Radiative heat transfer in inhomogeneous materials is governed by the photon mean free path. For long‐wave photons, the mean free path depends on the size and shape of the individual grains and, where applicable, of the voids. Long‐wave photons have long mean free paths and play an important role in the heat transfer. In some cases they cause the radiative thermal conductivity at moderately high temperatures to be larger than one would expect by extrapolating from highest temperatures (above 2000 °K) according to aT3law. Some special cases are treated including polycrystalline solids, densely packed powders, foams, and fibrous aggregates.