Improving the thermal performance of earthen walls to satisfy current building regulations

Abstract

Earth building materials offer architects, engineers and clients a low carbon walling solution for low rise properties (under 3 storeys). Unfortunately, the mixture of sub-soil and fibre known as Cob does not currently comply with the thermal aspects of many building regulations across the world. This paper designs and optimises a low-density cob mix intended to maintain the material’s low carbon credentials whilst conforming to the thermal aspects of building regulations. Samples of a range of unmodified subsoils, dug from the ground near to the sites of some prospective buildings are described. These subsoils are combined with a range of commonly grown fibres. Practical and laboratory measurements are undertaken on these mixes and the results are compared with pure subsoil and separate fibres to provide a model that can predict the thermal conductivity of a theoretical soil-fibre mix. It was found that fibre contents over 50% gave very little reduction in thermal conductivity. Furthermore, if the optimal low-density mix is combined into a single composite 2-layer cob wall this can offer a ready-made solution for compliant low-carbon energy-efficient low rise properties or the extension of existing historic buildings.