Novel second-degree solution to single zone mass-balance equation improves the use of build-up data in estimating ventilation rates in classrooms

Recent advances in international guidelines for indoor air quality and on-going work on guidelines for ventilation in Europe increase the need for routinely applicable measurement methods for indoor air quality and ventilation. Ventilation is the key determinant of the behaviour of pollution from both indoors and outdoors in buildings and therefore should be measured together with other indicators of indoor air quality.Various methods have been proposed to measure ventilation rates in different types of buildings. Utilizing carbon dioxide as a marker substance and applying several ventilation phase-based methods together allows for recognizing air recirculation and quantifying CO2 emission levels that have been identified as challenges in CO2-based methods, as well estimating the intrinsic variability of ventilation rates over various levels of occupancy. In schools CO2 based estimation of ventilation has often been applied to decay phase only, which is not representative of exposure of the school children while using time periods when the classroom is not occupied and when the ventilation may be enhanced, e.g., by opening windows. This paper presents a novel solution to the mass-balance equation facilitating the use of the build-up phase to solve air exchange rates. The new equation allows to avoid impacts of device calibration errors and in combination with previously used steady state and decay methods improves significantly representativity of ventilation estimates in schools.Practical implicationsCombined use of occupancy phase based ventilation assessment method improves the quality of CO2-based estimates of ventilation alone and as part of more comprehensive indoor environmental quality measurements.