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All-glass facades are popular with owners and tenants for a host of reasons, intuitive and otherwise. Daylight outperforms artificial light in enhancing indoor environmental quality, optimizing visual acuity, entraining circadian rhythms, and enhancing other aspects of physical and psychological health, creating optimal conditions for quality of life; these advantages are supported by the relevant scientific literature (e.g., Knoop et al., 2019) as well as common-sense observations. Human eyes and bodies evolved in response to the sun, not the fluorescent tube or the light-emitting diode, and ample fenestration is how buildings accommodate this aspect of our natural biophilia.
Image courtesy of Handel Architects
Winthrop Center, currently under construction, will add a distinctive pleated-glass profile to Boston's downtown skyline.
Yet the disadvantages of extensively glazed buildings are also substantial. In an era when the urgency of climate-change mitigation and adaptation is increasingly obvious, glass facades’ thermal performance is a challenge that must be met either through higher standards or through less reliance on glazing. Technologies that can raise the R-value of common double-pane glass from the R2 level to the R5-7 range are available, though not yet widely adopted; approaches that can perform as high as R14 are confined to niche markets. The last major advance, low-emissivity (low-E) coating, occurred four decades ago, eventually achieving a market share of about 50 percent in the commercial sector and 85 percent in the residential sector (Harris et al., 2022). Thermal bridging, particularly through aluminum frames that lack thermal breaks, makes even high-performing glass (measured at the center of a window or insulated glazing unit [IGU]) relatively inefficient when measured at the edge of the glass or on the full-unit scale. Operational carbon footprints are only part of the problem; considerations of embodied carbon underscore the importance of durable buildings and materials, and too many glazed buildings are anything but durable.
Image courtesy of Harris et al., U.S. Department of Energy, 2022
Figure 1. The 50-year trends in market share for four glazing types. Low-E coatings have seen rapid adoption since their introduction, while triple-pane IGUs have remained a small portion of the overall market. Commercial glazing shows similar relative trends in the adoption of low-E coatings and triple-pane glazing, but with lower overall adoption of both technologies. Source: Harris et al., 2022, based on figures from Selkowitz et al., 2018.
Since buildings account for roughly 40 percent of total U.S. energy consumption, and heating/ventilation/air-conditioning (HVAC) systems account for the largest share of total building energy use, about 35 percent (Selkowitz et al., 2018), reducing energy wastage through glass windows, doors, IGUs, and associated components can make a substantial contribution toward reducing HVAC reliance and cutting greenhouse gas (GHG) emissions. Yet efficiency upgrades face barriers in not only costs but market fragmentation, misaligned incentives for retrofitting of existing buildings, and limited awareness of the potential benefits among owners, engineers, and architects. Views among specialists in facades’ energy performance range from optimism toward imminent paradigm shifts to skepticism about the general desirability of glazed facades. Some argue that many commercial buildings are simply overglazed.
From one rigorous perspective, today’s facades–even the unitized curtain wall systems that have improved dramatically on thermal performance–are an essentially unsustainable technology. “We are, in my opinion, quite literally building tomorrow's problems today,” says Mic Patterson, PhD, ambassador of innovation and collaboration at the Facade Tectonics Institute (FTI) and lecturer at the University of Southern California School of Architecture. Patterson has become increasingly “wary of the optimism gene” and skeptical about incremental measures when conditions call for more radical transformations in design and construction practices. Too many buildings are designed for a short service life, he finds, with no plans for retrofitting, no repair capabilities comparable to the “plug-and-play” systems found in other product categories such as information technology or audio systems, and a perverse set of economic incentives that disregards energy and emissions.
Citing remarks by Antony Wood of the Council on Tall Buildings and Urban Habitat (CTBUH), Patterson says, buildings should not be designed for just a few decades; instead, “they should last until we’re done with them…We’re making a massive commitment of resources” in constructing high-rise buildings, Patterson observes, “and to not define a service life for them─60 years, or maybe a stretch to 100 years─is really shortsighted. I think we should be talking about hundreds of years, if not thousands, and even better, perpetual service life until we’re done with them. That calls for an entirely different kind of thinking…Embodied carbon is telling us that our systems that we’re designing aren’t nearly adaptable enough.”
During power outages, heavy HVAC dependence can compromise more than performance and comfort, adds Stéphane P. Hoffman, M.Arch., M.Eng., PE, principal, senior building science specialist, and vice-president at Morrison Hershfield, the Toronto-based engineering firm that served as principal investigator for a 2011 American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) report providing thermal-transmittance data on envelope details (Lawton et al., 2011). “Some of these highly glazed buildings are fine as long as the lights are on, but if you lose that power, they rapidly become uninhabitable,” even hazardous to life during disasters or severe weather.
