Abstract <p>One of the main elements contributing to heat loss in buildings is the exterior envelope, which can be improved by employing passive solar systems. This research aims to investigate the Barra–Costantini system’s performance through simulation to obtain the best thermal performance with varying system characteristics, enhance thermal comfort, and reduce energy demand. Three types of glass—single-layer, two-layer, and three-layer—along with various channel dimensions (0.05, 0.1, and 0.15 m) and vent configurations were simulated in a residential building in Mashhad. For a room with a volume of 33.6 m³, the maximum heat gained with a 15 cm depth channel and single-layer glass resulted in an energy saving of 29.1%. The room temperature did not exceed 2°C with this system. However, this system alone cannot provide thermal comfort for Mashhad. The level of thermal comfort, as indicated by the Predicted Mean Vote index in October, with and without this system, is –1.6 and –2.06, respectively. Thermal comfort is improved during hot periods of the year by keeping external vents open during the day with an open roof vent at night instead of keeping all vents open throughout the day and night. Additionally, an increase in the height of the Barra–Costantini channel resulted in a decrease in the amount of heat gained.</p>