An Investigation into the Thermal Conductivity of Cast Bio-Aggregate Wall Insulation in Experimental and Real-World Conditions

The construction industry is responsible for 34% of global greenhouse gas emissions, while space heating accounts for 45% of building energy use in OECD countries. In Europe, 85–95% of the housing stock expected in 2050 already exists, making the retrofitting of existing buildings essential in achieving net-zero targets. Bio-aggregate insulation, formed from plant materials and lime-based binders, is a vapour-permeable approach for improving the thermal efficiency of solid-walled buildings. However, evidence on its in situ thermal performance remains limited, particularly when cast directly onto walls. This study investigates the thermal performance of cast bio-aggregate insulation made from hemp, miscanthus and oilseed rape, and evaluates the influence of mix ratios and compaction density. Insulation samples (50 mm thick) were cast onto a double-skin brick wall and their U values were measured according to BS EN 9869-1:2014. U values were between 0.59 ± 0.03 W/m2K and 0.75 ± 0.03 W/m2K for the insulated wall for a 1:1 mix of bio-aggregate:lime. Varying binder ratios and compaction levels showed a significant association between density and U value (p < 0.01), but no separate effect of plant material or plant:binder ratio was found. Furthermore, heat flux was measured across walls in two houses insulated with hemp–lime and miscanthus–lime, and λ values of 0.067 W/mK and 0.070 W/mK were obtained for the bio-aggregate insulation materials. Cast bio-aggregate insulation significantly improves the thermal performance of solid walls, with density being the primary determinant of performance rather than the type of plant material.