Thermal Performance

While the roles of the building enclosure are varied and many, the facade’s thermal performance levels loom large in today’s environment of sustainable designs and increasingly stringent codes and standards.

Traditionally, glass has been the weak link in the chain, notably decreasing the facade’s overall insulating R-values, no matter how well insulated the other cladding and connecting elements.

“Heat loss and heat gain through large areas of glazing are some of the more significant performance challenges, particularly in high-altitude and alpine climates, along with controlling glare,” states McGowan.

Consequently, high-quality thermal breaks, which work to eliminate the thermal bridges through which cooling and heating energy freely pass, are absolutely essential.

Photo: Geoff Captain

At the 777 Aviation office building in El Segundo, California, a structural silicone-glazed curtain wall utilizes a polyurethane thermal break and injection-molded thermoplastic connector that joins interior and exterior members to deliver total thermal isolation.

In terms of energy codes in general, experts anticipate that California’s stringent Title 24 will be setting the standard moving forward and that many states will follow its lead.

As for what this means in practice, Matt Williams, associate principal and facade engineering leader, Arup, Los Angeles, explains that a prescriptive, code-based approach to Title 24 is predicated on a 40 percent window-to-wall ratio (WWR), which allows the non-vision or opaque areas of the facade to have a higher, better-performing thermal transmittance to counteract the lower performance of a glass facade. “Where facades have a higher WWR, with more transparency or vision areas, the opaque or spandrel areas need to perform even better to balance the reduced performance of the larger glazed areas.”

In order to provide the proper documentation for compliance, building teams will need to rely upon energy consultants and manufacturers. In addition, glazing systems suppliers, with the use of project-specific National Fenestration Rating Council Bid Reports, will need to ensure that the systems being specified meet code requirements.

Along these lines, Ronald Wooten, FMPC, director of product testing and certification, C.R. Laurence, Los Angeles, cautions against assuming that manufacturer-published thermal performance data applies to all comparable systems in a given project.

Similarly, most curtain-wall and storefront manufacturers publish best-case, center-of-wall data, but even though the same system may be used throughout the facade, the performance values can vary considerably based upon configuration or elevation.

It’s also important to note that code requirements for renovations will vary significantly from locale to locale. For example, Title 24 requires that prescriptive U-factors be applied to all alterations or additions to existing buildings, regardless of size. However, solar heat gain and visible transmittance requirements only take effect when 150 square feet or more of the fenestration is altered.

Ultimately, to keep up with these ever-changing code requirements, Paul Worthington-Berry, AIA, associate, Shepley Bulfinch, Boston, requests that manufacturers incorporate new components and materials into their systems.

Rising to the challenge, newer curtain-wall systems have two thermal breaks and incorporate thermal separators rolled into the aluminum extrusions. Furthermore, some manufacturers have added advanced plastic materials to create continuous thermal breaks in vertical and horizontal mullions to provide optimal thermal performance.

Another newer innovation uses ultra-thin glass as the inner layer of triple pane glazing. The product is much lighter so it can be used in a conventional frame and is therefore more affordable.

Furthermore, polyamide thermal breaks and insulating glass units with warm-edge spacers significantly reduce heat transmittance compared to traditional curtain walls. Curtain walls and window walls that incorporate metal panels or spandrel panels with insulation can also mitigate thermal bridging at building slab edges.

Another strategy for enhancing thermal performance is incorporating some insulation into the glazed wall assemblies via shadow boxes and insulated back pans, which can give the exterior appearance of an all-glass facade; although the trade-off is a more limited actual vision area, according to Schroter. In determining if the design will meet thermal requirements, Rogan says his firm utilizes analysis tools that can directly import their drawings and carry out thermal calculations and condensation checks. “This ensures our engineers find and solve the weak points in their designs before they are fabricated.”

Air, Water, and Moisture Protection

While the curtain-wall glass, opaque filler panels, and supporting frame function as basic air-barrier materials, the details of the interface of the glazing system with the opaque wall assembly is the most critical part of the design, explains George Blackburn III, AIA, BCxA, Blackburn Architecture, Carrollton, Texas. The second priority is specifying effective performance verification testing for air leakage of the wall assembly.

Ultimately, the continuity of the glass panel through the air seal is critical to the continuity of the air barrier.

In addition to air barriers, vapor barriers, flashings, and sealants must all be carefully detailed to work in concert with curtain-wall systems.

