Rain-Screen Facades Are More Than Skin Deep  

Water is a necessity of life. But in buildings, mismanaged water allowed to penetrate exterior walls is a pernicious problem for architects, occupants, and owners. It can wreak havoc on finishes and structural components, and adversely impact a building's market value.

The conditions that cause infiltration are straightforward. "For a leak to occur, you need water, a hole, and a force to drive it through," says Neil McClelland, Arup associate principal. But finding the exact source of water problems and correcting them after occupancy is not so clear-cut, and often disruptive and expensive. It is more effective to prevent water from penetrating facades during design and construction, and one strategy for doing so is a rain screen.

In the broadest sense, a rain screen is cladding divorced by means of a flashed and drained cavity from a building's weather-resistant barrier, says Tom Schwartz, president of engineering firm Simpson Gumpertz & Heger (SGH). According to this definition, the typical masonry cavity wall can be classified as a rain screen. In such a wall, the continuous air space between inner and outer wythes helps control some of the forces that drive water into the interiors of buildings, such as gravity, capillary action, surface tension, and momentum.

However, rain screens need not be made of masonry. The cladding can be made from almost anything as long as it is durable, including metal, timber, glass, or even some fabrics, says McClelland. Similarly, for the weather-resistant backup wall, a variety of materials are suitable, such as metal stud-wall construction, masonry, or concrete.

Bold beacon

Many architects have capitalized on the freedom that the separation of the cladding and weather-resistant barrier allows by reinterpreting cavity-wall construction. One recent example is the Diller Scofidio + Renfro−designed Institute of Contemporary Art, in Boston, which opened in December. A channel-glass-clad cavity wall encloses three sides of the upper level of the 65,000-square-foot, two-story waterfront building. This wall assembly, which includes a standard metal stud wall that acts as the true thermal and weather-resistant barrier, protects the art from natural light but allows the building to serve as a beacon at night, says Gregory Burchard, project manager for Perry Dean Rogers Partners, the executive architect.

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A channel-glass cavity wall illuminated from within helps Boston's Institute of Contemporary Art glow at night but shields gallery space from the sun. Photography: © Peter Vanderwarker

The 11-inch-wide pieces of channel glass are butted together and sealed at the flanges along their 14-foot lengths, except for an approximately 12-inch venting gap left at the bottom of each. At the base of the glass, a drainage slot allows the wall to weep. Between the stud wall and the channel glass is an 18-inch cavity, sized primarily for the light fixtures inserted between the two layers. White polyvinyl chloride (PVC) sheeting covering the stud wall provides a reflective surface for these lights, helping achieve the glowing effect the architects sought.

Classification conundrum

Although the ICA, along with other cavity-wall buildings, has cladding that is separate from its weather-resistant enclosure, some industry experts would not classify its facade as a rain screen because it is not a pressure-equalized system. According to a more narrow definition, a rain-screen wall is one designed to neutralize wind currents on the inside and outside surfaces of cladding so that pressure differences do not drive water through gaps or flaws in an assembly and into a building interior.

Walls designed with these pressure differences in mind are said to conform to the so-called "rain-screen principle" and are sometimes referred to as "pressure-equalized rain screens." Such systems are composed of an airtight plane protected by an open-jointed or vented cladding. Separating these two layers is a cavity or air chamber. The joints are sized so that air, but little or no water, can pass through as gusts buffet the cladding, equalizing the pressure on the exterior and within the cavity.

click images to view larger As part of a $4 million renovation of a 1960s-era office building in Denver, the architect replaced an original glass curtain wall and expanded metal facade (top) that had earned the building the nickname "the cheese grater." The new cladding (above left), an open-jointed terra-cotta rain-screen system, appealed to the client because it offered the warmth of brick without the weight. Window shading devices (above right) are made of terra-cotta square sections threaded on posttensioned steel rods. Photography: © Frank Ooms (above left and right); Courtesy 4240 Architecture (top)

Experts say that an effective air-barrier system-one that has low air permeability, structural strength, and continuity over gaps-is an essential component for creating the required airtight plane. As part of the backup wall, an air barrier reduces the flow of air through the assembly and greatly contributes toward reducing the air-pressure differential across the rain screen, according to the National Research Council Canada (NRC). "In a pressure-equalized system, an air barrier is absolutely necessary," concurs Schwartz.

