Effective thermal conductivities (ETC) under vacuum were computed numerically on 3D blocks of open cell foams either obtained by X-ray tomography or generated by Computer-Aided Design (CAD) with ideal geometries. For the first time a Monte-Carlo/Random Walk code accounting for the coupling of conduction in the solid phase and of radiation in the pore space has been used. The whole range of conduction/radiation ratio, parameterized by a Nusselt number, has been scanned; a law relating the ETC to this parameter has been obtained. In all cases the conductive and radiative contributions are additive. The slope of the radiative contribution to the ETC is found to display a distinct behavior, depending on. whether radiation or conduction dominates. The low-temperature regime has an emissivity-dependent ETC slope, while the high-temperature regime does not. The critical ratio between both regimes is related to the ratio between cell and strut diameters. In all cases, it is found that the ETC anisotropy decreases with temperature. Closing some windows enhances conduction parallel to the closing walls and reduces radiation perpendicular to them. This effect is shown to influence the ETC eigendirections in actual media. (C) 2016 Elsevier Masson SAS. All rights reserved.