Natural ventilation is an effective method for energy conservation while potentially improving indoor air quality. Furthermore, a window has a significant impact on energy consumption. The purpose of this study is to reduce building energy use by changing ventilation rates and window areas. We analyzed the energy consumption and CO2 emissions according to ventilation rates and window areas through using a computer program, HEED 3.0. The reference model had two windows of 1.2m×1.5m, and assumed there was no ventilation except for the minimum ventilation rate of 0.35 ACH. In order to determine the combined effects of changes in ventilation rates and window areas, three ventilation rates; 1.0 ACH, 5.0 ACH, 20.0 ACH, and three window areas were studied. The findings were as follows: annual energy consumption was reduced by 10.6%, CO2 emissions by 16.7% compared with the reference model by changing ventilation rates and window areas.