Window condensation in dwellings and offices can be very troublesome. It not only obstructs the view through the windows, but also runs off, accelerating the deterioration of paintwork, frames and furniture. In the present article, measurements and modelling techniques have been used to study the longitudinal temperature variation of double-glazed windows. It has been shown, herein, that the above temperature variation is primarily due to the buoyancy driven flow of gas within the sealed enclosure. This flow of gas alternates between the heated inward and cooled outward panes. The present work provides a physical model which can be used to calculate, with precision, the temperatures along the height of double-glazed windows. This method may be a prerequisite for the quantification of condensation, so commonly encountered in the European and North American climates.