Expanded PolyStyrene (EPS) foams are very widely used as thermal insulators, especially for the Building insulation. Therefore, the heat transfer in these materials has been extensively studied and optimised. Nevertheless, due to their very low density, the IR thermal radiation propagating through their porous structure still contributes significantly to the total heat transfer. In order to increase their insulation performances even more, the addition of particles strongly interacting with IR radiation in the polymer matrix is very promising. We have investigated this solution from the theoretical and experimental points of view. The influences of the presence of particles on both the conductive and radiative properties of the opacified foams have been modelled. A parametric study allows us to highlight the key properties required to make the opacifying particles efficient and to evaluate the corresponding potential savings. Alongside, several powders have been tested experimentally by incorporating them into polystyrene thin films. Spectrometric measurements conducted on these films allow us to identify their radiative properties, evaluate the reduction of equivalent conductivity and confirm the efficiency of the solution. This investigation then constitutes a useful tool for selecting appropriate opacifiers. (C) 2018 Elsevier B.V. All rights reserved.