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.