A study to measure energy performance and CO2 concentrations was conducted in two Hawai‘i classrooms to determine the impact of user decision-making on adequacy of fresh air. Using CO2 as a marker for indoor air quality and fresh air exchange, significantly different CO2 concentrations were observed in the two identical classrooms. In the Hawai‘i mixed-mode classrooms, ventilation and CO2 levels were dependent upon: user awareness of how fresh air was introduced; user training; availability of operable windows; and outside fresh air supply from the air-conditioning unit.