This paper aims to study the variability of indoor CO 2 concentration due to occupant behaviour and physical parameter uncertainties. A case study, conducted in a mechanically ventilated detached house, is presented with an uncertainty and sensitivity analysis (Monte Carlo method with a Latin hypercube sampling). Uncertainties related to occupant behaviour are described by combining four types of scenarios: occupation, generation of CO 2 per person, indoor doors, and outdoor windows' openings. The uncertainty analysis showed that despite an acceptable average room CO 2 concentration, large variations , due to input parameter uncertainties, are observed in CO 2 instantaneous concentrations. Moreover, during occupied periods, average value is relatively important (higher than 1300 ppm). Occupants spent around 30% of the time at CO 2 concentrations over 1500 ppm. Large output uncertainties are reached on the cumulative CO 2 concentration and time fraction spent over 1500 ppm. The sensitivity analysis highlights the strong influence of the parameters related to bedrooms (number of occupants, night generation of CO 2) and of the kitchen extracted airflow rate. It also shows that low-level air change rates in bedrooms are mainly caused by an incorrect air distribution in the building. Potential solutions to reduce both concentrations and uncertainties are discussed.Read less <