Relevance of CO2-based IAQ indicators: Feedback from long-term monitoring of three nearly zero-energy houses

There are a large number of indicators that use CO2 concentration as parameter to assess air stuffiness and, consequently, to asses IAQ. Their comparison is difficult since they are not usually linked to each other. The aim of this article is to compare the results of 10 CO2-based IAQ indicators and determine if they classify a house in a similar way during heating seasons. We propose a method to normalize the results based on the reference values of each indicator, and we highlight the sensitivity of the indicators to the choice of one occupancy scenario among several possibilities. The database used contains the CO2 concentration measured over 2–3 years in the living room and the parental bedroom of three new and occupied nearly-zero energy houses in France (COMEPOS project) with low-cost probes sampling every minute. The results indicate that the IAQ of the same house in the same heating season can be classified differently depending on the indicator and threshold chosen. Moreover, an indicator can show different results for the same room over the years. For example, the IAQ of the bedroom of House 2 is classified poor in 2017 and 2019 but good in 2018 according to the mean CO2 concentrations with a 1000-ppm threshold. The indicators also present different levels of sensitivity to occupancy scenarios, being the cumulative exposure the most sensitive by increasing up to 257% without an occupancy scenario, which highlight the importance of the systematic implementation of a standard occupancy scenario for the CO2-based IAQ performance indicators.