Indoor air quality is important to occupant health. Ventilation is a widely used technique for improving indoor air quality. However, improper ventilation results in heat losses up to 70.0% and is often the source of energy usage problems. According to the final report of the HOPE [11], the simultaneous assurance of ventilation, energy reduction and occupant health remains a design challenge. In addition, CO2 is a pollutant which is largely impacted by air exchange, the effects of which cannot be neglected in indoor air quality. Therefore, this study focuses on levels of CO2 and calculates the makeup air rate and recirculation air rate of indoor spaces. The optimum ratio of makeup air and recirculation air is evaluated in Korea to maintain acceptable indoor air quality and to save energy. (c) 2011 Elsevier B.V. All rights reserved.