Air change rate is often used as an important characteristic of indoor environmental quality, which significantly impacts human health. However, easy, effective, real-time and low-cost air change rate measurements in naturally ventilated resident buildings are still a huge challenge. This paper presents a method based on the release of a stable rate of the tracer gas CO2 given off by solid CO2 (dry ice) in an insulated box. In theory, the dry ice will sublime at a constant rate as long as there is sufficient dry ice in the box. Thus, the dry ice sublimation rate should remain constant at any time once the steady state heat transfer condition has been reached. This method can be applied with people present if necessary and can be used in almost all types of applications, including both naturally and mechanically ventilated buildings.