Research brings traditional sustainable building material into 21st century

An international research project led by the University of Plymouth has succeeded in bringing the ancient and sustainable building material Cob into line with modern thermal standards.

According to researchers, the use of cob – which is a mixture of earth, water and fibres like straw and hemp – has the potential to bring substantial reductions in carbon dioxide (CO2) emissions and construction waste compared to conventional masonry materials, as it is made with soil sourced from site. But traditional cob, which has been used for houses and other buildings across the south west of England and northern France for centuries, does not comply with the most up-to-date thermal building regulations.

Investigators on the project, the first phase of which runs until early 2019, have come up with a way to radically increase cob’s ability to help trap heat inside buildings. The technique involves two different grades of cob – one lightweight version with greater insulating properties, and one denser, stronger type – that are bonded together to form walls.

Researchers in the University’s Building Research Group have spent the last year trying to find a solution, working alongside the Ecole Superieure d’Ingenieur des Travaux de la Construction de Caen (ESITC), Syndicat Mixte du Parc naturel régional des Marais du Cotentin et du Bessin (PnrMCB), Earth Building UK and Ireland (EBUKI) and the Université Caen-Normandie (UCn).

They have renamed the material ‘CobBauge’, which is an amalgamation of the English and French words for the material.

Professor Steve Goodhew, who led the research, said: ‘What we’re doing is taking a robust vernacular material and bringing it right up-to-date. A large part of the University’s role in the project has been analysing and measuring the thermal performance of different types of soil and fibres, using the latest methods to optimise this ancient material.

Dr Jim Carfrae, lecturer in Environmental Building at Plymouth, added: ‘Anywhere you find heavy clay soils – like the south west of England – you’ll find cob buildings, and some famously picturesque Devon villages, places like Broadhempston and Inner Hope, are almost entirely cob.

‘Over the past year we’ve experimented with different mixes based on English and French soil, and come up with one each of the structural type, and one each of the lightweight thermal kind. The two together can be combined in a composite cob wall that will pass current regulations.’

The next stage of CobBauge, subject to further EU funding, will see at least two houses built in France and the UK using the new techniques. Researchers also aim to publish building guidelines for the new cob mixes and train hundreds of builders in their use.