Curtain-wall cautions

Exterior wall construction has evolved during the past century and a half. We have moved from massive masonry walls that support the floors of a building to slender walls that hang off the slabs. The heavy load-bearing masonry walls of the past absorbed rainwater and dried to the inside and outside. Contemporary glass and metal curtain walls rely on sealants and gaskets to prevent leakage to the interior. A small defect in these joint seals can lead to leakage into the wall system that likely includes gypsum wallboard and other materials that deteriorate quickly.

Modern glass-and-metal curtain walls have become popular because of their sleek appearance, light weight and, most importantly, the speed at which they can be used to close in a building. “Stick-built” or field-assembled systems are typically erected mullion by mullion, similar to a child’s erector set, and infill panels of glass, metal or stone are glazed into the openings during erection or shortly thereafter. “Modular,” or “unitized,” curtain-wall systems include large multipanel sections or modules that are assembled and glazed in the factory and placed on the building using a crane or hoist. Modules include gasketed mullions at the perimeter that interlock with the perimeter mullions of the adjacent modules. Both stick-built and modular systems generally require significantly less time to install than more traditional backup walls and cladding, an important factor during these times of high labor costs and aggressive construction schedules.

Glass-and-metal curtain walls can provide an attractive, durable and cost-effective cladding solution, but in many buildings, these wall systems are plagued with problems ranging from air and water leakage to falling trim covers.

This article discusses some of the most common curtain-wall design problems that afflict owners, architects and developers. A second article will discuss design issues, and the final article will cover fabrication and construction related issues and items relating to laboratory and field testing.

Promptly drain each opening
One of the most important design features is prompt drainage of each glazing pocket sill. Water will inevitably bypass glazing gaskets and exposed frame joints and will enter the glazing pocket. Even glazing that has an application of an elastomeric sealant between the glass and frame, or wet seal, will eventually admit water. Prompt drainage is a key consideration in curtain-wall design that is often misunderstood or ignored.

Frame corner seals typically consist of organic sealants, which, if properly constructed, contain the water in the sill glazing pocket until it drains. Sealants degrade from prolonged exposure to water, resulting in leakage through the frame joint and, eventually, leakage to the interior of the building. These frame corner seals cannot be maintained or repaired without significant disassembly of the curtain wall. Prompt drainage of the glazing pocket extends the life of these seals and, by extension, the service life of the curtain wall.

Insulating glass hermetic seals also typically consist of organic sealants which prevent water vapor from entering the sealed space. These sealants also degrade from prolonged exposure to water, increasing the risk of hermetic seal failure and fogging of the insulating glass unit.

Key features for promoting prompt drainage include the following:

Weep holes: Each sill mullion should have two or three weep holes, at least 3⁄8-inch in diameter. Weep holes should be offset from setting blocks and should never be plugged with sealant. Trim covers, foam baffles and weep covers can help prevent infiltration of wind driven rain.
Sloped glazing pocket: Glazing pockets can be sloped to the exterior to ensure that water moves quickly toward the weep holes. Strangely, this feature is often only available in custom designs.

Perimeter conditions
Proper detailing of curtain-wall perimeter conditions is critical to reliable facade performance. Many of our investigations of curtain-wall leakage show that the culprit is not necessarily the curtain wall itself, but the intersections with adjacent wall, roof and foundation systems.

We commonly find that a single line of sealant provides the only weather protection at these intersections. The key to reliable performance at curtain-wall perimeter conditions is to integrate the weatherproof barrier/underlayment and flashings of the adjacent building envelope systems with the curtain wall.

Jamb flashings should extend from the adjoining wall construction and return to the inboard face of the vertical curtain-wall mullion in line with the inboard leg of the sill flashing pan. An alternate location is to extend the jamb flashing into the inboard side of the glazing pocket. Designers often have to fight hard to get the jamb flashings installed properly due to varied construction sequences as well as the fact that multiple subcontractors might not want to coordinate concurrent work.
With proper detailing, however, perimeter wall flashing conditions can usually be designed to accommodate a variety of cladding installation sequences (see figure below). Some of these problems can be avoided by requiring the curtain-wall installer to install the perimeter flashing for the system.

Ideally, curtain-wall sills should be set into a water-tight pan flashing that is sloped to the exterior. The pan flashing should be integrated with pan flashings of adjacent wall systems if possible. Alternatively, watertight upturned legs/end caps should be provided to keep water from traveling laterally off the end of the flashing and draining behind weather seals. Interlocking horizontal “stack” conditions in modular systems need to be end-capped in a similar fashion. The horizontal portion of the sill flashing should not be penetrated by curtain-wall anchors. This often requires a modification of manufacturers’ standard installation practices.

Curtain-wall heads should be protected by a metal head flashing with a projected drip edge or a drip cut in the case of precast concrete construction. Installing the head flashing flush with the face of the curtain wall does not work well where hollow vertical mullions extend through to the top of the curtain wall and create a discontinuous surface against which to seal. A backup membrane flashing will provide redundant protection and can act as an air and vapor seal.

Continuity of thermal protection also is important and can often be provided through addition of spray foam insulation or other water-resistant insulating materials at these conditions.

Architectural features
Special architectural curtain-wall features that help provide a “signature” appearance are finding increasing favor in the marketplace. Bump-outs, sunshades, and other special features are rarely detailed to maintain continuity of the weather seals in many cases. The addition of such features late in the design process almost guarantees installation difficulties and future performance and maintenance problems.

Where components must penetrate through a curtain-wall system, they should be treated with the same level of care as the perimeter tie-in conditions. The continuity of air, water, vapor and thermal protection is often difficult to maintain around penetrations and complicated corners. Proper design detailing might require that discrete areas of modular curtain-wall assemblies be field assembled in order to create such continuity. Beware of frame designs that incorporate haphazardly applied features rather than custom-designed detailing, even if fabricators claim they “do it all the time.”

To achieve reliable weather resistance at special curtain-wall features, provide backup weatherproofing provisions, such as a membrane “boot” flashing or soldered metal collar.

Stepping a wall in and out of plane is often architecturally desirable, though this can create discontinuities in insulation and thermal bridges that increase heat loss and the likelihood for condensation to form on interior surfaces.  Such conditions are often a problem in highly humidified buildings such as museums and hospitals.

Where architectural features are required, include them in pre-construction performance mock-up testing. Complete testing before approving shop drawings for fabrication.