Phase change materials (PCMs) are being used as thermal mass in buildings to conserve energy. In this research, the application of appropriate PCMs to reduce building energy consumption and operating time of mechanical equipment during spring and autumn transition period in hot arid climate is studied. The study was conducted in buildings with diurnal occupation pattern. The simulation results were compared against measurements obtained from a calorimeter that was constructed for this purpose. The results show that PCM 29 degrees C and 5 air change natural ventilation rate reduce annual energy consumption by 15%. This means that while for 38 days in spring and 29 days in autumn, the mechanical equipment can be turned off. The thermal performance of PCM 25 degrees C is very well in spring. The appropriate PCM is the one with combined phase change melting point of 29 degrees C and 21 degrees C PCMs, which reduces energy consumption and increases the seasonal transition period 8% more than that of the single PCM 29 degrees C. (C) 2018 Elsevier B.V. All rights reserved.