This study is dedicated to the development of a numerical finite-element solution to determine the effective thermal conductivity of extruded polymeric foam with different levels of open and closed porosity. The implemented numerical method is derived from periodic homogenization techniques used for the simulation of laser welding of composite materials. For a validation purpose, it was applied to the calculation of the effective thermal conductivity of polyethylene extruded foam for which the input data (density, cell size and distribution, porosity ratio, and extinction coefficient) were characterized experimentally. The computed results were compared with experimental data obtained by the transient plane source technique and analytical results derived from the literature. The deviation from the experimental data is five times lower than that of analytical methods. Moreover, the proposed numerical solution allows reaching higher porosity ratio and makes it possible to reduce the computation time while ensuring the same compactness ratio regardless of the calculation.