Particulate media, among which some are porous, like fibers, powders, foams and others, are of considerable interest in many engineering applications. Heat transfer in such media is due to complex mechanisms involving interactions of radiation with conduction and/or convection. Improvements of the thermal performance of these materials, used, for instance, for thermal insulation, need a good understanding of the physical phenomena involved, combined to an accurate knowledge of the thermophysical properties responsible for these performances. This paper deals with the determination of the spectral properties : optical thickness, albedo and phase function shape parameters, that govern the radiative transfer in a semi-transparent medium. Results are presented for fiberglass and silica fibers-cellulose insulations.