High-rise apartments are a prominent type of residential buildings in Canadian cities. However, poor aging performance of existing apartments has led to high discomfort and energy consumption that must be addressed. Thermal energy storage is a potential energy retrofit measure that affects energy consumption by regulating radiant temperatures. The aim of this study is to evaluate the effectiveness of latent thermal energy storage using phase change materials (PCMs) integrated into walls and ceilings of apartment units. A composite PCM system comprised of two different PCM products with melting points of 21.7 oC and 25 oC is proposed and evaluated to provide a year-around thermal energy storage. A simulation analysis using Energy Plus is performed to investigate the impacts of the composite PCM system on indoor temperatures and energy use. An experimental study is conducted using two small scale test cells to monitor the performance of the PCM system in detail.