The world’s major advanced countries and the Republic of Korea are reinforcing efforts to realize Zero Energy Building (ZEB) by presenting the Green New Deal as a low-carbon leading strategy. In order to realize ZEB, efforts are being made to ensure that the windows and shells of buildings are insulated and air-tight, but no effort has been made regarding the indoor environment. For this reason, the Ministry of Environment continues to strengthen the recommended indoor air quality standards, but most occupants of residential buildings open their windows to improve indoor air quality. However, natural ventilation through the opening of windows may make indoor environments uncomfortable and increase energy consumption through heat loss and heat gain in extreme cold and hot conditions. Therefore, in this study analyzed window-opening behavior, which can increase indoor air quality and affect energy consumption; the amount of natural ventilation was analyzed using the carbon dioxide decay method. Based on the analyzed amount of natural ventilation, heat loss and heat gain were analyzed and compared with energy consumption.