An optimization study was conducted for an external office wall containing a phase change material (PCM) layer under intermittent cooling operation. The design parameters such as location, PCM layer thickness and phase transition temperature were optimized to maximize energy savings by efficient use of latent heat. Moreover, simulations were performed for a representative week of each summer month to evaluate the effect of incorporating PCM into the wall on thermal comfort and air conditioner operation. PCM located near the exterior did not save energy and might even increase the energy demand. It is advisable to locate the PCM layer near the interior. The optimum phase transition temperature was 25 °C irrespective of the PCM layer thickness, which equaled the upper temperature setpoint. At this phase transition temperature, the room and wall surface temperature was reduced before working times and during lunch breaks which enabled reducing the number of operation cycles of the air conditioner. Also, energy savings of up to 12.8% were attained for the PCM layer thickness of 23 mm as compared to a wall without any PCM. Any phase transition temperature above 26 °C negatively affected the energy savings due to the adverse effect of latent heat usage.