Condensation can form on and within a building enclosure system when its surface temperature falls below the dew point of the surrounding ambient environment. When interior environments operate at elevated relative humidity (RH) levels, the potential for condensation increases. Even with high-performance building enclosures, fenestration systems (particularly their glazing infill) are typically more vulnerable to surface condensation because they provide less thermal resistance (i.e. greater heat transfer) than properly designed and constructed opaque wall assemblies.

Providing warm air flow across the interior plane of a glazing system has long been well understood and employed, mainly to increase human comfort by eliminating down drafts. During heating seasons, providing forced warm air across the interior surface via mechanical provisions can also increase the surface temperatures of the glass and framing components, thereby reducing condensation potential on these exposed interior surfaces.

Conversely, installing window treatments and furnishings that isolate (or buffer) a window system from the influence of the interior ambient temperature can increase the potential for condensation on the now-cooler, exposed interior surfaces of the system. In certain circumstances, particularly in existing buildings, interior glazing systems (commonly referred to as storm windows) are installed directly inboard of the primary system to improve the overall fenestration’s energy efficiency. As this interior glazing also buffers the primary system from the interior temperature, the “storm” system must incorporate air seals to prevent interior moisture from migrating into the interstitial buffer zone. If airflow is not managed, that moisture can contact the now-cooler surfaces of the primary system, thereby increasing the potential for condensation.

This was recently observed at a humidified, mission-critical facility in the northeast that intentionally installed an interior “storm” window with an integral mini-blind inboard of the primary punched-opening glazing assembly; not to improve the fenestration’s overall energy performance, but rather to provide privacy while limiting blind access and maintaining a more sterile environment. During the first heating season after installation, widespread condensation formed on the interior glass surface of the primary glazing assembly.

Upon further review of existing conditions, it was determined that the interior storm windows were not airtight, exhibiting open joints at mitered frame corners and along perimeter interfaces. Additionally, the building was operated at a slightly positive air pressure relative to the exterior, which encouraged moist interior air to flow into the interstitial space between the primary and storm windows through the open frame joinery. Since the interior “storm” window isolated the primary glazing assembly from the influence of the interior temperature, the primary assembly’s interior surface temperatures were more influenced by exterior temperatures and thus reduced, making these surfaces more vulnerable to condensation caused by contact with moist interior air within the interstitial air space—an unexpected consequence of the design and installation.

 Authors

Jeffrey Sutterlin, PE, is an architectural engineer and associate principal with Wiss, Janney, Elstner Associates (WJE) in the New York and Princeton offices. He specializes in the investigation and repair of building enclosures, as well as peer review and consulting for new enclosure design. He can be reached at jsutterlin@wje.com.

David S. Patterson, AIA, is an architect and senior principal with Wiss, Janney, Elstner Associates (WJE) in Princeton, New Jersey. He specializes in the investigation and repair of the building enclosure, as well as peer review and consulting for new enclosure design. He can be reached at dpatterson@wje.com.