A CO2 concentration of more than 1000 ppm has been monitored in Taiwanese bedrooms during sleeping hours in the wintertime. The high indoor CO2 levels were caused by poor ventilation due to insufficient ventilation rates. This study sought to reduce the wintertime CO2 concentration level in a typical Taiwanese bedroom with less outdoor air to maintain thermal comfort. CO2 was used as an indicator to assess whether an adequate ventilation rate has been obtained to dilute or remove harmful pollutants. With the help of the thermal buoyancy effect, the CO2 generated in the bedroom was effectively removed by means of less outdoor air. Through computational fluid dynamics simulations, the appropriate window and transom locations with the corresponding outdoor air supply volume, as well as the lowest possible outdoor air temperature were identified.