This paper evaluates the thermal performance of a simple triple-glazed window filled with a layer of paraffin wax (PCM) to reduce heat transfer through building openings. This research compared the performance of a traditional triple-glazed window with air gaps. The comparison was made using numerical modeling and experimental approaches. The experiment took a full day, and technical-grade paraffin was used as a PCM to fill a 20-mm gap between triple-glazing units. The temperature of the inner glass, the exterior surface, and both gaps of the glazed window were measured. The thermal performance of a triple-glazing window was evaluated numerically using a finite volume algorithm. The results showed that the solar radiation reached its maximum at 580 W/m2 in March, and the internal surface temperatures of the normal triple-glazed window (TW) and integrated with PCM (TW-PCM) were 37.1 and 34 °C, respectively. The obtained temperatures of the TW-PCM decreased by 3.1°C compared to the TW. Furthermore, the TW-PCM demonstrated an increased time lag of 2 hours, effectively delaying the peak load.