New ideas in glass curtain walls, window walls, and storefronts

Driven by two distinct influences, a new generation of glass building enclosures has a unique trajectory—toward higher performance and sustainability. One of those forces is the work by building teams who effortlessly blend design analysis with the creation of novel system solutions. 

“We’re seeing a mix of façade design and sophisticated analytics, which help clearly establish the required performance, the expected benefits of the façade itself, and key metrics serving the aims of design teams, contractors, and owner-operators,” says John Ivanoff, an Associate Principal with Buro Happold’s Facades and Specialty Structures team.

In project examples around the country and in collaborative “hackathons” bringing together various AEC disciplines in the design phase, novel daylighting and thermal simulation tools allow fine-tuned evaluation of predicted enclosure performance, says Brad Pfahler, Project Manager and BIM Manager with architecture firm Studio Ma.

A second and equally forceful inflection for glass-intensive systems such as curtain wall comes from new local laws and stricter energy codes, including adoptions of statewide code updates, such as Pennsylvania’s in 2018, and a raft of more-stringent rules passed over the last three years in nine cities: Las Vegas, Mesa, Ariz., New York, Philadelphia, Phoenix, Reno, Nev., San Antonio, St. Louis, and Tucson, Ariz. 

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“Some of the regulations being put in place provide for penalties if buildings include all-glass façades and require larger mechanical systems,” says Buro Happold’s Ivanoff. “This has focused project teams on the relationship between façade and building energy performance, and as these higher-performance standards become less of an anomaly, they compel the private sector to adopt what was previously considered high-performance but has become more the norm.”

Building teams point to leadership from cities such as Phoenix, one of the first to adopt the 2018 International Energy Conservation Code (IECC) for residential construction in June of the same year. About 12 months earlier, Pennsylvania passed House Bill 409, bringing the state up to IECC 2015 standards and allowing Philadelphia to update to 2018 codes for commercial construction, which it did that June. 

“In New York City, a stretch code passed in 2019 that is 10% more stringent than statewide rules, and last year’s ambitious Local Law 97 requires the largest buildings to reduce overall carbon emissions by enough to meet the citywide goal of an 80% reduction by 2050,” according to MBB’s John Mealy, AIA, LEED AP. “Other municipalities have made codes stricter, too.”

With these two influences—call them codes and coding—teams are beginning to think of glass façade performance in new ways. For example, curtain walls can perform as filters of energy, light, and air rather than as barriers.

How does this work in practice? Studio Ma’s Pfahler describes the use of a daylight optimization analysis to study the orientation and shape of the façade system of glass and cementitious cladding with insulated metal for Arizona State University’s new ASU Downtown Phoenix Residence Hall and Entrepreneurial Center. While the enclosure design initially included horizontal projections for shading, the architects instead modeled precise angling of the glazing into a shallow sawtooth shape to both maximize interior daylighting with reduced glare while controlling solar thermal gain. 

“The client group desired more daylight and very clear, low-emissivity glass, and this change allowed us to adequately illuminate the interiors naturally while reducing glazing areas and orienting them in different ways without need for shades—and with a glass specification the university was seeking,” says Pfahler.

Similarly, tech-driven approaches inform new commercial building projects under construction today, such as the eight-story, mixed-use Corporate Commons Three complex on New York City’s Staten Island. “The daylight control strategy starts with the massing and is then enhanced by elements such as the fixed vertical fins we’ve designed for the upper floors of the east and west façades,” says Charles Thomson, LEED AP, Project Manager and Associate with CetraRuddy. “They provide shading in conjunction with the high-performance glazing, automated internal roller shades, and daylighting sensors on a window management system. Internal glare and excess heat gain are vastly reduced compared to a base case of manual shades, unarticulated façades, and lower-performance glazing.”

A variety of other building technologies and material innovations are being considered for glass-dominated enclosures. Building teams begin the design process with overarching considerations of design, installation, and long-term maintenance.

