Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE
Indications of potential thermal performance problems with exterior wall components are typically expected during the winter, but they can appear any time of year. In the example illustrated here, isolated areas of significant condensation on a curtain wall’s exterior surface occurred in mid-summer, raising the question of what factors led to this condition.
The condensation was noted during implementation of exterior sealant repairs to address water leakage attributed to the curtain wall (but unrelated to thermal performance) on a newly constructed residential high-rise in the northeastern United States. The observation was made on a warm and humid summer day, with an ambient dewpoint near 21 C (70 F). The building’s interior was being readied for occupation, with the temperature maintained at an unusually cool 18 C (65 F). It is important to note the building was designed to operate under a positive pressure relative to atmosphere. The condensation was seen in the morning, before the curtain wall had the opportunity to be warmed by the sun.
The condensation’s presence on the exterior is an unusual phenomenon at any time of the year in the northeast, but particularly curious during the summer. Two possible causes of the extensive exterior condensation were suggested:
- either a direct thermal short exists within the curtain wall; or
- an air path(s) is present in the wall assembly’s air/vapor barrier.
In addition to leading to the observed exterior condensation during the summer, either one of these conditions can result in the formation of condensation or frost on interior surfaces of the curtain wall, or within the exterior wall construction, during the winter—this can result in damage to interior finishes and increase the potential for organic growth.
The curtain wall incorporates thermal isolation technology that separates the interior and exterior framing components to reduce thermal conductance through the system (an attribute usually designed to improve thermal performance
during cooler ambient temperatures). A foil-faced product was used to insulate the stud space at areas adjacent the curtain wall system and within spandrel areas; the foil facing served as the air/vapor barrier for the wall assembly. Joints and terminations in the foil-faced insulation were specified to be sealed with aluminum tape to maintain continuity in the air barrier system.
Considering this wall construction, the fact the building is operating under positive pressure, and the isolated occurrence of the condensation, the condensation was likely due to breaches in the air/vapor barrier. Under positive pressure, these breaches allow the cool interior air to bypass the air/vapor barrier and thus influence the temperature of the curtain walls exterior components such that it falls below the dewpoint of the ambient outdoor air.
Confirmation of this hypothesis is needed through further investigation. However, if it is correct, the potential for the formation of condensation or frost within the exterior wall construction during winter conditions is likely and of concern.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.
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 firstname.lastname@example.org.
David S. Patterson, AIA, is an architect and senior principal with the Princeton, New Jersey, office of WJE, specializing in investigation and repair of the building envelope. He can be contacted at email@example.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 firstname.lastname@example.org.