Hydronic radiant floor systems enhanced with phase change materials (PCMs) could achieve significant energy savings while improving the thermal comfort of occupants in lightweight buildings. However, successful integration of PCMs typically requires comprehensive numerical analysis due to their complex nature. This study aims to investigate two scenarios for optimal integration of PCM into the hydronic floor heating system of a highly glazed study room exposed to cold weather conditions. Scenario 1 includes optimization of two design variables, including PCM melting temperature and thickness. Scenario 2 encompasses optimization of seven design variables, including PCM melting temperature and thickness, insulation thickness and thermal conductivity, floor thickness, thermal conductivity, and solar absorbance. Both scenarios are optimized for the six supply water temperatures, ranging from 35.6 °C to 45.6 °C with a 2 °C step. The overall findings suggest that successful integration of PCMs into the hydronic heating system requires a comprehensive solution tailored for the specific application. Thus, scenario 1 and scenario 2 achieved the highest total energy savings of approximately 17.7％ and 20.5％ for the lowest supply water temperature of 35.6 °C, whereas under both scenarios the supply water temperature of 43.6 °C provided the best thermal comfort. Furthermore, scenario 1 achieved more substantial cooling energy savings over the wider temperature range (41.6–35.6 °C) compared to scenario 2 (39.6–35.6 °C). The findings also suggest that the addition of the insulation layer in the second scenario reduced the thickness of PCM and payback period for more than 70％ compared to the first scenario.