Glass façade toolset

“The first and most valuable opportunity is optimizing building orientation and ideal exposures, which can have a significant impact on performance and interior quality,” says Cindy Bubb, RA, LEED BD+C, a Project Architect with Studio Ma also involved in the ASU tower in downtown Phoenix. “For an east-west-oriented building, we can assume an approximately 3% improvement in energy use over an equivalent structure oriented north-south. This reduces energy loads and mechanical system sizing, demonstrating the high value considering façade orientations first.”

The second element is a teamwide focus on glass system specifications and detailing to advantage energy efficiency and environmental control, says Emir A. Pekdemir, an Associate with Buro Happold in its Sustainability & Analytics group. “Good envelope design can make efficient HVAC systems viable and actually reduce overall first costs,” he says. “Yet this can only be apparent with a multidisciplinary approach and when all parties are talking to each other. There are still silos out there; the time is ripe to break them apart.”

As an example, Buro Happold worked collaboratively with Perkins and Will on a double-skin façade for Northwestern University’s new downtown building, the 12-story Simpson Querrey Biomedical Research Center. Workstations and offices line the exterior next to a double-glass curtain wall, which includes shading devices that automatically raise or lower to adjust for glare. 

“The prominent south façade is a very sunny side, so to maximize transparency and good views for the floor-to-ceiling glazed wall we employed a double-skin façade, which is rare in North America,” says Jane Cameron, FAIA, LEED AP, Associate Principal with Perkins and Will. “It’s a passive, open-air system, so as the cavity heats, awning or hopper vented glazing units open out, protecting from extremes of heat and cold.” The façade cavity has catwalks for maintenance access and an array of perforated venetian-type blinds on sun-activated sensors, all tied into the mechanical systems and building controls. 

To regulate performance, Buro Happold’s team, led by Chicago Principal Matthew Herman, analyzed architectural design concepts and studied the controls needed to operate and maintain all contributing elements, Cameron explains. Marrying the mechanical, structural, and enclosure systems, the façade team provided specifications along with control system parameters, coordinating with mechanical engineers and the commissioning agent on needed adjustments when the building opened.

Northwestern’s multistory double skin contributes to a sustainable solution, along with daylight-responsive lighting controls, green roofs for reduced heat absorption, and a “blue roof” that retains up to six inches of stormwater for irrigation at grade. The facility is tracking to be LEED Gold certified and 33% more efficient than a baseline code-compliant building meeting ASHRAE 90.1–2007. “In my experience, nothing beats a well ventilated, double-skin façade with automated interstitial cavity blinds,” says Buro Happold’s Pekdemir. In spite of its higher first cost, “it checks all the boxes.”

In other instances, building teams are comparing the benefits of external shading or operable exterior blinds to reduce heat gain and energy loads, says Studio Ma’s architect Bubb. “Operable blinds can increase maintenance requirements, so we often look for opportunities like self-shading and even the use of trees and vegetation, though their benefit takes time to accrue as the shade plants grow,” she adds.

Maximum glass

Another dominant driver in the industry is the relentless push for maximum glazing ratios and its dueling nature with the requirements of energy codes that are becoming ever so stringent, says Pekdemir. With carbon emissions jumping into the decision-making matrix, building teams are watching first costs more carefully than ever in their ROI calculations. “And not so surprisingly, this has an impact on coating choices,” he says.

In cooling-dominated climate zones, low-emissivity (low-e) coatings are gaining favor over the highly reflective glass types employed in past years, says Studio Ma’s Pfahler. With a surface dynamic that emits low levels of radiant thermal energy, low-e glass minimizes UV light and heat transfer without a major penalty for visible light transmission, or VT.

Using low-e coatings along with other techniques, such as fritting, the building team can calibrate how much the glass reflects, absorbs, and transmits sunlight. In many cases, glass specifications improve performance by selecting the application of the low-e coatings and fritting to more exterior glass surfaces, in combinations that improve the re-radiating of heat. 

