Natural ventilation is an effective method to save energy required to condition buildings and to improve indoor air quality. Computational fluid dynamics (CFD) was used to model single-sided buoyancy-driven natural ventilation in a single room with a heater and door. The velocity and temperature profiles at the doorway agreed fairly well with published literature that includes Mahajan's experimental [2] and Schaelin et al's numerical studies [1]. The 2D and 3D models predicted the neutral level with a difference of 5.6 % and 0.08 % compared to the experimental results, respectively. Using solutions at the doorway, heat transfer rates were calculated. More realistic situations were studied considering conduction, various ambient conditions, wind speeds, and additional heat sources and furniture in the room. The heat loss through the wall was modeled and the airflow and temperature within the room showed no significant changes despite modeling conduction through the walls. Various ambient temperatures and wind speeds were tested, and the neutral level height and total heat transfer rate through the doorway increased with decreasing ambient temperatures. However, the neutral level did not significantly change as wind speeds varied. Total heat transfer rate at the doorway became positive, that is heat transferred into the room, with wind speed. Lastly, the effect of additional heat sources (mini-refrigerator, monitor and computer) and furniture (bookshelf, desk, chair and box) on airflow and heat transfer in the room was analyzed by comparing with a simple case of a room with a heater. Large velocities and high temperatures were predicted in the vicinity of the heat sources. However, the spatially averaged velocity and temperature did not change significantly despite additional heat sources. The room with furniture was modeled at lower ambient temperature, where the spatially averaged velocities were larger and temperatures were lower than the simple case. The room heated up and reached its thermal comfort level, but the velocities exceeded the maximum acceptable level set by ASHRAE guidelines [8]. Wind was considered simultaneously with the lower temperature, and the room was cooled faster with wind. However, the room was never able to achieve the comfortable level both in velocity and temperature.