Apartment buildings in major Canadian cities make the largest share of the residential stock. However, their poor performance is often characterized by indoor thermal discomfort conditions and high-energy consumptions. Therefore, decreasing the energy demand of high-rise residential buildings through energy retrofits is critical. In the recent years, the adoption of thermal energy storage has often been proposed to stabilize indoor temperatures and to reduce the energy demand for space conditioning in buildings. In this study, the benefits of passive latent thermal energy storage systems are investigated by integrating phase change materials (PCMs) into walls and ceilings of apartment units. A hybrid PCM system with a thickness of 2 cm composed of two PCM products with melting temperatures of 21.7 °C and 25 °C respectively, is investigated. The effectiveness of this composite PCM system is evaluated by quantifying the changes in indoor temperatures and energy demands for apartment buildings in the climate conditions of Toronto and Vancouver. A parametric analysis considering the building window to wall ratios, orientations and temperature set points is performed using simulations done with EnergyPlus^TM. The results show that the composite PCM system improves the indoor temperatures in both climates by reducing temperature fluctuations and balancing temperature peaks. Overall, the adoption of this system results in the reduction of cooling energy demand by 6% in Toronto and 31.5% in Vancouver. The results showed the benefit of applying two PCM melting points in one zone as an effective method for annual indoor temperature regulation, especially in Toronto.