This balancing can optimize the relationship between aesthetic appearances as well as VT and solar heat-gain coefficient (SHGC), for which the codes set minimum requirements. “In practice, available coatings in the market can mostly only satisfy two of these asks,” says Buro Happold’s Pekdemir. “You either get a high VLT to SHGC ratio—with greenish tints—or you get clear-looking glass with a high VLT, along with a higher SHGC.”

Multiple specifications of glass coatings and substrates can be employed for different building floors or exposures, adds CetraRuddy’s Thomson. “For the fully unitized curtain wall glazing on Corporate Commons Three, we chose two coating-substrate combinations to reinforce the massing,” he says. “The building is a rectangular block in plan, but it slopes to the south in section to self-shade the south façade while allowing sunlight to reach the ground on the north of the site.” With this in mind, the project team selected a low-iron glazing with a neutral look for the ground and second floor and an extra-clear substrate with a silverish, high-transparency solar control formulation on upper floors, giving the building a uniform appearance while also contrasting the glazing selections.

In addition to low-e treated glass panels including application by spray pyrolysis and magnetron sputtering vapor deposition, or MSVD, other glass coatings are gaining interest by adding beneficial features, says Pfahler. “There are valuable advanced coatings out there, including a biomimetic formula applied to the #1 exterior surface that mimics lotus plants to reduce streaking and dirt on windows,” he explains, describing the highly hydrophilic surfaces. “You can just hose the exterior off, and in some places the rain also cleans the surfaces, which primarily helps in reducing maintenance needs especially for less accessible surfaces, such as skylights or atrium glazing.”

Other emerging coatings include vanadium dioxide, which provides self-cleaning properties as well as a means to block thermal radiation escaping in cold climates and to enhance anti-glare performance in sunny conditions. Another is a class of transparent coatings called organic photovoltaics (OPV) that turns windows into glass or polycarbonate solar panels, and the material can be applied to paints and flexible substrates also. These will add to other PV glazing products to increase net-zero-energy operations, says Pfahler: “Photovoltaic glazing will change how we make buildings in coming decades.”

Performance by details and layers

In other cases, building teams can rely on glass interlayers for performance advantages. Three commonly
employed materials are polyvinyl butyral (PVB), a proprietary formulation known by the acronym SGP, and ethylene-vinyl acetate (EVA), which is widely used for indoor glazing assemblies such as partitions. Tested for UV absorption, VLT, stiffening and tear strength along with other mechanical properties, these interlayers for laminated glazings also reduce sound transmission through glass façades and storefronts, which can be a key consideration.

“Due to acoustic requirements for the west façade, we chose to laminate the outer lites with a clear, noise-reducing PVB interlayer,” says CetraRuddy’s Thomson, describing the design for Corporate Commons Three. Yet strength and stiffness also guided the specification: “The sloping façades have PVB interlayers as well to meet the local building code for sloped glazing,” he adds, noting that bending deflection and load bearing capacity are key considerations in curtain wall systems sloped more than 15 degrees from vertical.

Other challenging aspects for design and specification as well as effective installation of glass walls include the framing, edge conditions, and transitions or terminations in the glazing assemblies and framing systems. Details for these framing and glass edges can play a significant role in energy efficiency as compared to center-of-glass performance, generally increasing in importance as window sizes decrease. Overall, however, to achieve high-performance glass façade, vertical fenestration requires thermally broken frames, at least dual-pane glazing with one low-e surface, as well as high-performance thermal breaks, warm-edge spacers and, in some cases, a second low-e coating surface or argon-filled glazing.

The edges of glass are the thermally weakest point of insulated glazing units (IGUs), says Buro Happold’s Pekdemir. “Warm-edge spacers reduce the conduction in this area which can be very important, especially if the simulated condensation analysis predicts lower than dewpoint surface temperatures around there,” he says, adding, “Spoiler alert: With thermally broken frames everything can come down to the performance of the edge spacer, when all other elements are spec’d to be high performance.”

