The application of phase change materials (PCMs) in smart window technologies offers a promising strategy for improving building energy efficiency and enhancing indoor environmental quality. However, climate-specific performance optimizations—particularly evaluating local commercial PCM formulations in semi-arid conditions with high diurnal temperature variations—remain insufficiently explored. This study investigates the thermo-energy performance of PCM-integrated double-glazed windows for residential buildings in Karaj, Iran, using validated EnergyPlus simulations with the Conduction Finite Difference (CondFD) algorithm. Unlike generic studies, this research evaluates locally available PCM formulations across various phase-change transition temperatures (21°C, 24°C, and 28°C). Results demonstrate that PCM-integrated glazing substantially reduces both heating and cooling energy demands. Specifically, annual cooling electricity loads decrease by 6.6% to 7.5% (with PCM24 offering the highest cooling performance), while annual heating natural gas consumption drops by 36.8% to 39.8% (with PCM21 yielding the greatest heating savings). The PCM21 configuration achieves the highest total annual energy saving of 4,063.1 kWh (representing a 17.5% reduction in overall building energy consumption). Comparative analysis indicates that integrating PCMs into glazing under semi-arid conditions yields up to a 10% higher heating load reduction than reported in humid or cold climate baselines, driven by substantial diurnal temperature fluctuations. These findings identify optimal phase-change threshold criteria for semi-arid Iranian climates, providing actionable insights for energy-conscious architectural design and building envelope retrofits.