“The technical definition,” Arbour notes with reference to federal and state highway-authority standards, “is a concrete with a compressive strength higher than 17,000 psi,” with a typical tensile strength of 2,000-4,000 psi depending on density. Given UHPC’s strength, ductility, and flexural capacity, he says, “if you were to compare a Liquid Wall panel with a precast concrete panel, we need about 15 percent of the material to achieve the same r esult. As compared to standard concrete, we have an 85 percent lower carbon footprint, because in a standard concrete panel, you need five or six inches of high-strength concrete with rebar. If you want to insulate that panel, you’re going to a 12- or a 15-inch-thick panel with two wythes of concrete.”

Photo courtesy of Arbour Group LLC

Figure 1. Rendering of a facade using the Liquid Wall UHPC curtain-wall system.

The Liquid Wall prototype used in the Center for Architecture project was Lafarge Concrete’s Ductal, which Arbour notes was “the sole source of this type of material for many years”; there are now multiple forms of architectural UHPC available, including (but not limited to) Steelike, Taktl, Bekaert’s Dramix, Hi-Con’s CRC i2, and Cor-Tuf, “the commercialized version of the material that was developed by the Army Corps of Engineers.” Noting that “in facades, UHPC has been used as a decorative material, primarily,” rather than a complete building envelope, he sees it increasingly finding infrastructural applications thanks to its high density, waterproof and noncorrosiveness: water utility systems, replacement jointing in bridge decks, decking overlay projects in roads and bridges, and others.

The Liquid Wall framing system can incorporate energy-harvesting components such as fluid-based passive solar or transparent PVs. With Liquid Wall panels installed through the same procedures as unitized aluminum curtain wall, Arbour expects that ease of construction will combine with its thermal metrics and the material’s configurability to make it an attractive option for building owners seeking long-range facade performance. “If, by using UHPC instead of aluminum and installing the Liquid Wall,” he says, “you’re reducing your energy consumption in your building by, say, 35 percent, by the time you make that improvement down the road, you’re going to get yourself to net zero or better.”

TAKE IT TO THE BRIDGE

Thermal bridging in glass curtain walls is a persistent yet preventable problem. “The less metal you see, in some ways, the more efficient the curtain wall is,” says Vermeulen, noting that thermal breaks built into aluminum profiles “perform much better, and they’re increasing the insulation properties of curtain walls.” The glass industry has responded to decades’ worth of thermal concerns with numerous effective technologies (low-emissivity coatings, frits, multiple glazing, gas or vacuum insulation, and others), yet thermal transmission where glass meets metal is the next frontier. “We’re doing just about as much as we can do with the glass,” Patterson notes, “but the frames are a problem. They can represent 15 percent or more of the facade area.”

Thermal-break technologies are currently a subject of intensive research and development, Patterson observes, calling attention to his FTI colleague Stéphane Hoffman’s work developing guidelines for their use (Morrison Hershfield 2021) and applying the Thermal Energy Demand Intensity (TEDI) metric, which “loops the facade system into the equation” and addresses design features affecting a building’s overall heating or cooling demand more specifically than Energy Use Intensity (EUI). TEDI, defined as the summation of space and ventilation heating output divided by modeled floor area and reported in kWh per square meter per year, is used in Canada, Massachusetts, and other areas as a component of code-compliance pathways.

Thermal breaks made of polymer with fiber reinforcement (fiberglass, carbon, aromatic polyamide, or other materials) can reduce thermal bridging substantially and improve comfort for users near the curtain wall, though the use of these products in the U.S. is not yet widespread. Value engineering in the narrowest penny-pinching sense, Patterson has found, is often the culprit. “The architect will spec full thermal breaks,” he says, “aggressive thermal breaks. And the project will go to bid, and the facade contractor or the general contractor or both of them will approach the owner and say, ‘Hey, we can drop this price by 10, 12, 15 percent if we go to a thermally improved system as opposed to a thermally broken system. They’ll dumb it down.” Plastic thermal breaks are also reportedly hard to fabricate or install, drawing resistance from some contractors.

Another strategy Patterson finds promising is simply increasing the ratio of glass to thermally bridging materials, using megapanel systems larger than the typical 4-by-8-foot or 5-by-10-foot dimensions of a unitized curtain-wall panel. “You still have the issue of the performance of the frame,” he says, “but it becomes less a part of the whole surface area, and that bigger union can be highly insulated, and they’ll use punch windows. It’s much easier to change punch windows over time; as you get better technology and the existing products degrade over time, you can change out a punch window pretty easily compared to a whole unit of a facade system.” Systems that embed inexpensive networked sensors in the panels, tracking performance, any problems such as breakage or intrusion, and (for operable windows) the number of times opened and closed, signaling support teams if maintenance is required, are under development by Schüco, connecting the facade to the building management system much as an organism’s skin communicates with its nervous system.