“It helps to have multiple envelope components manufactured by a single source,” states Worthington-Berry. “This insures that these systems have been time-tested to provide compatibility and performance. Working with a single-source supplier allows the architect to collaborate with them on critical details.”

In terms of what to look for to ensure a tight enclosure, Schroter notes that transition detailing—not to mention facade system material interfaces, corners, and terminations—are often the most critical and a sometimes overlooked component in successful facade performance. Consequently, it’s important to evaluate how and where the air, water, and vapor barriers are employed in all adjoining assemblies to ensure consistent detailing, continuity, and material compatibility.

“Water infiltration systems for the building envelope follow the principle of overlapping or cascading elements that drain or weep any moisture out of the system through gravity,” explains Williams.

As a simple test, Blackburn recommends drawing with a pencil along the air, water, and thermal barriers to practically see if there is continuity.

“Where curtain walls often fail is at the interfaces with adjoining building elements,” agrees Rogan. “These interfaces should always be the focus of architects and engineers in the design stage, but it’s also important that general contractors assign responsibility for the interface with one of the trade contractors carrying out the work. We find it useful to color-code our drawings indicating the watertightness line and the airtightness line, so it’s easier for the trade contractors to understand how to properly interface between systems.”

He adds that air, water, and moisture performance should be tested off-site prior to installation. By minimizing the need for applying wet sealant on-site, the project team is less reliant on field workmanship, which is important, as sites can be wet, windy, dirty places where it is difficult to manage quality.

To help support tight enclosures with curtain walls, specifiers recommend pressure-equalized systems with interconnecting gaskets or wet seals to deliver the needed weather barrier continuity.

“This allows the pressure-equalization chambers to eliminate outside forces upon water infiltration and enable the curtain wall to perform as a high-performance rainscreen,” Worthington-Berry explains.

Another challenge with all-glass facades is dealing with the floor-line exterior air and interior smoke seals. Offering some advice, Schroter suggests placing unitized wall assembly stack joints at or slightly above the floor line and providing a back-sealed backpan filled with curtain-wall insulation to best maintain the floor-to-floor closure.

“The floor line smoke and interior air seals can be sealed to the face of the slab, the backpan, or the foil-faced insulation to ensure the visual continuity of the exterior glass at the floor line while maintaining the floor line seals,” he explains. “Interior ‘false’ mullions can be used to provide a horizontal closure for the backpan while allowing the glass to continue vertically uninterrupted across the facade at the floor lines.”

Keleher shares the following technical best practices for optimal water and moisture protection in curtain walls:

• Specify pressure-equalized and compartmented systems and silicone or PVC zone dams with silicone sealant.
• Cope the end portion of the pressure plate screw spline at the perimeter of curtain-wall framing members to allow for the installation of continuous sheet membrane flashing. Use exterior-glazed system to allow for inspections and connection of the air and water barrier into the glazing pocket.
• Bring the adjacent weather-barrier membrane into the glazing pocket. Cope the ends of pressure plate screw splines at the top and bottom of vertical mullions to allow continuous installation of membrane flashing into the glazing pocket. Do not just seal between the curtain-wall tube and the adjacent jamb as is typically shown on manufacturers shop drawings.
• If the above is not possible, avoid “F” anchors, and cap top and bottom of vertical mullions.
• Provide end caps on snap covers so that secondary sealant has a surface to adhere to, if sealant is shown to be adhered to snap covers.
• Design three weep holes in each horizontal pressure bar: two outside of the setting blocks and one between them, per the Glass Association of North America, unless the setting blocks are designed to pass water.
• Use backpans for spandrels and metal panel and stone panels. Leave a ¼-inch gap for warm indoor air. Insulation should be on five sides of the backpan, stopping 1-inch from the glass.
• Each glazing cavity should be compartmented and separately weeped (with water and insect-resisting baffles) through the horizontal pressure plates to the space behind the snap covers and outside via weep slot holes in the bottom of the snap covers.

Honing in on the parapet interface, Keleher notes that at the point where the curtain wall “flies by” (i.e., extends past the roof system to create a parapet), thermal issues can be significant. “Careful design by a building enclosure consultant and equally careful execution by the contractor is necessary to make these designs work.”

For above the curtain wall and out under the soffit, it’s important to ensure that the air, vapor, and thermal barriers are continuous. “Floors over soffits can be very cold in the winter and the source of discomfort for occupants, so use of a closed-cell insulation is important,” says Keleher. “Or, even better, the creation of an interstitial space that is heated to keep the floor slab from getting cold.”