CASE STUDIES
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Seattle Justice Center, from the southwest. The innovative double-envelope facade is ventilated to cool the large southwest-facing glazed area. Photo: Tim Griffith
Double Envelope Properties:
Curtain Wall Properties: Seattle Justice CenterNBBJArchitects must be mindful of multiple concerns in designing building facades. There are always trade-offs that must be made in optimizing building facade performance and the challenge lies in balancing conflicting criteria. A desire to maximize transparency, daylight, and views, for instance, can often be at odds with the need to minimize solar heat gain and reduce air-conditioning loads. Orientation is also an important factor. Site constraints or features, such as surrounding buildings or vistas, often dictate easterly or westerly facades, which are difficult to shade from low-angle morning or afternoon sun. For the design of the Seattle Justice Center, NBBJ had a public client with high expectations. The project brief emphasized daylight penetration, views, and outdoor connections, but they were not to be achieved at the expense of increased air-conditioning or energy expenditure. The project goals were to create a work environment that provided high occupant satisfaction, set an appropriate, open image for the City of Seattle, and address sustainable building design principles. The Justice Center features many green elements including a planted roof, the use of recycled materials, a rainwater recycling program, and energy conservation. All the sustainable building features were chosen and developed to express and emphasize good building practices. The City of Seattle set the goal to demonstrate and encourage such strategies for other projects. These programmatic goals had to be addressed on a difficult site. Constrained on three adjacent blocks by high-rise buildings and sharing the block with an existing six-level parking structure, the Seattle Justice Center opens to potential daylight and views, of the new City of Seattle campus and Elliot Bay, only to the southwest. Lot depth also made it difficult to provide daylight deep in the building. NBBJ therefore explored the use of a double-skin glazed thermal buffer to offset these site conditions. Existing research illustrates that vented, double-skin facades are an appropriate approach to maximizing the positive qualities of glazing while minimizing its negative energy impact and potential for thermal discomfort, especially on easterly or westerly glazed facades. Simply put, a shading device within the double skin can absorb solar gain and re-radiate it as heat trapped in the cavity. Exterior apertures at the wall's top and bottom induce air movement. The heat's natural tendency to rise pulls fresh, cool air in at the bottom while exhausting hot air out the top. Controlling the capture or venting of this trapped heat dictates cavity air temperature and, in turn, the inner glass surface temperature. A related advantage of double-envelope facades—especially when external shading elements are not possible for technical, aesthetic, or maintenance reasons—is the control they offer against direct solar gains, particularly in a building's perimeter zone. For instance, operable shading located in the double-envelope cavity, as opposed to internal blinds located in the internal perimeter zone, stop direct solar heat gain before it enters the perimeter zone space and becomes an air-conditioning load. When open, the shades provide high transparency when direct sunlight is not an issue, and they can be closed to provide shade from low-angle sun. Direct solar heat is absorbed and re-radiated when trapped within the cavity of the double skin, inducing a thermal stack, which is the driving mechanism for the cavity ventilation. The double envelope, in short, can offer significant advantages with regard to thermal energy control and daylighting if properly designed and operated. Yet, just as external shading devices reflect a particular building site, climate, and orientation, a vented double-skin facade must be matched to the climate, orientation, and building program in order for it to perform well. Double-skin systems are prevalent in European climates, but less common in the United States. NBBJ studied the overall building envelope in light of local energy codes and standards, and the specifics of different building facades and their impact on the adjacent interior perimeter zones, to demonstrate their applicability.
View of double envelope with interior light shelf (left) and interstitial space in the double envelope (right). Photos: Kerry Hegedus The design had to be adapted to the Seattle context and climate. NBBJ teamed with Arup in developing their preliminary facade proposal. With sophisticated computer modeling, interdependent parameters could be understood to keep the facade's initial cost in alignment with the building budget, as well as meeting all the other criteria established in the building program. Experience has shown that in benign climates such as Seattle's and in projects like the Justice Center, with a high percentage of core floor area to perimeter zone floor, saving significant amounts of energy from advanced facade design is difficult to achieve. The low cost of energy typically inhibits realization of overall building energy savings, offsetting the anticipated additional facade system's first costs. Not surprisingly, while the highly effective facade system proposed for the southwest-facing facade indicated a 33% energy savings in the adjacent perimeter zone, preliminary estimates indicate this to result in only a 2% overall building energy savings. As with any project, the quality of the environment desired must be weighed against cost. In this case, the impact was felt in the initial cost of the final vented, double-skin facade. Cost estimates indicate that the final vented, double-skin solution would be $320,000 ($17.50 per square foot of facade) greater than a traditional glazed facade. The previous technical analyses allowed the design team to achieve the aesthetic goals of the project, increase perimeter floor-to-ceiling heights, and bring more daylight deep into the building; these variables were ultimately difficult to quantify in a life cycle cost analysis. Measured on its own, the double-envelope, thermal-flue facade has limited energy benefits (considering the limited area and orientation). But considering the orientation, client goals, aesthetic considerations, and the Seattle Justice Center's overall sustainability strategy, the thermal-flue concept is seen as an essential feature of the project, extending beyond its measurable qualities.
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