Windows can cause significant thermal energy gains or losses in buildings. Focusing on wintertime, a simple method for analyzing and discussing energy flows through glazings is presented. The impact of the glazing quality, the facade orientation, and the severity of the climate on the ratio of solar gains to thermal losses through glazings are shown. As regards the passive solar heating of buildings, the glazing quality is represented by the ratio of the total solar energy transmittance to the thermal transmittance (g/U). The severity of the climate is determined by the ratio of the interior-exterior temperature difference to the solar irradiance (Delta theta/I). In this study, the method is based on monthly mean values of interior-exterior temperature difference and solar irradiance. Not at least because the approach is straightforward, it might also be valuable for educational purposes. For eight case study locations in Europe namely Bucharest, London, Madrid, Moscow, Rome, Stockholm, Warsaw and Zurich, charts are presented, which display condensed information on the energy performance of glazings. At a given location, the gain-to-loss ratio varies by a factor of roughly 30 depending on the glazing quality and the facade orientation. At all locations and facade orientations, modern triple glazings perform best and enable net gains at south facades in December even in Moscow and Stockholm. (C) 2011 Elsevier Ltd. All rights reserved.