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By Werner Lang and Thomas Herzog

Although a dual-layered glass facade may be configured in many ways, it is essentially a pair of glass skins separated by an air corridor ranging in width from a few inches to several feet. The glass skins may stretch over an entire structure or a portion of it, or a single layer of glass may arch, tentlike, over several buildings with conventional or glass facades. The main layer of glass, usually insulating, serves as part of a conventional structural wall or a curtain wall, while the additional layer, usually single glazing, is placed either in front of or behind the main glazing. But no matter how the layers are arranged, they make the air space between the layers work to the building's advantage-primarily as insulation against temperature extremes and sound.

Dual-layered glass facades offer many indirect advantages as well, particularly the twin-face facade, which allows natural ventilation in an environment with high winds, such as high-rise buildings. This type, the most popular in Europe, enables users to control their working environment while helping to eliminate "sick-building syndrome," which can result from an over-reliance on air-conditioning.

In winter, the glass layers enhance the heat-insulating functions of the facade owing to the comparatively higher surface temperatures of the inner surface of the facade. Moreover, according to studies by Drees & Sommer, environmental engineers in Stuttgart, two-layer facades improve sound insulation properties by 5 to 30 decibels, depending on the layout of the floors.

THE AIR SPACE BETWEEN THE LAYERS OF GLASS IMPROVES INSULATING QUALITIES.

All types of dual-layered glass facades offer a protected place-within the air gap-to mount shading and daylight-enhancing devices such as venetian blinds and louvers. Sheltered from wind, rain, and snow, these shading devices are less expensive than systems mounted on the exterior. The protecting layer of glass makes it possible to operate the devices year-round and in any weather. Maintenance and replacement are also simplified by the gap. In addition, advanced daylighting systems, such as reflective blinds, prismatic components, and light grids, which are used to control daylight and heat gain, also are better installed in the gap.

Beyond the practical considerations, these crystalline facades often possess a fragility and weightlessness that make more conventional wall systems seem oppressively solid. The glass admits copious quantities of light, so that the interior appears barely enclosed.

Architects in central Europe are increasingly using dual-glass facades to create these often-ethereal spaces while reducing energy consumption. According to some estimates by environmental engineers, certain types of ventilated facades show energy savings of 30 to 50 percent. But much confusion exists in the U.S. and other countries about the function, construction, costs, and appearance of these facades. A successful application addresses this type of facade not as an isolated subsystem of a building, but as an integral part of the whole building-its construction, its technical equipment, and the overall energy balance.