Seeing Double_feature, AGG, JulyAugust 2011

Volume 25, Issue 4- July/August 2011

feature

Seeing Double
Double-Skin Façade Designs on the Rise in North America
by Jeffrey Vaglio and Mic Patterson

We’ve heard about them, some of us have seen them, fewer of us have actually worked with them, but that may be about to change. In spite of the adverse economic conditions, double-skin façade (DSF) applications have actually increased; they’re part of the green trend that continues to thrive in the down economy. So what are they, what’s the point and can you expect to see one in your backyard anytime soon? Well, it depends a little on where you live, but with recent applications in major metropolitan areas including New York, Boston, Chicago and Los Angeles, the chances are that there may be one not too far from your doorstep.

DSFs are simply a strategy for improving building envelope performance through the introduction of a second glazed layer, thereby creating an airflow cavity between the two.

The application of the technology in the United States has been a long time coming. Although early examples of DSFs exist here—the Occidental Chemical Center in Niagara Falls, N.Y., in 1980 is but one example—the major development and implementation of the technology took place in Northern Europe through the 1990s and 2000s. Numerous completed examples of a great variety of DSFs were driven by legislative mandates for improved energy efficiency in buildings. The impetus for the initial development of DSFs was not only thermal comfort and energy efficiency; it was also about acoustical performance, as these façades mitigate sound transmission through the glazed building envelope. This is still a very good reason for their use, especially as our dense urban environments become increasingly populated with residential dwellings. Nonetheless, thermal performance and natural ventilation have been the more recent drivers of this advanced façade technology.

It’s All About the Cavity
Let us provide some background on DSFs. The cavity is useful for a few things. First, it acts as a thermal and acoustical buffer between the inside and outside environments. Second, the cavity can be employed in various ways to provide airflow and even building ventilation. Third, the cavity provides an optimal space for the location of shading devices: outside the inboard skin so that solar radiation is stopped before penetrating into the building, yet shielded from the elements by the outboard skin. If the cavity is deep enough, it can also house mechanical equipment and maintenance platforms. Cavity depth ranges widely—from about 4 inches to 6 feet—among the various built DSFs. It should be no surprise then, that the applications of DSFs are most often categorized by variations in cavity design and behavior. Specifically, ventilation type, ventilation mode and cavity partitioning are the most commonly used criteria.

The ventilation type refers to the driver of airflow within the cavity, which can include natural, mechanical and hybrid systems. The ventilation mode refers to the airflow pathway from intake to exhaust. The five common ventilation modes are outdoor air curtain, indoor air curtain, air supply, air exhaust and buffer zone. The diagrams in Figure 2 trace the pathways characteristic of each mode. Finally, DSFs are most usefully classified by the cavity partitioning strategy employed in any given design. The four primary cavity configurations are box window, shaft-box, story-height (corridor) and multi-story (see Figure 3). Each configuration possesses unique attributes of design, performance and application. The multi-story types tend to be the deep cavity systems, while the other configurations typically utilize much shallower cavities.

Trends in DSF Applications
In a recent evaluation of 23 existing applications, the most common DSF cavity partition configuration in the United States is the multi-story (70 percent) and the most common ventilation mode is the outdoor air curtain (74 percent). The multi-story DSF cavity has no horizontal or vertical divisions, and may encompass an entire elevation of a building façade. Intake air openings are placed at the bottom of the cavity, with exhaust openings at the top. Ventilation of the cavity can be induced naturally through the stack effect (as the cavity air warms it rises and is exhausted through the top vent, in turn drawing air into the cavity through the bottom vent) or mechanically assisted as required to prevent overheating of the cavity air. The more advanced designs utilize this cavity behavior to provide ventilation to the building. The Richard J. Klarcheck Information Commons at Loyola University in Chicago (see Figure 4) utilizes this effect in a west elevation DSF. In this application the stack effect is augmented by offshore winds that act to draw air from the cavity at the top vent.

Multi-story DSFs can provide a unique, highly transparent aesthetic, abundant daylight, a thermal buffer, enhanced acoustical performance and can contribute to building ventilation. Potential disadvantages include flanking sound and odor transmission through the cavity, overheating of the cavity air if ventilation is inadequate and building code issues with respect to fire-safety because of the lack of fire-safing between floors. Design flexibility is greater with the multi-story DSF types than with any other category. Many variations are conceivable, and this DSF type has been applied on educational, museum and healthcare facilities, among other building types.

The evolution of DSFs in the United States exhibits other emerging trends. An alternative to the multi-story system is the increasingly popular box-window type, with a cavity depth at the shallow end of the spectrum, typically in the range of 4 to 8 inches. This DSF type is configured as a modular, prefabricated, unitized curtainwall system appropriate for high-rise buildings. The location of mechanized systems, such as shading products, within the cavity of the DSF means that the cavity must be accessible for maintenance purposes, significantly complicating the design of a unitized façade system. The lack of a hermetic seal in the unit means that airborne particles and moist air potentially can infiltrate the cavity, resulting in dirt and condensation on the inner glass surfaces and further escalating the maintenance requirements. Current development efforts are aimed at addressing these issues. In addition to high-performance unitized curtainwall systems capable of cladding an entire building, box-window configurations can be developed as discrete window or window wall units, and have been used as a façade component in office, residential and hospital projects where the floor plan is subdivided into many repeating units (offices, condos or patient rooms).

