Volume 46, Issue 2 - March 2011

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Seeing Double
Glazing Contractor Sees Applications of Double-Skin Façades on the Rise
by Jeffrey Vaglio and Mic Patterson


We’ve heard about them, some of us have seen them, a lot 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 City, 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, however; it was also about acoustical performance, as these façades mitigate sound transmission through the glazed building envelope. 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 first to guide glazing contractors’ entry into 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. It turns out that 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 1 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 2). 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.

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 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 easily configured as a modular, prefabricated unitized curtainwall system appropriate for application on high-rise buildings. 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 floorplan is subdivided into many repeating units (offices, condos or patient rooms).

Double-Skin Installation
Assembly and installation issues with DSFs range as widely as the system variations. Unitized double-skin curtainwall systems can be complicated by the need for panel operability to accommodate maintenance needs. Prefabrication may include the installation of shading devices and controllers as part of the unit assembly process. Once the units are assembled, installation proceeds much the same as with conventional units. An exception is that the units are typically heavier, which may preclude lifting several simultaneously.

Multi-story DSFs present quite another scenario. Because of the long spans typically involved, these applications often will have exposed structural systems, sometimes requiring architecturally exposed structural steel (AESS) standards. This type of work is often unfamiliar to glazing contractors and steel fabricators alike, and is rightly regarded as a specialty item. In fact, many of the multi-story DSFs referenced above make use of structural glass façade technology, including the use of frameless glass systems, as a support strategy for the exterior skin. The interior skin is often a conventional curtainwall or storefront type system. The issue is with the exterior skin, its means of support and the required cavity work. The cavity often incorporates maintenance platforms, shading devices and potentially other mechanical components such as operable vents. These may or may not be included in the façade contractor’s scope of work. The cavity depth is an issue of particular concern; the deeper the cavity the easier it is for workmen to operate with all the required equipment. Cavity depths less than 30 inches begin to seriously constrain ease of movement for the workmen.

A particularly elegant way to support the outboard skin is with the use of a cable net. This presents a new set of challenges to the façade installer relating to the pre-tension requirements that must be applied to the cable system. The magnitude of force is typically high enough that hydraulic jacking equipment is required to achieve the required cable prestress. Tensioning a cable net is not generally as simple as moving from one cable to the next with a tensioning device; the progressive tensioning tends to alter the previously tensioned cables, resulting from the residual effects to the supporting boundary steel. Cable tensions must be confirmed with the use of an appropriate tension metering device. The installer should request a detailed installation method statement from the façade designer, and carefully consider the cost impacts in the estimate of work.

Access is a consideration on any façade job. If there are maintenance platforms in the cavity and they are installed before the outboard skin, they can be used during installation. If not, temporary platforms may be required within the cavity. Depending upon the glazing system design, workers may be required on both sides of the skin.

A final consideration for the façade contractor is commissioning. The requirement for system commissioning of advanced façade designs, DSFs among them, is becoming increasingly common, and is something that progressive façade contractors should prepare for. While commissioning requirements will vary between jobs, it is essentially a process of validating that the façade is installed and funcfunctioning as intended. With operable and dynamic components integrated into the façade design and critical to the intended function, commissioning processes are vital in assuring the building owner of future performance.

Future Developments and Conclusions
Arguably, the most compelling future application of DSF technology is in building retrofits. Realizing energy consumption and carbon emission reduction goals established by various green platforms will require energy retrofits to a large percentage of the current building stock. Many of the early glass curtainwall towers built during the 1960s and 1970s were originally constructed as single-glazed facades 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.

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 (transparency and the manner in which it enriches our built environments with daylight and view) assure that glass 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. 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. 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 facades, is unclear. We need to make a more aggressive and sustained effort in the attempt to better understand how these experiments in advanced façade design are actually performing. DSF technology, however, is but one strategy to the challenge presented by façade performance. There are others and there will be many more. The needed solutions will involve collaboration between the profession, academia and industry, and will require ongoing 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. Their opinions are solely their own and do not necessarily reflect the views of this magazine.

USG
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