applied system using lime mortar and chopped lime that can actually ensure the building acts as a carbon safe.

Unlike conventional materials that emit thousands of tonnes of CO2 during manufacture, the hemp transforms the gas, locking in the carbon and emitting oxygen as the plant grows. By the time it has been mixed with the lime mortar and spray applied on building projects its producer’s estimate some 50 kg/sq m will have been captured in this way.

This carbon capturing process can be used to dramatically reduce the carbon footprint of an individual building.

Lime Technology calculate that on a small two-storey extension at its own offices – the first spray application of its kind in the country – the use of 117 cu m of its Tradical Hemcrete helped save 38 tonnes of carbon dioxide from being emitted into the atmosphere.

By using the material Lime Technology worked out that the net amount of carbon locked up in the building, after emissions from brick, plaster, mortar and render had been accounted for, totalled 13 tonnes.

A conventional building of the same size could see an average of 25 tonnes of carbon dioxide emitted.
At the Centre for Alternative Technology in Machynlleth, mid Wales, the material is being spray applied on the three-storey Wales Institute for Sustainable Development environmental education centre. The project will be capable of cutting emissions of carbon dioxide by as much as 500 tonnes.

“These are very serious tonnages we are talking about,” says Ian Pritchett, managing director at Lime Technology. “There are two ways in which it helps cut carbon emissions. The first is by physically locking up carbon dioxide during the growing process, the second is through savings made against the non-use of conventional high embodied energy materials.”

Good thermal mass

At the CAT centre the material will be sprayed onto a relatively standard timber frame with a timber-based permanent shuttering system. This will retain the sprayed material, with the final profile being some 500 mm thick, although this can be slimmed down to 300 mm.

“It is a thick profile,” admits Mr Pritchett, “but it will deliver an extremely low U thermal efficiency value. It will be around 0.134, which means that it will hardly need any heating at all. We used a 500 mm profile in our office and even in winter it does not need any extra heating.”

But it also boasts a good thermal mass which ensures the buildings will not overheat in the summer, will not be compromised by any thermal bridging and will help designers hit green building targets.

“At 500 mm thickness it will meet Code for Sustainable Homes Level 6. It has a very serious thermal performance,” says Mr Pritchett.