APPROPRIATE LEVELS OF TECH

Perhaps the lowest-hanging fruit is retrofitting buildings to replace single-pane glass, which Selkowitz says accounts for about 40 percent of the glass in U.S. buildings. “Most of the glass that’s in place now will still be in place 20-30 years from now,” he says. “Unless we take some action, a good chunk of it will still be single-glazed, and if it’s single-glazed, it almost certainly is going to be a big energy and carbon problem, and comfort as well.” Triple glazing is code in Europe, and Selkowitz, after extensive research on the available and developing options (see Figure 2), finds that the “thin triple” variant, minimizing the additional weight (and embodied carbon) that leads some specifiers to opt for double instead, has much to recommend it for adoption here.

Source: Selkowitz 2024; courtesy of Stephen Selkowitz

Figure 2. Families of highly insulating glazing options, with center-of-glass thermal performance ranges: current, emerging, and future markets.

Opinions on this choice vary. “Triple glazing is a great option for a lot of retrofits [and] new projects,” comments Tucker, “but there have been some recent questions about triple glazing, as the double-glazing units have almost caught up closely with some of the film technologies to the triple glazing. The net effect is that because you only have two pieces of glass, it’s a lighter curtain wall; therefore you need less backup and structure to hold it up, and when you’re taking into consideration the whole embodied carbon story, you need less material brought on site.”

The less weight the third layer adds, the stronger the case for triple glazing. While advanced insulation approaches yielding impressive R values (in the 15-20 range) still have drawbacks other than cost–Selkowitz cites optical issues with aerogel and VIG (haze and visible spacers, respectively)–the R values around 10 with triple and 15 with quad strike him as a worthwhile improvement. In addition, “on the VIG, if you’re using a high-temperature furnace to seal the edges, you’ve got an energy input into the process. It’s not in the glass, but it’s embodied in the manufacturing unit.” Discrepancies between center and edge-of-glass performance can also cast doubt on claims of high R values. “The reality is, first of all, that the actual window numbers are going to be much lower, unfortunately, because of all the framing edge effects. So if you get the R 20 at no extra cost, that’s great, but if you’re paying a lot more to get the R 20, and then you’re degrading it at the frame, you haven’t optimized the window product; you’ve optimized the glazing.” Considering the practical context of these choices, Selkowitz says, “My argument is that the R 10 to 15 triple/quad is good enough for essentially any application you can conceivably think of.”

Windows are commonly replaced at roughly 50-year intervals (compared with 10 to 15 for lighting systems or 25 for HVAC), Selkowitz says, and their lasting effect on building performance implies a need for aggressive increases in the facade retrofit rate, he says, perhaps an order of magnitude or more. DOE has announced a competition for secondary glazing systems to address this need (US DOE, “DOE launches $2 million prize”). “They’re trying to put a niche in the market that hasn’t seen a lot of action, is most cost-effective, and if done well can have really positive impacts,” he says. Winners of the first phase, in which companies submit concepts, will be announced imminently; the second phase is prototype development, and the third will include installation in a building.

Although secondary glazing (“essentially storm windows for commercial buildings”) has received less attention than heat pumps as a decarbonization strategy, Selkowitz points out synergies between them: “If you’ve got a heat pump in a building with single glazing, first of all, it’s going to be much larger than it needs to be, which means more expensive. Secondly, it’s going to have a greater demand on the grid, because the peak heating is going to be 5 or 6 am on a cold winter morning, and even if you have PV out there, it’s not going to be working at five in the morning. So our argument is that if you want to decarbonize, and you want to go to electric heat pumps, the first thing to do in every one of those buildings is make sure that that glazing is replaced. And here’s where that secondary glazing comes in, because it will typically not be cost-effective to pull out all the windows and replace them.” Some products are already available for interior retrofits, including stretch plastics, rigid plastics, or a fiberglass frame around thin glass.

Building-integrated PVs (BIPVs), a perennial hopeful among energy-optimizing facade technologies, strike Selkowitz as promising in the abstract and practical only for specific circumstances and scales. On the positive side, the cost of generating PV power has dropped dramatically in the past decade and is now below that of fossil-fuel sources. Positioning and orientation of panels, however, remain suboptimal on vertical facade surfaces, though better on rooftops or in large-scale field systems that enjoy economies of scale. BIPVs also need AC/DC conversion (since PVs produce direct current) and battery storage. “There are specific buildings and applications where it makes a lot of sense,” he summarizes, “but as a general, mainstream, large-scale solution for most buildings, I don’t see it.... Motorized shades, electrochromics, and BIPV all have the challenge of integration of power, wiring, controls, sensors, with the rest of the building.”

