Natural ventilation is an effective method to simultaneously improve indoor air quality and reduce energy consumption in buildings, especially when indoor temperature is close to ambient temperature e.g. the transitional seasons in Germany. Heat loss due to opened window and ventilation effectiveness ratio were analytically modeled. Following that, the effects of thermal buoyancy on the steady classroom airflow and thermal stratification comfort as well as the contaminant dispersion were discussed. Classroom displacement ventilation and its thermal stratification as well as indoor air quality indicated by the CO2 concentration have been investigated concerning the effects of supplying air temperature and delivering ventilation flow velocity. Representative thermal comfort parameters, percentage dissatisfied and temperature difference between ankle and head have been evaluated. Subsequent energy consumption efficiency analysis illuminates that classroom energy demands for natural ventilation not only in transitional seasons but also in winter could be decreased with the promotion of the ventilation effectiveness ratio for heat distribution when the natural ventilation rate maintains a constant, and with the shrinking of the ventilation effectiveness ratio for heat distribution when the supplying air temperature is not variable. Detailed fitting correlations of heat loss resulted from opened window and ventilation effectiveness of natural ventilation inside the classroom have been presented. (C) 2013 Elsevier B.V. All rights reserved.