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Beyond the straw bale

Hybrid grass beats straw as building material and offers prospect of commissioned plants for specific projects

If you think building in straw bales maxes out your sustainability brownie points, think again. Researchers in Wales have developed a hybrid grass that they say reduces embodied carbon and minimises the impact on farming, compared to regular straw. Now they are seeking industry partners to help build the first building using bespoke bales developed in trials.

Scientists at Aberystwyth University found that by cross breeding different varieties of miscanthus grass, a biomass energy crop commonly use in coal-fired power stations, it attained properties suitable for construction, such as low U values and structural rigidity. The project was carried out in collaboration with miscanthus supply chain development firm Terravesta.

As a perennial crop, miscanthus’ root structure holds soil together in winter, preventing soil erosion and reducing flood risk, compared to annual crops, like the wheat grass used in straw-bales.

Miscanthus can be grown on low grade marginal land not suitable for food production – the biomass crop currently covers over 8,000 ha of marginal land in the UK – making it attractive to farmers struggling to grow barley, oil seed rape or wheat, and increasing the potential for large scale harvesting for construction.

In addition, levels of carbon sequestration are comparable, or lower, than wheat, helping reduce embodied carbon in building, say the researchers.

Judith Thornton, research development officer for agriculture and environmental sciences at Aberystwyth University, told RIBAJ: ‘We have spent thousands of years domesticating crops for food, to improve texture, taste, size etc, but they have never been domesticated for use as building materials. The use of hybrid miscanthus is a great way to diversify the potential end uses of the crop, which is grown on land not suitable for food production … from climate change perspective, regardless of the end use, it is a good environmental option.’

This year’s harvest was used to bale up enough two-string bales to build a two storey house. Now Terravesta is looking for an investor to fund construction of a miscanthus house or commercial building using the bales, which it plans to donate to the project.

Miscanthus bales satisfy UK building regulations, exhibiting good insulation values and structural capability as building blocks. ‘We can build load-bearing miscanthus walls using the traditional straw bale building method, and the surface can directly take internal clay plaster and external render,’ said Bee Rowan, straw bale building course leader at the Centre for Alternative Technology, based in Machynlleth, Wales.

But miscanthus bales may only be the tip of the iceberg when it comes to potential uses in construction. The university is now looking to ‘engineer’ a ranges of hybrids tailored to specific uses.

Thornton comments: ‘There’s lots scope to develop plant-based building materials. Today it’s possible to buy loft insulation, fibre board panels or renders made of hemp, flax or sheep's wool etc. but none of those have had the plant specifically designed for the end use. In the future, an architect specifying a building will be able to ask a plant breeder to design a plant with the specific characteristics they require.’