The thermal conductivity through bulk materials in porous or granular form is governed by the type of gas in the gaseous phase, and its pressure. At low pressures, the thermal conductivity of a gaseous phase is reduced, with this behaviour governed by the Knudsen number, defined as K n = l / d , where l is the mean free path of gas molecules and d is the typical gap size of the space filled by the gas. In a granular material d corresponds to the characteristic size of the gaseous phase in the pores or intergranular spaces.