Estimation of infiltration rate using occupant-generated CO₂: Filtering process for uncertainty reduction

The occupant-generated CO₂ concentration decay method has garnered significant attention because it eliminates the need for repeated gas injections and enables air infiltration rate calculation using only indoor and outdoor CO₂ monitoring. This approach facilitates easy calculation of the infiltration rate based on the decay history of the indoor CO₂ concentration, which increases owing to occupants' respiration and decreases after their departure, using the mass balance equation. However, depending on the expertise of the researcher, the selection of an inappropriate decay period can lead to either overestimation or underestimation of the infiltration rate. This paper discusses three key considerations for ensuring reliable infiltration rate estimation using occupant-generated CO₂ and presents a filtering process. The increased measurement uncertainty in the later phase of CO₂ decay, when small changes in concentration affect the accuracy of infiltration rate estimates is discussed, and a filtering method to improve reliability is introduced. The overestimation caused by non-uniform CO₂ concentrations during the initial phase of decay is then addressed, suggesting a filter to correct this. Finally, the optimal point for minimizing measurement uncertainty in the final calculation is identified. The proposed process is evaluated using long-term field measurements and validated by comparing the calculated infiltration rates with those obtained using traditional tracer gas decay methods. This process allows daily variations in infiltration rates to be reliably estimated using only CO₂ monitoring, offering a valuable tool for improving indoor air quality and reducing energy consumption through integration with HVAC systems and energy simulations.