Indonesia has been experiencing population growth in the last few decades. On the other hand, achieving thermal comfort in a hot and humid climate is challenging. This paper aims to evaluate the effect of window-opening patterns on occupant thermal comfort in low-cost apartment buildings in Medan, Indonesia. The objectives of this research are: 1) to observe the window-opening pattern of occupants; 2) to investigate the effect on indoor air temperature, relative humidity, occupancy hour, activities, orientation, and the usage of active means; 3) to analyse the correlation between window-opening pattern and thermal comfort. The methodology used in this research is a survey of questionnaires and data logging on indoor temperature and humidity monitoring. One case study building was investigated in this study. It is called Rusunawa Kayu Putih, a publicly rented vertical housing unit operated and managed by the Ministry of Public Works and Human Settlement in Medan. The results showed that the opening window pattern was influenced by the occupancy hours, activities such as cooking, showering, sleeping, and smoking, and the praying time. It is also observed that the occupancy hours might not be correlated with the hourly indoor air temperature. In terms of thermal comfort, about 50% of participants experienced comfort during rainy and dry seasons, but only 20% of them opened their windows throughout the day. It is summarised that residents achieve thermal comfort by using mechanical ventilation rather than natural ventilation.