Phase change materials (PCMs) reduce energy consumption and improve indoor thermal comfort in buildings. Various numerical models have been developed to evaluate the thermal performance of the PCMs integrated into building enclosures. To remain computationally efficient, these models typically adopt a trade-off between the ability to investigate complex and realistic configurations and the prediction accuracy of the heat transfer associated with the phase change. This study developed a model to simulate a room with three-dimensional heat conduction in PCM-integrated walls and to analyze the surface heat balances with high resolution. The model was validated using experimental data of a passive solar test cell with a non-uniform and dynamic thermal environment. The model was then applied to investigate the PCMs integrated into the test cell during the summer. The PCM with a narrow phase change temperature range and a thickness of approximately 10mm was highly effective in reducing the indoor temperature fluctuations; the peak surface temperature was reduced by up to 6°C in the sun patch, and the operative temperature fluctuations decreased by up to 2.6°C. The PCM integration could be limited to the sun patch trajectory on the wall surfaces to optimize its utilization and limit the installation cost.