This study investigates the thermal and optical performance of a double-glazed window (DGW) system incorporating a thin layer of solid-solid phase change material (SSPCM) under the climatic conditions of Montreal, Canada. Transient simulations are conducted for the hottest and coldest days of the year under both sunny and cloudy conditions. A parametric analysis evaluates the influence of window orientation (North, East, South, and West) and the SSPCM transition behavior on the optical and energy performance of the fenestration system. Natural convection (NC) effects within the glazing cavity are included to capture buoyancy-driven airflow. Results indicate that neglecting NC in winter leads to an overestimation of heat energy by 12% to 19%, while its effect in summer is minimal, allowing it to be excluded to reduce computational cost (4–5 times) without compromising accuracy. During summer, the SSPCM undergoes full phase transitions across all orientations, maintaining transparency during standard office hours but without significantly enhancing thermal performance. In contrast, winter simulations demonstrate measurable energy savings due to the SSPCM latent heat storage, which helps retain indoor warmth as exterior temperatures drop. The south-facing configuration provides the highest winter energy savings at 8.2% and the longest duration of visual transparency. The optimal design is a south-oriented DGW system with a 15 °C transition temperature SSPCM applied to the interior pane, offering the most effective configuration. It combines enhanced winter thermal efficiency with consistent daylight access, making it especially suitable for commercial applications seeking improved energy performance and year-round visual comfort.