The walls containing phase change materials (PCM) play a significant role in the air conditioning of buildings. However, one of the characteristics which limits the use of PCMs is their low conductivity. It is possible to solve this problem by coupling a metal matrix with the PCM. The aim of this study is to determine the optimal size of a wall which allows a complete change of state, and a homogenization of the temperature without a too fast change-of-state rate. To study the thermal behavior of the wall, we have carried out a numerical simulation using the COMSOL® software in considering a unit cell. To describe the change of state, the method of the equivalent heat capacity has been used. Paraffin has been chosen as PCM (melting temperature 24.5 °C). The influence of the nature and the volume fraction of the metal were determined. The case of honeycombs made of aluminum has been specifically treated.