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By Werner Lang and Thomas Herzog

Partitioned gaps

Horizontal and vertical partitions divide the gap between the glazing layers in the partitioned-gap facade. This division influences fire protection, sound insulation, and natural ventilation, depending on which partition-gap system is used. Types include the following:

Corridor facades, one of the most commonly used twin-face types, are divided with horizontal partitions at each floor. Fresh-air intakes and exhaust areas are located on each floor; the openings for each level are often placed above one another, alternating in a horizontal direction with the openings for the exhaust air. The construction of corridor facades demands more than an undivided twin-face facade due to the additional elements for the partitions and the number of ventilation openings necessary for each floor. The functioning of the facade improves, however. Overheating in the upper reaches of the building is reduced, as is noise transmission and smoke and fire spread.

Shaft-type facades have vertical divisions in the gap between the glass layers, which create shafts for moving the extract air out of the building. Ventilated sections between the shafts draw fresh air into the building through double windows.

Natural ventilation is better with shaft-type facades than corridor facades; fresh air enters the building via openings in the outer skin in the lower part of the double window. The exhaust air leaves the room in the upper part of the double window and enters the shaft through openings in the upper part of the partition between the shaft and the double window. As in a solar chimney, buoyancy forces due to temperature differences induce an upward flow of air in the shaft, thus improving air movement within the building by creating lower air pressure in the double window. Compared to other types of twin-face facades, shaft-type facades present problems with fire protection, noise, and the mixing of fresh and foul air.

TWIN-FACED FACADES CAN COST TWICE AS MUCH AS STANDARD CURTAIN WALLS, BUT ARE OFTEN WORTH THE EXPENSE

Double-window facades are more complex to build than other types of twin-face facades, because the gap between the inner and outer glass layers is divided vertically and horizontally. The facade consists of separate window units that have no immediate interaction with adjacent units. Separate ventilation openings are positioned with an offset in order to prevent the mixing of fresh and foul air.

Commonly used in office buildings, double-window facades reduce smoke and noise transmission within the air space due to the vertical and horizontal partitions. On hot, sunny days, temperature extremes within the facade are minimized by frequent ventilation openings in the outer skin. As in other twin-face facades, natural ventilation is possible much of the year. Other advantages of twin-face facades, such as nighttime cooling of the interior, mean considerable savings on cooling loads.

At what cost?

The various types of multiple-glass facades offer architects ways to enhance occupant comfort, reduce energy demand, and provide natural light and natural ventilation. But despite these advantages, they are seldom used outside central Europe. Experience and extensive testing by the designers of these structures prove that the facades can be easily adapted to other regions besides central Europe; indeed, they are occasionally used in parts of Japan and China.

The principal drawback in Europe and elsewhere is cost. In central Europe, twin-face facades are about twice the price of conventional curtain walls. In the U.S., they are likely to be four to five times more expensive. The extra costs are racked up by the expense of engineering these systems, the amount of special glass required, and an unfamiliarity with these systems among the trades, which leads to higher installation costs. In addition, mechanical engineers must think creatively (which often leads to more time and higher costs) about the heating and cooling loads and the type of hvac system used.

Despite the higher investment, twin-face facades are common among high-rises in central Europe, principally because energy prices are significantly higher in Europe than in the U.S., resulting in a faster return on investment.

But there is more to evaluate when considering a dual-glazed system than the hard costs. In central Europe, studies conducted by employers as well as engineers show that the satisfaction and productivity of those who work in buildings with these facades is higher, thanks to the many benefits that twin-face facades can offer, including natural ventilation, daylighting, and greater control over the workplace atmosphere. Depending on labor costs, the higher investment might be worthwhile.