Designing building enclosures

Designing and delivering a reliable and durable building enclosure can be a challenging task.

Modern building enclosures separate occupied space from the outdoor environment through a complex integration of various building systems and components. The building enclosure functions, in part, to control bulk water infiltration, vapor diffusion, air flow, and heat transfer. With various climate and project-specific considerations that need to be addressed from schematic design through construction, designing and delivering a reliable and durable building enclosure can become a very challenging endeavor.

The authors discuss two case studies; the first, a newly constructed building that required remedial work on the building enclosure fairly soon after construction, and the second, a rehabilitation project that had challenges of retrofitting new windows into an existing historic mass masonry wall. The case studies highlight how insufficient considerations of both liquid and vapor moisture and thermal control short circuits through the building enclosure led to condensing windows and water intrusion to the building interior.

Case study 1: Condensing windows

Construction of a seven-story, 342,000 sq-ft health-care facility on the East Coast was nearing completion, and the building was partly occupied in the winter of 2008 when condensation began to appear on the window frames and glazing. We were contacted to assess the cause and the extent of condensation and recommended remedial options to address the problems. Field investigation revealed the following key findings:

* The building had punched aluminum framed windows with double glazing set directly into precast concrete facade panels. Only a few were separated from the concrete with plastic shims.

* Moisture-laden air was condensing and water droplets were forming on the window frames at the sill and glazing stops (Figure 1). This moisture was pooling on the interior window stools. Condensation was not reported to have occurred on the heads or jambs of aluminum-framed windows.

* Condensation was most severe on the north building elevation and we were tasked with designing remedial repair options only for this elevation.

The thermal improvement options highlight remedies that can be evaluated and implemented in an existing facility. In this case, the windows were installed directly on the highly thermally conductive precast concrete panels with the thermal breaks in the window frame located outboard of the thermal insulation plane of...