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Mixed-up systems

slaton patterson sutterlinFAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE
The control and management of rainwater on and within a curtain wall assembly is critical for long-term performance and durability. Water-control strategies—most commonly drainage or barrier (exclusion)—can vary by system type and glazing technique. Different elements of the same system (e.g. operable vents and louvers) can use different strategies to control water.

It is important the curtain wall is designed to minimize the potential for internal water accumulation. While water head within a curtain wall should be anticipated for short durations (e.g. heavy wind-driven rain events), long-term accumulation and storage of water within the assembly can lead to durability issues and leakage to the building interior. Edges of insulating glazing units (IGUs) should not be directly exposed to water for a prolonged period of time, as such exposure can increase the potential for premature edge seal failure, as well as that for delamination and discoloration of interlayer laminates and surface-applied coatings. Additionally, unintended water penetration into internal components without drainage provisions may lead to chronic water leakage. If a water exclusion control strategy is specified for an element of a curtain wall assembly, a clear understanding is needed of how each surrounding component manages water that enters it, as well as whether any backup drainage mechanisms exist in the barrier elements should the primary deterrent fail.

On a recent project under construction, the stick-built curtain wall assembly was designed as a drainage system, but its operable vents were designed as barrier systems. The outboard surface of the insulating glass infill was adhered to the sash frame with shop-applied glazing tape (intended to function as a seal against water penetration) and secured in place with an interior snap-in glazing bead.

Water leakage observed at the metal joinery between sash frame and interior glazing bead during water testing. This indicates water migration into the glazing pocket of the sash, which was not designed with drainage provisions.
Photo courtesy WJE

Recognizing the potential vulnerability of this sash construction, the contractor elected to install field-applied cap bead seals between the glass and sash frame as an added deterrent to water migration into the glazing pocket. Due to this exclusion strategy, drainage provisions were not incorporated within the glazing pocket of the sash frame. However, despite the added feature, in-situ water penetration testing performed on the curtain wall resulted in water leakage at a number of operable vents.

Close-up examination at leak locations revealed breaches in the glazing tape that oddly enough aligned with discontinuities in the cap bead seal, providing a direct path for water to enter the sash glazing pocket. Absent drainage provisions, water then entered the interior of the building through the metal-to-metal joint between the sash frame and interior glazing bead. IGU manufacturers rarely honor warranties against premature edge seal failure when installed in glazing pockets without drainage provisions.

In this case, the addition of a cap bead is considered a benefit to a system otherwise reliant on glazing tape alone; however, the cap beads must be of proper profile, continuous, and carefully installed without defects to be effective. Additionally, if weep holes had been installed in the sash frame to drain incidental water breaching the cap bead and glazing 
tape, leakage to the interior would likely not have occurred.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.

David S. Patterson, AIA, is an architect and senior principal with WJE’s office in Princeton, New Jersey. He specializes in investigation and repair 
of the building envelope. 
He can be reached at dpatterson@wje.com.

Jeffrey N. Sutterlin, PE, is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. 
He can be reached at jsutterlin@wje.com.

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