The present study investigates experimental cooling from the ceiling using phase change materials (PCMs) in Yazd, a city characterized by a hot and dry climate. A one-fourth scale model of a real room, measuring 4 m × 3 m × 3 m, was employed for the analysis. To evaluate system performance and the impact of PCM on energy consumption reduction, three configurations were considered: a simple PCM system, a PCM system with a fan (PCM-F), and a PCM system with a mini cooler (PCM-C). Additionally, to assess the influence of window configurations on ventilation, temperature, and comfort conditions within the model, three scenarios were examined: two open windows, one open window, and no windows. The economic analysis compared the two systems with the fan and mini cooler against a full mechanical cooling system without PCMs. Furthermore, CO₂ emissions and environmental impacts associated with the systems were also evaluated. The results indicate that the presence of PCMs in the ceiling, due to heat absorption during phase change, leads to a temperature reduction of 5 to 10 °C in the ceiling and a 3.2 °C reduction in the average room temperature compared to the scenario without PCMs. The findings demonstrate that ceiling cooling with PCMs significantly contributes to energy consumption reduction during peak hours of cooling demand. Specifically, the PCM-F system results in a 92% reduction, and the PCM-C system leads to a 71% reduction in total cost compared to the reference mechanical cooling system. Additionally, the PCM-F system achieves approximately a 36% reduction, and the PCM-C system results in a 34% reduction in environmental impact relative to the reference full mechanical cooling system.

Roofs are the building envelope component most exposed to solar radiation in hot–arid climates, and phase change materials (PCMs) are widely used to buffer the resulting heat gain. However, almost all reported PCM-roof studies use flat roofs, leaving the interaction between roof curvature and PCM performance largely unexamined. This study numerically compares flat and barrel-vault roof geometries integrated with PCM for a single-room residential model under hot–arid climatic conditions, using three-dimensional transient computational fluid dynamics (CFD, ANSYS Fluent) with solar ray tracing. A two-stage design first isolates the effect of roof geometry (flat vs. barrel vault, both fitted with the same PCM) and then the effect of PCM thermal properties (PCM1–PCM3, with melting ranges of 29–34, 27–29 and 34–36 °C, in the barrel-vault geometry). On the fourth simulated day, once a periodic state had been reached, the barrel vault reduced the peak indoor air temperature by 1.7 K and the 24 h mean by 1.5 K relative to the flat roof. Among the PCMs, PCM3 gave the lowest peak indoor air temperature (29.5 °C), whereas PCM2 gave the lowest 24 h mean (25.30 °C against a 32.30 °C ambient mean) but the highest peak. All three PCMs entered their melting ranges every day, but only PCM1 completed a full melt–freeze cycle. The results show that roof curvature and PCM melting range selection are two separate levers for passive cooling of buildings in hot–arid climates.