The NRC recommends compartmentalizing the cavity, so that as wind pressure varies across the building face, air does not flow from high-pressure areas to lower-pressure areas, carrying water with it. Generally, smaller and more rigid compartments can respond more quickly to pressure changes than those that are larger and more flexible, explains Madeleine Rousseau, a council research officer and coauthor of numerous papers on rain-screen systems.

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In the Denver building's new wall section, the insulation is placed outboard of the studs, eliminating thermal bridging and enhancing energy performance.

There is considerable industry debate about how best to size compartments. Efforts to create modeling tools especially tailored for this purpose are under way but not yet validated, says Rousseau. Compartment configuration is another area of discussion. However, NRC wind tunnel studies indicate that compartments should be smaller at locations vulnerable to large pressure variations, such as corners and parapets.

Weight-loss strategy

Despite the promise of performance advantages, worries about water penetration are often not the determining factor in the selection of a rain screen over a more traditional cladding system. For example, when 4240 Architecture renovated a 32,000-square-foot office building in Denver's Cherry Creek neighborhood, the need for a lightweight system made a terra-cotta rain screen a good choice. Because the concrete floor slabs of the four-story building, constructed in 1963, were not up to current code, the architect was limited to systems that weighed no more than 10 pounds per square foot. Even after reinforcement of the structure at column locations with shear collars, the use of brick, the client's preferred material, was not an option. "That is what led us to terra-cotta, which has the warmth of brick," says Geoffrey Brooksher, AIA, 4240 senior associate.

click images to view larger Parts of a research facility for Brandeis University will be clad with fiber-reinforced cement panels. Although the assembly will be open-jointed, it will not be compartmentalized and is not designed to achieve pressure equalization. The architect was drawn to the system because it allows expression of the north facade (top) as components that hang from the building, in keeping with the glass curtain wall planned for the south facade (above). Renderings: Courtesy Payette (right two)

During the nine-month, $4 million gut renovation project completed in spring 2006, the contractor, PCL Construction Services, reduced the building to its structural frame, removing interior partitions and finishes. It replaced the original glass curtain wall and expanded metal sunscreen facade-which had earned the building the nickname "the cheese grater"-with a wall assembly composed of standard 6-inch metal-stud framing, sheathing, a combined vapor and air barrier, rigid insulation, and the 1¼2-inch-thick terra-cotta panels hung on an aluminum angle and support rail system.

The relative position of the materials provided energy-performance benefits, according to Brooksher. Because the cladding system was lightweight, the architect was able to pull the rain screen away from the face of the slabs and provide enough space outboard of the stud wall for 3 inches of rigid insulation. The configuration eliminated thermal bridging, or loss of heat through the studs, and increased the insulation's effectiveness by about 30 percent, he says.

click images to view larger The primary function of the vertical steel fins that project from the reveal between the aluminum panels cladding the High Museum is to add visual interest to the facade surface. However, the fins also act as air-chamber delimiters, slowing the flow of air gusts through the cavity. At the parapet level, the aluminum panels transform into skylight sunshades. Photography: Courtesy Arup

To prevent pressure variation across the cladding surface, and to help the cavity achieve pressure equalization, each panel is surrounded by a 3¼16-inch open joint, and the cavities within the four facades are divided into discrete compartments with aluminum closures, or delimiters, at the building's corners. The team decided not to pursue further compartmentalization because the basic wind-loading calculations required by code for any facade system, rain screen or otherwise, indicated that the building face did not have a large variation in wind pressure, according to Brooksher. However, he adds, "In larger buildings, compartmentalization is a bigger issue."

