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By Werner Lang and Thomas Herzog

Twin face is best

A typology based on the function and construction of the dual-glass facades classifies the various systems and clarifies the different terms used in the design of the multilayered glass skins. There are three principal types of dual-layered glass facades: buffer, extract air, and twin face, which differ significantly with regard to ventilation and potential reduction of a building's energy demand.

Buffer facades have been around for almost 100 years and are still in use. They were developed before insulating glazing was invented to increase the heat- and sound-insulation properties of facades without reducing the use of daylight. They consist of two layers of single glazing mounted 10 to 30 inches apart. As with insulating glass, the gap between the two layers of glazing is sealed. Fresh air gets into the building through separate box windows, cut into the two layers of glazing. An early example of this type is the curtain wall of the 1903 Steiff factory in Giengen/Brenz, Germany. A modern example is the 1983 Hooker office building in Niagara Falls, N.Y., by Cannon Design in Grand Island, N.Y. This recent building improves on the model: warm air is exhausted through openings at the top of the facade, while cool air is drawn into a gap at the bottom.

EXTRACT-AIR FACADES WORK WELL FOR BUILDINGS IN WINDY AND NOISY LOCALES

Extract-air facades, popular in the 1970s and 1980s, consist of a second single-glazed skin placed inside a main facade of insulating glass. This type of facade is useful for buildings in windy and noisy locations or in places with fumes, where fresh air is supplied by the HVAC system because natural ventilation via operable windows is not possible.

The gap between the two layers of glazing becomes part of the air-conditioning system, since the "used," warm air is mechanically exhausted through openings in the inner skin and pulled into the gap of the facade. The warm air from the room heats the inner layer of glass, improving comfort and reducing heat lost through the facade during cold weather. From there, the extract air is led to heat exchangers (within the HVAC system) so the warm air tempers incoming fresh air and saves energy.

Shading devices may be mounted within the gap, keeping solar gains outside the occupied areas of the building in the summer, where they may be exhausted immediately. In this way, the extract- air facade lightens the cooling load. To supply the necessary air changes mechanically, however, demands energy and prevents occupants from adjusting the temperature of their individual spaces.

A better choice, and the most innovative and commonly used system today, is the twin-face facade. Developed in the late 1980s, this facade consists of a conventional curtain or massive wall system within an outer skin of single glazing. In most cases, the secondary glass skin is a non-load-bearing curtain wall made of a single layer of safety or laminated glass. It can also be made of insulating glass to enhance the thermal properties of the twin-face facade.

The width of the gap between inner and outer skins varies from 6 to 30 inches, depending on the space needed to maintain the facade and on the type and size of the shading system within the gap. Twin-face facades are primarily distinguished by openings in the skins that allow for natural ventilation. The outer glass skin blocks the wind, permitting high-rises or buildings in exposed settings to have access to fresh air. As a result, windows on the interior glass layer can be opened without gales sweeping through the space. Where external noise is a problem, remote openings allow fresh air to enter while minimizing sound.

The role of glazing

Combining a variety of glazing types with very different functional characteristics enhances the function of dual-skin glass facades. Early versions, such as buffer facades, used single glazing for the outer and inner skins.

Extract-air facades employ insulated glazing for the outer skin and single glazing for the inner skin. That's because this type was developed so that the warm "used" air passes through the gap in the facade, tempering the inner layer of single glazing, while the outer layer of insulating glazing minimizes heat-transmission losses.

Twin-face facades combine a single-glazed outer skin with an insulating-glass inner skin. While the function of the outer skin is primarily to protect the contents of the gap and the inner layer from rain and wind, the insulating glass on the inner layer prevents heat-transmission losses during cold weather. It also offers a range of construction variations for the outer skin, leading to easier architectural expression. For example, the substructure of the outer layer of glass is normally composed of aluminum or steel cantilevers. The glazing is then fastened with a point-fixed system or top and bottom rails.