2021 Volume 67 Issue 4 Pages 167-177
Assuming the use of semi-carbonized chips under the floor for thermal insulation, wood (Douglas fir heartwood) was semi-carbonized (torrefied) at heating temperatures of 270 to 300 ºC and oxygen concentrations of 1 to 4%. Changes in thermal insulation capacity, hygroscopicity, strength properties, and so on were investigated using the mass loss caused by semi-carbonization as an index. The thermal conductivity decreased linearly as the mass loss increased, and the rate of decrease was about 0.75% per 1% mass loss. The main cause of the decrease in thermal conductivity was the increase in porosity due to the decrease in density. However, the heat transfer model analysis revealed that the thermal conductivity of the substance (heat bridge) decreased by up to 5% due to the decreases in hygroscopicity and substance density. Although the compressive strength and shear strength decreased to about 1/4 when the mass loss reached 40%, the pulverization rate due to repeated drop impacts remained below 10%. Assuming that a mass loss of 20% or more ensures decay resistance, the semi-carbonization condition that achieves both thermal insulation and impact resistance was judged to be in the range of 20 to 40% in terms of mass loss.
