Behnisch Double-Wall Facade
In winter the air cavity remains sealed, as the sun naturally heats the air to create a buffer between the interior and exterior; in summer the cavity is freely ventilated. A computerized building system automatically adjusts the sunshades for optimal solar protection, although each individual occupant may override the system, as desired. Likewise, occupants can control the degree of ventilation in each office. When a window is opened, a sensor automatically switches off the heating and cooling supply to that space, thereby increasing energy efficiency and avoiding waste.
The Building Envelope as Selective Filter
Because of the continuous fluctuation of all environmental factors across time, the building wall must be visualized not as a simple barrier but rather as a selective, permeable membrane with the capacity to admit, reject and/or filter any of these environmental factors. All building walls have always acted in this fashion, of course. Modern scientific knowledge and technical competence merely make possible much higher, more elegant and precise levels of performance than previously.
James Marston Fitch's depiction of the building wall as a selective two-way filter in American Building, first published in 1948, remains a useful analogy, particularly given today's growing concerns about energy efficiency and sustainability.
Fitch envisioned the building envelope as analogous to the skin of the human body, which adaptively responds to the external environment in an effort to maintain optimal internal conditions. But the capacity of the human body to accommodate the range of climatic conditions evident in the natural world is of course limited, and we therefore require what Fitch termed an ameliorating "third element."
This third element, which he calls a "meso-environment," acts as an interface between the microenvironment of the body and the macroenvironment of the external world. There are two primary manifestations of the meso-environment: clothing and buildings. As Fitch explains, each can be tailored to meet the requirements of specific people in particular situations (and, by the way, each may be subject to the whims of fashion). Clothing and buildings simply operate at different scales.
The curtain wall should act as a selective filter, purposefully controlling the flow of heat, light, air, water, and sound, as well as the subsequent impact that these elements exert on interior spaces. Examples of such filtering include the control of views into and out of a building, the transmission of natural light, the ventilation of interiors, and the regulation of rainwater.
At the most basic level of performance, however, a curtain wall should be structurally sound. The components of the curtain wall system must be designed and built to resist anticipated loads, such as those induced by wind, seismic activity, and the potential impact of objects or people against the wall. The most significant load on a curtain wall is often lateral wind load (except in cases where blast resistance is required).
As glass tends to dominate the contemporary curtain wall, the overall thermal performance of the wall often comes down to the selection and detailing of the glass panel, which can be an inherently poor thermal insulator. The glass treatments can greatly affect the way that glass conducts heat and transmits or rejects solar energy. Reflective and low-e coatings, consisting of microscopically thin layers of metals deposited on the surface, significantly improve the shading coefficient and thermal insulating properties of glass. These coatings selectively filter (transmit or reflect) the various wavelengths of sunlight to fine-tune the facade's performance.
In addition to specifying appropriate glass products and mullion configurations, designers continue to experiment with multilayered glass skins as a means of creating a more sophisticated selective filter. The main concept in this type of system is the creation of an air plenum, between two layers of glass, that acts as a moderating buffer between interior and exterior environmental conditions. The two layers may be positioned on either side of a single mullion, separated by several inches, or they may exist as two separately framed curtain walls, spaced several feet apart.
In periods when interior heating is required, the air cavity remains sealed and the air is heated naturally by solar gain. This warmed air can then be used either as a passive buffer, which reduces the need for mechanical heating, or as preheated intake air for the HVAC system. During cooling periods, the air cavity can be ventilated to provide a continuous flow of fresh air that can be routed into the interiors.
The area between the layers of glass creates a convenient and protected location for sunshades, which can be adjusted seasonally or daily to provide the optimal balance of views and shading, blocking unwanted solar energy before it strikes the inner glass wall and making large expanses of glass more feasible from an energy standpoint. The multilayer approach also offers the benefit of improved sound control.
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Scott Murray is an architect and assistant professor at the University of Illinois at Urbana-Champaign, where he teaches graduate seminars on innovations in building envelope design.
This article is excerpted from Contemporary Curtain Wall Architecture by Scott Murray, copyright © 2009, with permission of the publisher, Princeton Architectural Press.
Project: Terrence Donnelly Centre for Cellular and Biomolecular Research (Toronto, Ontario, Canada)
Owner: University of Toronto
Architects: architectsAlliance and Behnisch Architekten
Structural Engineer: Yolles Partnership
Mechanical/Electrical Engineer: HH Angus & Associates