AJ Celebrezze Federal Building to be
Re-Clad Using Double-Skin Technology
by Ellen Rogers

The AJ Celebrezze Federal Building in Cleveland, built in the mid-1960s, is undergoing a transformation that includes a double-skin technology designed to create a more energy-efficient structure. Interactive Design Inc. (IDEA), led by partner Charles Young, was chosen as the project architect. IDEA’s design plans will utilize a glass double-wall technology it says has never before been used on a high-rise in the United States, though common in Europe. The technology is designed to upgrade the perimeter structure to improve the costs of operating the building and minimize temperature variances throughout the edifice utilizing advanced systems available for cladding. The project is funded through the American Recovery and Reinvestment Act (ARRA).

“In the planning phase, it became obvious to us that the design needed to utilize solar orientation to drive the architectural concept, as seven months out of the year Cleveland experiences relatively cool to cold temperatures,” says Young. “We determined that the Celebrezze building was an ideal candidate for double-wall technology, a sustainable green and advanced technology system, which utilizes double glass lites to insulate the building and maintain visual purity.”

According to architects, a new external curtainwall will be placed approximately 2 ½ feet beyond the face of the original building skin. In this way, the existing façades can be retained and used as the internal portion of the double wall resulting in a thermal blanket similar in concept to a thermos bottle. This approach helps ensure minimal disruption to the existing tenants in the building by utilizing the existing façade to serve as a protective barrier between the façade construction process and building employees.

In designing the structure the IDEA team looked at how the building responded to the sun’s light. Downtown Cleveland is planned on a grid, similar to other major cities, such as Chicago, and in both cities the grid runs parallel and perpendicular to the lakefront. While the Chicago lakeshore runs more or less south to north, the Cleveland lakeshore runs southwest to northeast roughly 35 degrees off latitude. As a result, the building receives sun exposure on all four façades throughout the year.

In response to this solar orientation, the design addresses north and south facing façades differently. The east and north façades receive mostly oblique angled light in early morning and late afternoon. The façades for these two sides are transparent with perpendicular interior fritted glass fins that filter and modulate this oblique light. Glass edges are detailed to float over the existing building to highlight the new high-performance skin and the original façade structural frame.

The south and western façades receive direct midday solar exposure. These façades have been developed with a double-wall construction to provide an insulating envelope in winter months and to reduce heat within a barrier zone in summer months. Horizontal light shades are utilized within the cavity to reduce direct sunlight on the interior tenant space. Glass is treated with a graduated frit pattern from almost opaque to clear from top to bottom to further reduce direct glare and heat loads on the building systems and improve tenant comfort.

Future Developments
Arguably, the most compelling future application of DSF technology is in building retrofits (see sidebar on page 10). Realizing energy consumption and carbon emission reduction goals established by various green platforms, such as the White House Agenda and the 2030 Challenge initiative, will require energy retrofits to a large percentage of the current building stock, many of these programs should include façade retrofits. Many of the early glass curtainwall towers built during the 1960s and 1970s were constructed originally as single-glazed façades with low visible light transmittance glass; they were poor energy performers from the beginning and now are approaching something very close to old age. The addition of a second skin may prove to be a viable approach in some, if not many, of these buildings, providing greater economy, modernizing the appearance and improving energy performance.

Conclusions
DSFs are one strategy of an emerging advanced façade technology that includes new glazing materials, improved framing systems, progressive techniques and novel designs. That unique attribute of glass assures it will remain a predominant material in the building skin.

Glass, however, as we well know, is a poor thermal and acoustical insulator, and these negative attributes threaten to limit its use in this same context. It is imperative that we, as an industry, do not adopt a defensive position in an attempt to protect a vested interest. We must embrace the mandate for improved energy efficiency and reduced carbon emissions in buildings, and deliver solutions that optimize façade performance and nullify these negative attributes of glass. This will assure the benefits provided by the unrestricted, but appropriate use of glass in the building envelope. The ultimate viability of DSF technology, and the role it will play in future building façades, is unclear. We need to make a more aggressive and sustained effort in the attempt to better understand how these very interesting experiments in advanced façade design are actually performing. The needed solutions will involve an intensifying collaboration between the profession, academia, and industry, and will require ongoing investment in research and development by all stakeholders.

Jeffrey Vaglio and Mic Patterson are both PhD candidates in the School of Architecture at the University of Southern California. They are employed by Enclos, the national curtainwall firm headquartered in Eagan, Minn., and work out of the firm’s Advanced Technology Studio in Los Angeles.