A large inquiry campaign began in 1985 on 100 social housing estates and 2,334 families were visited. The housing estates were selected according to their age, location, type of building (dwelling/apartment) and heating system. The main purpose of this paper is to explore the data received concerning window use in order to find the effect of the inhabitants on the ventilation rate. The summertime and wintertime situation was analysed and differences between individual dwellings and apartments were also investigated. The results of this study can be summarized as follows: - a rather good description of window use in Belgian social houses was achieved. - the impact of window use on the ventilation rate is estimated.