In highly energy-impervious building envelopes, windows serve as major portals for the entry and exit of light and heat. BIDR architects Yair Etzion and Evyatar Erell (2000) have designed a novel combination of window glass - known as a reversible glazing system - that selectively modulates the admission of light and heat according to the season of the year.


 

 

Reversible glazing system. The double-paned device can be seen at the center of the window frame rotated some 45 degrees on its axle, with ventilation openings at top and bottom. After the glazing is properly positioned for winter or summer, the window fastener at the left is locked in place.



The window is built around two closely positioned glass panels, one of which is conventional glass while the second strongly absorbs sunlight and heats up. The hot window is naturally cooled by a ventilation path between the two glass panes, which removes heat via convection.

In summer, the window - which is built on a rotating axle - is positioned so that the absorptive panel faces outside. Solar energy is thus primarily absorbed by the exterior panel and heat is released back into the atmosphere by the flow of air through the interwindow space. Solar energy does not penetrate to the inner space, although about 25% is transmitted for illumination. In the winter, the window is rotated so that the absorptive glass faces the building's interior. The solar energy that it absorbs is released as heat to the inside air, helping to keep the house warm during cold winter evenings.

This novel glazing system has a practical benefit. When regular windows are used in winter to admit direct sun for passive heating, this radiation - and particularly its ultraviolet portion - takes its toll on furniture upholstery, carpeting and woodwork, causing them to fade and reducing their useful life. Since the reversible window traps most of the sunlight in the dark absorptive glass, the room is heated but interior finishes are protected. In summer, the double window provides an additional benefit, as for all practical purposes is is shaded without the use of shutters, which restrict vision and may limit natural indoor illumination.​