Some manufacturers' trepidation that raising the performance bar (and thus cost) of envelopes would lead to replacement by opaque walls has not been borne out by experience elsewhere, Sanders notes. German codes have included fenestration performance minima for decades, for example, and German buildings feature ample, high-quality glazing (Sanders “Are windows...” 2022). Rigorous codes and “the Passive House-type stringency that we're starting to see in western Canada, in British Columbia,” she adds, are driving local development of unitized wall systems addressing the spandrel thermal-bridging problem. Firms in Canada and the U.S. have produced prefabricated systems (Speedwall by Flynn/Northern Facades; ONEWALL by MiTek/Benson) that combine pre-installed windows and insulation panels, with thermally broken carrier systems, pressure-equalized compartments, and a menu of finishes, colors, textures, and window configurations. One system claims a “thermal R20 or greater, U value of .05 [BTU/(h⋅ft2⋅°F)] or less with 5-1/2” insulation.”

The Passive House system requires an energy budget of no more than 4.75 kBTU per square foot, comprising total energy uses for the building as modeled by system-specific software. The envelope is one of five aspects this system addresses, says Gabrielle Brainard, AIA, CPHC, adjunct assistant professor at Columbia University's Graduate School of Architecture, Planning, and Preservation. To meet the target, “you need to look at the insulation of your envelope, your airtightness, quality of your windows and doors, energy recovery as part of your ventilation system, and right-size heating and cooling; usually it's all electric. And that's it, and it's saying, 'Okay, we don't care how you meet this energy budget, but meet this budget, and provide also good indoor air quality.'” Strong codes like New York City's Local Law 97, she adds, are moving toward Passive House-style carbon budgeting for existing buildings.

“There's a lot of technology out there—triple glazing, vacuum-insulated glazing, a really high-end range of coatings—that can help mitigate solar heat gain, [so] that you can easily meet the Passive House requirements,” says Hoffman. “The pace of construction and the ease of construction come from unitized construction, and no one does unitized in North America better than the glazing industry, and that's why you see so [many] curtain walls going up and window walls going up, because the unitized construction makes it easy for the general contractors; it helps the schedule by being able to manufacture these panels off-site and then just hang them from the building.

“Unfortunately, these systems do not perform well thermally, especially in the opaque areas of the building...Every square foot of opaque wall that you're trading off against vision creates a deficit. You're going from an R15-R18 expectation of the opaque wall and trading it off against maybe an R4, maybe an R8 if you go to triple-glazed, so it creates that deficit that you have to make up.” Newer prefabricated panels, Hoffman says, “achieve much higher performance in the opaque areas of those walls than the traditional unitized curtain wall, where the insulation for these opaque sections of the panel lives within the frame and therefore sees significant thermal bridging.”

Support for better products and systems under the 2022 Inflation Reduction Act (IRA), Sanders suggests, is part—only part—of the solution. “Everybody is talking about heat pumps right now, which is great,” she says; “we need good efficient heat pumps; but the problem is, a heat pump isn't going to be able to work very well if you've got a superhighway of heat flowing out of your envelope and it can't keep up. So in order to enable heat-pump technology, we really have to focus on the building envelope.”

Diminishing returns from incremental improvements in low-E glazing are probably inevitable, Sanders infers, considering the proximity of visible and infrared frequencies of radiation that these materials address. “Ultimately, we're fighting the laws of physics in static low-E coatings; we've gone from single to double to triple-silver low-E in the quest to reduce solar heat gain while reducing thermal conduction. Since half the sun's heat is in the visible spectrum, if you want to cut out the solar heat, you don’t just have to cut out the near infrared; you have to start cutting down the visible light transmission, too.” Single-silver low-E coatings let in enough sunlight to be effective in northern climates by blocking a lot of the near-infrared radiation and letting much of the visible light through (for passive heat gain), as well as doing its primary job of reducing conductive heat transfer; double-silver low-E coatings “try to reduce solar heat gain further by taking off as much as the near-infrared radiation as they could without taking out too much of the visible light”; triple-silver low-Es (often referred to as solar control low-E) continued to reduce solar heat gain, and by doing so sacrificed more visible light transmission. “This is why dynamic glazing is so attractive, because it allows you to change that solar-heat-gain coefficient when you want to...It helps you bridge that issue with the solar-heat-gain/visible-light-transmission conundrum.”

With less technological sanguinity, Brainard says, “Personally, I think architects should be using less glass...I've done a lot of projects that are thermally improved double-glazed curtain wall systems in New York, in this ASHRAE Zone Four. Those don't perform very well thermally; you're lucky if you get R4 out of that. And in my experience, that's the industry norm in the U.S. right now.” She attributes the prevalence of all-glass facades to economic and historical factors: unitized curtain walls' interlocking extrusions have allowed high degrees of cost-efficient prefabrication, and the desire for glazed facades dates to Le Corbusier, “who really understood and worked with the sun.” Yet, “the application of his aesthetic by others after him was not necessarily as cognizant of the role of the sun in terms of massing and siting.”

