During the period of redevelopment of many museums in Austria and Germany in the last two decades, much emphasis was laid on the improvements of house technique as well as on reducing energy losses in winter. The latter was done sometimes by replacing the traditional double-pane chest windows with single casement windows with insulation glass. But the double-pane chest window is an intelligently developed part of historical museum buildings. If properly repaired, sealed and maintained, and then retrofitted with a physically adequate shadowing system, such windows are the perfect solution to the opposite climatic stresses of winter and summer. One of the unexpected consequences of these refurbishments, however, is the fact that sun-exposed (but unsuitably) shadowed windows, together with the exhibition lighting, considerably increase the indoor temperature in summer. We could quite often observe the same problems under sky-light roofs after the original green coloured glasses were replaced with translucent insulation glass panes. In this paper I will illustrate that through an improperly adapted chest window, the surface temperature of the inner glass pane can rise above 60°C. On the other hand, it is possible to reduce the surface temperature of the same sun-exposed window down to 31°C at most, only by means of an adequate arrangement of different sheets, coated panes and through natural ventilation of the window chest. With a physically adequate shadowing system and a variable lightning concept, the running costs of an exhibition in summer can be reduced to a factor of ≥3.