The use of phase change materials (PCMs) in the building envelope has emerged as an attractive passive design strategy that can be used to improve the energy performance of buildings. This paper discusses the effect of using PCMs on the thermal performance of shaded and unshaded lightweight structures in different Algerian climatic zones. A simulation-based optimization was carried out by coupling EnergyPlus and GenOpt tool, to determine the optimum melting temperature of PCM when integrated into the building envelope. Results show that the shaded building with PCM is a preferred design model in majority of Algerian climates when it is free floating. The annual percentage of thermal comfort ranges between 44.13% and 59.11% in cities where the PCMs showed positive potential. Furthermore, the optimization results show that the PCMs achieved a modest cooling energy saving for mechanically-ventilated buildings in summer, especially in hot-dry climate with optimal melting points equal to or close to 28 degrees C. The control of solar heat gain by using shading devices is critical to enhance the thermal performance in summer. In winter, the PCMs achieved a noticeable reduction in the heating energy requirements where it reached a maximum of 63.23% in the city of Tamanrasset. The shading device is not recommended during the winter because it hinders the use of solar radiation for passive heating. During this season, the optimal melting points of PCMs are generally bounded between 18.50 degrees C and 21.25 degrees C. When considering both periods, the most advantageous design varies from climatic zone to another.