Designing building enclosures

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

01/26/2011


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. Building Enclosure Case Study 1We 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 the building enclosure. The resulting thermal bridges and short circuits created cold spots on the window frame that led to condensation. It is imperative that designers recognize the importance of aligning the thermal breaks within the window frame system with the thermal insulation plane within the building envelope.

Two strategies can be employed to reduce the frequency of window condensation problems that do not include repositioning the window to align with the thermal insulation plane with the wall. They are: reduce the interior relative humidity, or increase the surface temperature of the window frame. Eliminating condensation using the former option would require reducing indoor relative humidity to an estimated 20%, which was not acceptable for this health-care facility. The client requested that the selected remedial repair option fully eliminate condensation on the window frame. In addition, since the majority of the rooms were already occupied by patients, extensive remedial work that involved window repositioning or modifications to the HVAC system to accommodate linear diffusers near the windows was not an option.

Given the circumstances and the limitations, the following remedial options were deemed suitable:

  • Passive approach (option 1):Provide a heat fin, consisting of two 1⁄8-in.-thick L-shaped aluminum angles below the solid surface windowsill with thermal conductive gel, to improve heat delivery to the window frame at the sill.
  • Active approach (option 2): Provide heat trace cable as a direct means of heat delivery to the window frame (Figure 2).

Using advanced numerical 2-D computer simulation tools, we optimized the configuration of the aluminum heat fin(passive approach), and determined the power requirements for the heat traceable (active approach). To validate modeling results, mock-ups of options 1 and2, as well as the typical as-built condition, were constructed and surface temperatures were monitored. The measured surface temperatures showed that the aluminum heat fin elevated the temperature of the window frame at the sill by 5 F, Building Enclosure Case Studysignificantly reducing the occurrence of condensation but not eliminating it.

The heat trace system, on the other hand, could be designed so that it would maintain the surface temperature of the window frame above the dew-point temperature of the interior air at all times (Figure 2). The active heat trace approach of direct heat delivery to the window frame proved to be successful in the mock-up installation. A comparison of measured and predicted (modeled) surface temperatures showed a very close correlation. A cost analysis performed to estimate the annual energy expenditure of maintaining surface temperatures of the window frame above the dew-point using the heat trace system proved feasible. The heat trace system option was implemented and has been operating successfully for the past two winters.

Case study 2: Water leakage issues

Building enclosure renovations encompassing integration of new and old components (i.e., addition of new thermally broken windows into existing punched openings) can pose numerous challenges and require diligent scrutiny of details at the design stage of the project. In the current economy, more retrofit projects of existing building infrastructure are being undertaken with the intent of improving their performance.

An existing warehouse building on the East Coast was being renovated to include commercial and residential spaces. The building enclosure consisted of multi-wythe brick masonry and new thermally broken aluminum windows set into punched openings. Building Enclosure Case Study 2Retrofitting new modern windows into an existing warehouse building is not always straightforward; some level of leakage was tolerated in many old warehouses, whereas in new office and retail space, water leakage is not acceptable. This change of use type is becoming more prevalent in the context of sustainable design of reusing existing infrastructure. Challenges exist in water tightness improvements as well as thermal efficiency and air infiltration/exfiltration control to meet in many cases historic preservation needs and improved energy efficiency.

Prior to completing the construction, moisture intrusion was observed at the new window sill and in the field of brick masonry. We were asked to examine moisture intrusion and propose remedial repair options. During our investigation, we reviewed the design documents, reviewed the installed replacement windows in the field, and performed water testing, and found the following:

  • The windows were installed in a continuous sill receiver with end-dams. Weep-hole covers were present at the window systems’ weeps. No subsill flashing was provided.
  • The windows were installed in one of the following three configurations: configuration Type I—two single-hung windows; configuration Type II—four single-hung windows; and configuration Type III—two fixed windows bordering two single-hung windows.
  • The brick masonry window sills consisted of a rowlock course of brick masonry. Some portions of the sill on the building interior were covered with a cementitious parge coat, which was cracked and gouged at several locations.
  • At several locations, the rowlock bricks were deteriorated and mortar joints were in poor condition. The mortar joints deterioration included surface erosion, palling, and cracking.
  • Single-stage sealant joints were installed on the exterior and interior side of the window perimeter as the air and weather seals. The rough masonry openings for the windows did not provide a uniform substrate to install the sill track or provide an easy substrate for sealant joints to be installed against.

Our investigation revealed that water readily penetrated the building enclosure in the field of the masonry (i.e., deteriorated and cracked mortar joints and brick) (Figure 3) and through the wall-to-window transitions (i.e., discontinuities in perimeter sealant joints). Water penetration will continue to occur in the field of the masonry through cracks, spalled or missing mortar, and gaps where mortar has debonded from the brick units, and it is likely that substantial remedial effort will be needed to reduce or eliminate it.

At a minimum we recommended: tuckpointing eroded and deteriorated mortar joints on the exterior and interior of the brick masonry to reduce the quantity of water allowed to penetrate; removal and replacement of brick masonry at the perimeter of punched openings to provide a better substrate for sealant joints; and consideration of head, sill, and jamb flashings to address the waterproofing shortcomings at the window openings. The sill flashing must cover the row-lock brick in front of the window at the sill and be well integrated with the jamb flashing via transition of the jamb flashing membrane into the pan. The sill pan must also have an interior return leg as well as enddams that are fully sealed or, in the case of copper or stainless steel, soldered.

It is important to realize that water penetration resistance of the masonry can be improved with more substantial repair options such as replacement of large section of brick masonry, as tuckpointing is a temporary solution as part of a periodic maintenance program. In addition, a penetrating sealer can be applied on the surface of the masonry after completion of tuckpointing to impart greater water repellency and reduce absorption into the masonry. Penetrating sealers do not mitigate water intrusion through cracks in the masonry, and require periodic monitoring and maintenance (i.e., reapplication). Note that as with any existing structure, periodic and repetitive ongoing maintenance(in this case, including all of the items listed above) over the life of the structure is needed, and that these types of maintenance programs will never fully address the shortcomings of the building, but instead provide the building with periods of slightly improved to adequate performance.

Summary

Modern building enclosures consist of multiple materials and components integrated in an effort to control, in part, the flow of moisture and heat. In buildings with high indoor relative humidity, the building enclosure design is especially challenging and the risks associated with deficient design are likely to have a greater consequence. The case studies presented highlight the design shortcomings and the related problems that manifest themselves shortly after construction completion. Substantial effort and costs are typically associated with investigation of such problems and the development of remedial repair options. More versatile and cost-effective options can be performed in the early design stages of the project to prevent the occurrence of such performance problems.


Totten is a senior project manager and has more than 13 years of experience in the fields of structural engineering, building technology, and building science. He has concentrated his expertise on the evaluation and analysis of heat, air, and moisture transfer. Pazera is a Staff II and has experience in investigation, repair, design, and rehabilitation of building envelopes. He has participated in numerous building envelope investigations, including the investigation of condensation at high-humidity buildings, for which he designed various options for thermal improvement options.



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