In this study, the effect of phase change materials on the time lag, decrement factor and heat-saving is examined numerically. The calculations are conducted for four different cities located at different climatic zones in Turkey, considering both summer and winter conditions, in order to explore the potential heating and cooling energy savings by employing phase change materials. A solar-air temperature, which is a function of time and solar radiation, was taken into consideration as external boundary condition for each city. The results of the present study show that employment of phase change materials in walls of the buildings has a pronounced effect on the time lag and decrement factor. It is concluded that a significant amount of heating energy can be saved and thermal comfort can be enhanced considerably by incorporating phase change materials into external walls. However, a proper phase change material must be selected, considering different climatic conditions.