Because the RSTIB is a light weight porous mate-rial, porosity has a significant effect on the thermal conductivity of the boards. The thermal conductivity of all boards was plotted as alinear function of boards’ density, as shown in Fig. 2. Notably, the higher density of RSTIB is associated with the higher thermal conductivity of the boards. As the board density in creases, the solid substances are in creased and the voids are decreased. Additionally, the thermal conductivity of a solid substance is much higher than that of air within the voids. This leads