Expanded polystyrene (EPS) foams are one of the most widely used thermal insulators in the building industry. Owing to their very low density, both conductive and radiative heat transfers are significant. However, only few studies have already been conducted in the modeling of heat transfer in this kind of medium. This is due to their complex porous structure characterized by a double-scale porosity which has always been ignored by the previous works. In this study, we present a model of one-dimensional steady state heat transfer in these foams based on a numerical resolution of the radiation-conduction coupling. The modeling of the conductive and radiative properties of the foams takes into account their structural characteristics such as foam density or cell diameter and permits us to study the evolution of their equivalent thermal conductivity with these characteristics. The theoretical results have been compared to equivalent thermal conductivity measurements made on several EPS foams using a flux-meter apparatus and show a good agreement.