From Corbusier's incorporation of shading in Unité d'Habitation, Chandigarh, and other projects through Lever House's need for retrofitting, Morphosis's use of an exterior scrim at its San Francisco Federal Building and others, “we've known since we started trying to use glass that it has major problems with thermal performance,” she continues. “A lot of the technical innovations that have happened since then have been just trying to undo these fundamental challenges of glazing...Insulating glass is inherently trying to make up for the fact that a single pane of glass works really poorly thermally.” One building where she finds a dynamic glazing system effective is LaGuardia Airport's new Delta terminal.

“The thing that people don't talk about,” Brainard continues, is that the demand for glass facades is “driven by the extreme efficiencies of this type of system for a commercial-development paradigm.” Certain improvements for thermal bridging, warm-edge spacers of plastic or silicone foam and plastic thermal breaks, raise questions of market acceptability: “A lot of consultants are wary of specifying those, [asking] 'Are these tested for the loads that we're going to see in a high-rise situation?'” Retrofitting glazed systems often makes more performance sense than economic sense; despite successful examples such as reclads by MdeAS (see Case Studies), she observes more buildings where “curtain wall is just not fixable once it's installed” because structural interlocks, seals, and other components create “a puzzle where all the pieces have been locked together, and unless you take the whole thing apart, changing out the framing is impossible...I think we can appreciate the genius of these prefabricated facade systems, but also recognize that they're really not future-proof, and they have a limited lifespan.”

WINTHROP CENTER: PASSIVE HOUSE IN AN OFFICE TOWER

Photos courtesy of Handel Architects; Facade Tectonics 2022 World Congress

Winthrop Center, Boston (left); Winthrop Center under construction (center); and the unitized curtain wall system (right).

Designed to be the world's largest office building to meet Passive House standards, Handel Architects' Winthrop Center demonstrates that the energy-accounting system originally aimed at residences can apply to commercial towers as well. In this mixed-use tower in downtown Boston, it is the 750,000-square-foot Class A commercial segment, not the housing, that applies Passive House strategies —energy-recovery ventilation, continuous insulation, and high-performance doors and windows—to save an estimated 60 percent in energy over the local average for comparable LEED Platinum buildings; Boston's typical Class A building stock uses 150 percent more energy. According to Handel's presentation to the 2022 Facade Tectonics World Congress, the energy budget will be 53 kBTU per square foot per year. The developers are pursuing international Passive House certification along with LEED Platinum and WELL Gold certification.

Winthrop Center (under construction at this writing) is a tower-on-podium structure with a T-shaped floorplan in the office segment, topped by a lozenge-shaped residential volume (left and center). The building uses a custom triple-glazed unitized curtain wall system with 600,000 square feet of glass, 10,000 frames, and 1,250 vents; with thermal breaks between the exterior walls, it has a U value of 0.22 BTU/(h⋅ft2⋅°F) for the vision glass and an R value of 18 for the opaque portion (right). The curtain walls feature vertical pleats that will give its profile a prismatic appearance on Boston's skyline. The pleated walls are also functional, as their bay-window spaces orient large portions of the office windows to the northwest, reducing solar heat gain. Vertical white metal fins on the eastern office wing play off the vertical concrete motifs of a nearby icon, Paul Rudolph's Blue Cross/Blue Shield building. Floorplans will place 95 percent of all workstations within 35 feet of a window.

Winthrop Center has downsides in some Bostonians' view: its planned height has been reduced to avoid disrupting flights from Logan Airport, and it breaks a local law against casting shadows over Boston Common and the Public Garden. By extending the expertise that Handel and energy consultants Steven Winter Associates gained from the Cornell Tech project into a different sector, however, it will contribute to this cold city's performance standards and set a precedent for Passive House commercial projects nationwide.

Candidates For The Next Paradigm Shift

Wider adoption of both low-E coatings and triple glazing are necesssary but not sufficient to make curtain walls a thermal asset rather than a liability. Newer technologies with game-changing potential are available, though not yet economically acceptable to most decision makers. Whether any will escape the boutique niche depends on a mix of professional recognition, subsidies, and hands-on experience.

The IRA incentivizes adoption of various technologies, from ground-source heat pumps to carbon-capture projects to one directly affecting building envelopes: electrochromic glass, long hailed for controlling solar heat gain and glare in summer and allowing daylight to reduce HVAC dependence in winter. Through an investment tax credit of 30 percent applied to the costs of dynamic glass, plus a bonus 10 percent for products with domestic content and/or projects on a brownfield site or other “energy community” (Schurle et al., 2022), the Dynamic Glass Act, incorporated in the IRA, may bring electrochromics closer to cost parity with traditional glazing and shading.