Using Designer's Simulation Toolkit DeSTh developed by Tsinghua University, research was carried out on the energy consumption of a typical residential building in Ningbo, P. R. China, with a focus on the heat transfer coefficient of external windows facing every direction. Results show that variation of the heat transfer coefficient of external windows in all directions has a substantial effect on energy consumption for heating but little effect on energy consumption for cooling. It is difficult to decrease cooling energy consumption by decreasing the heat transfer coefficient of external windows in summer. The heat transfer coefficient of external windows facing south has a significant effect on building energy consumption, but the windows facing north have less effect, and those facing east and west have the least impact. Decreasing the heat transfer coefficient of external windows facing north and south from 4.7 to 3.2 produces the most favorable performancecost ratio for energy saving investments. Reasons for the decrease in the heat transfer coefficient of external windows leading to increased air conditioning energy consumption were discussed. The conclusions are useful for guiding energy efficient building design in other subtropical cities.