Wall components primarily intended to serve other purposes can sometimes do double duty as air-chamber delimiters. This was the approach followed by Renzo Piano, in collaboration with local architect Lord Aeck Sargent, when the two firms created a villagelike complex to expand Atlanta's High Museum. The High project, completed in late 2005, has a rain-screen system composed of 4-foot-by-12-foot aluminum panels surrounded by 3¼4-inch reveals and hung about 2 inches off the building's weatherproof stud backup wall. Steel fins run vertically between each row of panels and project beyond the edge of the facade.

Although Piano conceived the fins primarily as a device for casting shadows on the building surface, they also break up the cavity internally and slow the flow of wind gusts through it. Because of the relatively low height of the museum's complex of buildings (about 75 feet) and the protected courtyard space that their configuration creates, a separate compartmentalization strategy was not necessary, according to McClelland of Arup, the project's facade consultant. "If it were a 40-story building, we would have been more concerned," he says, echoing Brooksher's comments about 4240's Denver project.

A distinctive feature of the High's rain screen is its integration with the devices that shade the museum's signature round light scoops. The top row of aluminum panels bend above the parapet to transform into sail-like screens that prevent direct sunlight from entering the galleries below. Arup performed three-dimensional analysis and ordered a full-scale mock-up of the panel to identify any inherent weaknesses in the unusual shape.

The diversity of components and the number of construction trades involved sometimes presents a challenge for rain-screen projects like the High expansion. At the High, three primary subcontractors were responsible for the building envelope-one for the stud backup wall, another for the steel fins and their attachment to that backup wall, and a third for the aluminum panels and their frame-complicating coordination of stud locations and the rain-screen structure. "The facade works as a system but is delivered as separate parts," says McClelland.

The choice of a rain screen also had construction advantages, points out John Starr, AIA, Lord Aeck Sargent partner in charge of the High project. Because the architectural envelope is divorced from the weather barrier, quick erection of the building's enclosure is possible, he says. Rain screens are particularly well suited for a museum because it is a building type with a high percentage of solid wall and few windows. "The basic box can go up and be weatherproofed quickly," he says.

In contrast to the High, a planned sciences research facility for Brandeis University, near Boston, has large expanses of glazing. The south facade of the 200,000-square-foot building, planned for completion in 2011, will be almost entirely glass curtain wall with sun-shading devices. But Payette plans to clad other areas with open-jointed, fiber-reinforced cement panels. Although one material is transparent and the other opaque, both are part of a strategy to detail the building as a series of layers, says Kevin Sullivan, AIA, principal in charge of the project. "Expressing these materials as components hung [from the building] was appealing from an aesthetic point of view," says Sullivan.

The High's designers built a mock-up of the skylight sunshade to identify any weaknesses inherent in the sail-like shape. Photography: Courtesy Arup

The panels will be mounted on aluminum furring channels and anchored to a backup stud wall. Although there will be an approximately 3-inch gap between the panels and the rubberized asphalt membrane that serves as an air and vapor barrier, the assembly is not compartmentalized and not designed to achieve pressure equalization, according to Michael Louis, principal of SGH, the project's facade consultant.

The Brandeis project's panels will dissipate some of the kinetic energy of wind-driven rain. However, the real defense against water penetration will be the membrane adhered to the stud-wall sheathing, says Louis. The project is now in design development, and SGH is detailing window penetrations and connection points between the panel assembly and the glazed curtain wall to make sure the air and vapor barrier is continuous, and properly detailed flashing is provided.

Flashing can be the Achilles heel of an otherwise well executed exterior wall system, according to Schwartz, Louis's colleague at SGH. He stresses the importance of careful detailing, correct material selection, and proper installation. Schwartz recommends, for example, that through-wall flashing extend beyond the building face, and points to the unfortunate but common practice of specifying flashing material with only a 5 to 10 year life span in an assembly expected to last half a century. "It is critical that the expected service life of the flashing match that of the rest of the building," he says.