From forest to fabric: thermal characterisation of UK hardwood and softwood insulation

This study evaluates the technical viability of UK-grown timber, specifically four hardwoods (alder, birch, poplar, sycamore) and two softwoods (sitka spruce, larch), for the production of wood fibre insulation (WFI). Analysis revealed significant interspecies variability in fibre length; sycamore and poplar exhibited the longest fibres, while alder exhibited the shortest. Notably, several hardwood distributions were comparable to the softwood benchmark, sitka spruce. Laboratory testing indicated that while fibre length influences thermal performance, variables such as density, porosity, and orientation remain critical factors. Thermal conductivity (λ) measurements showed that homogeneous sycamore achieved the highest performance, achieving 0.041 W/mK, marginally surpassing softwoods such as larch and sitka spruce, which achieved 0.042 W/mK. Investigations into hybrid formulations (80/20% to 50/50% softwood–hardwood blends) demonstrated that increased hardwood content typically elevates thermal conductivity. However, mixed hardwood blends (alder, birch, sycamore) remained competitive. When comparing results, 50/50 blends outperformed 80/20 blends, yielding values of 0.0405 and 0.042 W/mK, respectively. These findings confirm that UK-grown species are suitable for WFI production. Utilising local hardwoods offers a strategic pathway to diversify material streams, reduce import dependency, and mitigate both operational and embodied carbon within the built environment.