During the first eight months of 2008, measurements of occupant behaviour and eight environmental variables was carried out in 15 dwellings. Logistical regression was applied to infer the probability of open window as a function of the outdoor temperature. The results were compared with the findings in the literature. The measured variables just prior to an opening/closing event were compared to variables where no events occurred. Indoor air quality and solar radiation where found to be the main drivers in the occupants’ determination of when to open a window. The indoor air quality and outdoor temperature affected when the window was closed and finally the time of day had an impact on the window opening behaviour of the occupants.