This consideration is driving many curtain wall and storefront designs for commercial and institutional facilities, says Thomson, whose firm is active in large-scale higher education, cultural, commercial, and multifamily projects. “The curtain wall framing we select for projects like the mixed-use office complex in New York is a thermally broken system,” he explains. Working with the fabricator ISA and installation subcontractor FA Group, the team developed details to mitigate thermal bridging primarily by using non-conductive materials within the system. “All spacers within the IGUs are warm-edge spacers to enhance the thermal performance of the façade,” says Thomson.

The key with thermal bridging is to consider the climate zone and whether the space inside is actively humidified. Heat loss through thermal bridges becomes more challenging the higher the temperature difference is between indoors and outdoors, explaining its focus for colder climates. With actively humidified spaces, the stakes are higher due to condensation risk, says Pekdemir: The key is to keep thermal resistance continuity aligned with other insulating materials and penetrate it as few times as possible. While some designs will require structural elements to protrude through the thermal resistance layer, building teams can develop details in the design phase that mitigate the impact of the penetrations on overall thermal performance.

Reconstruction and active solutions

In some cases, an advanced framing solution can allow for effective reconstruction approaches that solve thorny retrofit situations.

For the renovation of the existing 1967 high-rise structure Manzanita Hall at Arizona State University in Tempe, Ariz., for example, the building team led by student housing developer American Campus Communities along with the architects Studio Ma and SCB dealt with substantial variations in the existing conditions. Says Pfahler, the residence halls supported by cast-in-place, post-tensioned slabs exhibited large variances in floor levels of as much as +/- 2 inches. The team worked with a specialty subcontractor to design and fabricate custom extrusions and gaskets to install on top of the slab curbs from the inside, effectively compensating for any gaps caused by floor level disparities.

Not only did this system help make the 784-bed project viable, it also allowed the building team to place the window wall system behind the building’s iconic, structural V-shaped lattice façade. An alternative solution—effectively hanging a new enclosure exterior over the white concrete lattice—would have created a new identity and options for tuning the façade performance, but in the process obscuring the recognizable, expressive campus architecture that had become a focal point for first-year students.

The new exterior skin solution is inspired by the native manzanita tree, an evergreen whose new growth is a deep red bark that contrasts its bleached-white old growth. Similarly, Manzanita Hall’s old white concrete lattice structure is left intact, while the new glass-and-metal skin lends a warm, vibrant contrast to the existing structure.

As with adapting curtain wall to existing structures, glass wall systems are also designed for active adaptation to changing environmental conditions or occupant needs—or both. This “tuning” of the glass façade performance includes variable transparency and opacity glazings that are activated by thermochromic (heat-based), photochromic (light intensity-sensitive), and electrochromic means.

The last category, the electronically tintable glass used for curtain wall, storefront, and windows, can be directly controlled by building occupants or tied into a building management system. “Electrochromic allows us to bring intelligence to the façade for better building operations and energy performance,” says Pfahler, “and end-users can treat the façade openings just like dimmers.”

Building teams have used electrochromic curtain walls and glass panels to maximize solar energy and minimize glare and unwanted heat transmission, in some cases netting cost savings over a building life cycle by reducing overall energy loads by about 20% and peak energy demand by up to 26%. Electrochromics can also eliminate the need for blinds, and provides for self-adjusting opacity and tint based on solar radiation or other needs, says Pekdemir.

On the plus side, he says, these capabilities can benefit sustainability and wellness goals in building projects, though he contends one minus is color rendering of electrochromic glazings, which tends to be better controlled in standard glass. “I’m a big fan of disruptive technology,” says Pekdemir. “I wish this high-tech solution was catching on faster than it currently has.”+