Tropical region such as Darwin has similar weather patterns throughout the year, thus creating higher energy demands in residential buildings. Typically, buildings consume about 40 per cent of the total energy consumption for indoor heating and cooling. Therefore, building envelopes are linked with design strategies such as the use of thermal energy storage and phase change materials (PCM) to minimize this energy consumption by storing a large amount of thermal energy. Primarily, PCMs are targeted by researchers for use in different components of buildings for thermal efficiency; thus, this study aimed to provide a suitable PCM to optimize indoor thermal comfort and minimize the cooling loads of residential buildings in tropical climates through simulation of a tropical climate building and provide optimum thickness for the selected material. Microencapsulated PCM mixed with gypsum in wallboards were used to reduce the cooling load of a building located in Darwin. The cooling load of the building was calculated using Revit software. A comparison of the cooling load of the building was carried out using PCM-incorporated wallboards of thicknesses of 0 cm, 1 cm and 2 cm in Energy Plus software. The total cooling load decreased by 1.1% when the 1-centimetrethickness was applied to the wall, whereas a 1.5% reduction was obtained when a 2-centimetre-thick PCM layer was applied. Furthermore, the reduced cooling loads due to impregnation of the PCM-based gypsum wallboard gave reduced energy consumption. Ultimately, the 2-centimetre-thickness PCM-based gypsum wallboard gave a maximum reduction in cooling load with a 7.6% reduction in total site energy and 4.76% energy saving in USD/m2/year.