Heat transfer properties from ambient up to extremely high temperatures are a key feature of advanced thermal protection and thermal exchange materials - like ceramic foams or fiber assemblies. Because of their porous nature, heat transfer rests not only on conduction in opaque solids and on convection in pores, but also on radiation trough pores. The precise knowledge of the thermal behavior of these materials in these conditions is an issue. In a "virtual material" framework, we present a computational simulation tool for heat transfer in such materials, combining solid-phase conduction and linearized radiative transfer in open or closed radiating cavities with opaque interfaces. The software is suited to working in large 3D blocks as produced e.g. by X-ray CMT or by image synthesis. An original MonteCarlo mixed random walks scheme accounting for both diffusion and radiation is presented and validated. The application to a real image of a fibrous medium is described and discussed, principally in terms of the influence of the diffusion/radiation ratio on the effective (large-scale) diffusivity tensor. (C) 2015 Elsevier Ltd. All rights reserved.