Building refurbishment, through incorporating phase change materials (PCMs) into building fabrics, has been considered to be an effective way to reduce the energy consumption and related carbon emission of buildings. At the same time, it can also help to reduce the extreme heatwave risks in non-air-conditioned buildings. This study investigates the potential applications of PCMs to be integrated into buildings to reduce heat stress risks during extreme heatwave periods through numerical simulations. This study uses 2009 weather data of Melbourne, a city that regularly experiences heatwaves in summer. A detached single-storey house, without an active air-conditioning system, is refurbished with the installation of macro-encapsulated Bio-PCM (TM) mats as inner linings of walls and ceilings. Dynamic thermal simulations have been undertaken to reveal the performance of, and factors that influence, the adoption of PCM to reduce heat stress during heatwave periods. Discomfort index has been used as an indicator for measuring the indoor heat stress risks. The results showed that PCM refurbishment can effectively reduce the indoor heat stress risks, indicating a significant advantage in improving the occupant health and comfort. The selection of suitable phase transition temperature, and amount of PCM, is critical for this application to be effective. Appropriate selection of PCM with better ventilation design could reduce the severe discomfort period by 65% during extreme heatwave conditions. While the thermal energy storage of PCM reduces the indoor heat stress, night ventilation enhances the cool storage of PCM. (C) 2016 Elsevier Ltd. All rights reserved.