Anisotropic thermal conductivity as a function of wood density: experimental data comparison across eight wood species

Abstract Wooden cubes from eight European species – spruce ( Picea abies ), pine ( Pinus sylvestris ), larch ( Larix sp.), oak ( Quercus sp.), beech ( Fagus sylvatica ), ash ( Fraxinus excelsior ), birch ( Betula sp.), and poplar ( Populus sp.) – were tested to investigate the relationship between thermal conductivity and density. Thermal conductivity was measured in the longitudinal, radial, and tangential directions on a total of 215 conditioned samples, each species covering a broad density range. Linear regression models were fitted for each species and anatomical direction. The gradients of regression models revealed species- and direction-specific differences. Longitudinal thermal conductivity showed the greatest density dependence, with spruce, birch, and poplar exhibiting gradients up to 40 % steeper than oak. In contrast, oak displayed the highest gradient in the radial and tangential directions, while other species showed up to 19 % (radial) and 13 % (tangential) lower values. The results demonstrate that, beyond density, anatomical direction and species-specific properties must be considered in thermal conductivity modeling of wood.