Adequate supply of fresh air is essential to provide a healthy, safe and comfortable indoor environment in buildings. Currently, the majority of the residential buildings in mild climate European countries, such as Portugal or Spain, remain naturally ventilated. This fact has raised concerns in the building sector regarding the indoor air quality present in these buildings as fresh air supply mostly relies on occupants’ window opening behavior and personal habits, which can vary significantly from one apartment to the next. In this context, this work presents the indoor CO2 concentration levels registered during more than one year in the bedrooms and living rooms of eight apartments of a naturally ventilated residential building located in Porto, Portugal. Monitored CO2 concentrations confirmed relevant periods of time with inadequate indoor air quality, exhibiting great variations between apartments. Differential occupants’ window opening behavior, personal habits as well as presence of internal CO2 sources (e.g. smokers) were stated as the major reasons behind these wide discrepancies. Results suggest that, in some situations, implementation of mechanical ventilation systems in residential contexts should be promoted in order to guarantee adequate IAQ at all times and regardless of outdoor weather conditions or occupants’ window opening behavior.