An investigation into the thermal and hygric performance of bio-based wall systems

Renewable bio-based materials are emerging technologies within the construction industry, providing inventive solutions for reducing material waste and the carbon footprint of an inherently carbon-intensive sector. Previously they have been commonly investigated in isolation and there are a significant lack of case studies demonstrating the performance of bio-based materials in entire external wall designs. This study evaluates the thermal and hygric performance of five novel bio-based external wall systems, featuring highly innovative combinations of insulation, structural and façade materials. Large scale wall panels were designed, constructed and tested within a bespoke Large Environmental Chamber (LEC) to determine thermal performance and hygric behaviour using the heat flow method. Hygrothermal simulations with WUFI® Pro were conducted to assess condensation and mould risks. Insulation materials used were wood fibre, mycelium, sheep's wool and blown-in cellulose. Structural options were timber, Cross Laminated Timber (CLT) and also light-gauge steel, while facades were bio-based-polymeric composite material or bio-based treated timber cladding. Thermal transmittance and conductance were calculated along with time series analysis of temperature and relative humidity. The U-values of the five walls ranged from 0.13 W/m²K to 0.22 W/m²K and modelling simulations were in broad agreement with the practical results, with four of the five walls fulfilling an 0.18 W/m²K benchmark value based upon building regulations. The significance of the study is in the demonstration of the feasibility of a range of bio-based external wall designs, combining emerging material technologies, innovative design and building physics to contribute to the development of bio-based construction.