Another variable affecting the range of smart-window systems is system interoperability or the lack of it, Selkowitz says. “There’s a group at Lawrence Berkeley Labs that’s working on an automated facade-control package that in theory would simplify the problems so that any manufacturer of the operating hardware could tie into the software; that software would tie into the rest of the building. But every company wants their own.... If every company that made smart glazing and automated shading were all part of the same team or group, and they had a standard way of integrating all that into building HVAC, the world would be a lot simpler.” The HVAC realm’s BACnet protocol offers one model for coordinating the various smart-glass systems, he suggests, implying that the facade marketplace might follow the precedents of industries that have overcome similar issues, e.g., 19th-century railroad track-gauge standardization. “In this particular case, the guys at Berkeley are working on the underlying techy stuff. I don’t know that anyone’s trying to get into the market side of it and actually see if you can get those guys in the same room together.”

Shading is another well-recognized, frequently low-tech strategy for incremental control of heat gain or emissivity (see Case Studies, “Malibu High School”). Not all shading systems, however, are equivalent. Stephan Schütz, executive partner at Gerkan Marg and Partners (GMP), points out that “what we face a lot in the U.S. is a single layer with a double- or triple-glazing facade, then an inner sun-shading layer,” which has the disadvantage of admitting some heat into a building, though it reduces light and shadow, improving comfort more than thermal performance. A double-skin facade with an outer single layer and inner double or triple layer, plus a screen or louvers for shading in between and natural ventilation from outside (sometimes with a double floor cavity), Schütz says, is a more effective German technology invented in the 1990s, “a very good example for a building envelope.”

From GMP’s work in Beijing, where air pollution was a serious problem, Schütz continues, the firm developed a closed variant it calls the climate active facade (see Figure 3), inverting the position of the layers so that the double or triple glazing is on the outer facade, ventilation occurs from the inside, and exhaust air is drawn through the cavity. “You use, in summertime, the relatively cold air of the interior space to get sucked into the system,” he says, “and then it’s warming up when it’s going up to the ducts. So you need air-conditioned ducts along the facade, and the advantage is that the inner glazing is much colder than the outer glazing, so the comfort for people working in the building is much higher. The same effect occurs in wintertime,” with relatively warm interior air entering the cavity to warm the inner layer. The double layer saves energy, and the high degree of glazing saves electricity on lighting. American colleagues at SOM, KPF, and other firms, Schütz says, use this technology; GMP’s CYTS Plaza tower in Beijing (2006) was the first climate-active facade in China.

A fourth variant (Figure 4), the closed-cavity facade, has a completely closed and unventilated cavity, providing a high degree of insulation and integrating the shading system in the cavity, protecting the operating mechanism; this system allows nearly zero exchange of heat and cuts cleaning costs roughly in half. “It’s a quite expensive solution,” Schütz notes, relying on prefabrication; “we always try to avoid the installation of the facade on site, like the classical mullion-transom facade [often seen] in the U.S.” These units can be installed from inside, requiring no scaffolding.

Image courtesy of Gerkan, Marg and Partners

Figure 3. Climate active facade.

Image courtesy of Gerkan, Marg and Partners

Figure 4. Closed-cavity facade.

On the China State Construction Engineering Corporation (CSCEC) headquarters in Guangzhou, Schütz says, GMP and the German climate engineers Transsolar Energietechnik combined the mechanical system and the facade system, integrating 6,100 square meters of PV panels into aluminum exterior louvers to harvest approximately 600 megawatts of electricity per hour, “roughly a quarter of the energy needed in the building.” A building-high solar chimney integrated into the structure provides large-scale ventilation; office windows are automatically opened at night to create the shaft effect. Two zones (28 stories) of the 37-story building are mechanically ventilated, necessary only from June to September or October each year, saving about 400 megawatts per hour per year; the nine-story top zone is completely naturally ventilated by the chimney effect. The building is constructed but not yet in use, he says, so the energy metrics are based on simulations, but if post-occupancy measurements match the models, it will reduce carbon by 58.9 percent and overall energy by 61 percent, earning a description as “a nearly zero-energy building” by the China Association for Building Energy Efficiency (CABEE) as well as a LEED Gold rating and a three-star (top) rating in China’s equivalent Green Building Evaluation Standard.