Progress in the thermal efficiency of glazing has been steady, though it still has far to go to help the nation reach its environmental goals. Proportions of glazing types, driven by technological improvements, code evolution, and EnergyStar incentives, have changed dramatically over the past five decades, particularly in the residential sector (Figure 1), where double-pane windows rapidly overtook single-pane during the 1970s and 1980s, followed by low-E double-pane windows replacing clear double-pane. The energy savings are a component of broader nationwide reductions in energy wastage due to improved efficiency during the same decade (relative to how much energy would have been squandered if 1975 standards for efficiency and structure had remained in effect), which outstrip the effects of growth in renewable energy sources (Figure 2).
Image courtesy of Stephen Selkowitz
Figure 2. Reduced energy intensity has had 30 times the impact of growth in renewables in the U.S. from 1965 to 2018. Source: Selkowitz S. New IGU Technologies for Carbon Reduction and Zero Net Energy. National Glass Association conference, Jan. 26, 2022.
There is arguably considerably more low-hanging fruit in the commercial sector, where an estimated 41 percent of the existing building stock still has single-pane glass (Guidehouse 2021). If thermal efficiency continues to improve, it is conceivable that windows can approach zero net energy or even theoretically become net positive. Passive House standards and practices, common in northern Europe but not integrated into code in the U.S., offer models that specialists find worth emulating, though caveats exist about embodied carbon in materials often used in Passive House projects, offsetting gains in operational carbon (Bernstein 2022). The British Columbia Step Code, for example, a performance-based system using a Thermal Energy Demand Intensity [TEDI] metric, approximates Passive House-level rigor. There are reasons to view the glass, as it were, as either half empty or half full.
All-glass facades are popular with owners and tenants for a host of reasons, intuitive and otherwise. Daylight outperforms artificial light in enhancing indoor environmental quality, optimizing visual acuity, entraining circadian rhythms, and enhancing other aspects of physical and psychological health, creating optimal conditions for quality of life; these advantages are supported by the relevant scientific literature (e.g., Knoop et al., 2019) as well as common-sense observations. Human eyes and bodies evolved in response to the sun, not the fluorescent tube or the light-emitting diode, and ample fenestration is how buildings accommodate this aspect of our natural biophilia.
Image courtesy of Handel Architects
Winthrop Center, currently under construction, will add a distinctive pleated-glass profile to Boston's downtown skyline.
Yet the disadvantages of extensively glazed buildings are also substantial. In an era when the urgency of climate-change mitigation and adaptation is increasingly obvious, glass facades’ thermal performance is a challenge that must be met either through higher standards or through less reliance on glazing. Technologies that can raise the R-value of common double-pane glass from the R2 level to the R5-7 range are available, though not yet widely adopted; approaches that can perform as high as R14 are confined to niche markets. The last major advance, low-emissivity (low-E) coating, occurred four decades ago, eventually achieving a market share of about 50 percent in the commercial sector and 85 percent in the residential sector (Harris et al., 2022). Thermal bridging, particularly through aluminum frames that lack thermal breaks, makes even high-performing glass (measured at the center of a window or insulated glazing unit [IGU]) relatively inefficient when measured at the edge of the glass or on the full-unit scale. Operational carbon footprints are only part of the problem; considerations of embodied carbon underscore the importance of durable buildings and materials, and too many glazed buildings are anything but durable.
Image courtesy of Harris et al., U.S. Department of Energy, 2022
Figure 1. The 50-year trends in market share for four glazing types. Low-E coatings have seen rapid adoption since their introduction, while triple-pane IGUs have remained a small portion of the overall market. Commercial glazing shows similar relative trends in the adoption of low-E coatings and triple-pane glazing, but with lower overall adoption of both technologies. Source: Harris et al., 2022, based on figures from Selkowitz et al., 2018.
Since buildings account for roughly 40 percent of total U.S. energy consumption, and heating/ventilation/air-conditioning (HVAC) systems account for the largest share of total building energy use, about 35 percent (Selkowitz et al., 2018), reducing energy wastage through glass windows, doors, IGUs, and associated components can make a substantial contribution toward reducing HVAC reliance and cutting greenhouse gas (GHG) emissions. Yet efficiency upgrades face barriers in not only costs but market fragmentation, misaligned incentives for retrofitting of existing buildings, and limited awareness of the potential benefits among owners, engineers, and architects. Views among specialists in facades’ energy performance range from optimism toward imminent paradigm shifts to skepticism about the general desirability of glazed facades. Some argue that many commercial buildings are simply overglazed.
From one rigorous perspective, today’s facades–even the unitized curtain wall systems that have improved dramatically on thermal performance–are an essentially unsustainable technology. “We are, in my opinion, quite literally building tomorrow's problems today,” says Mic Patterson, PhD, ambassador of innovation and collaboration at the Facade Tectonics Institute (FTI) and lecturer at the University of Southern California School of Architecture. Patterson has become increasingly “wary of the optimism gene” and skeptical about incremental measures when conditions call for more radical transformations in design and construction practices. Too many buildings are designed for a short service life, he finds, with no plans for retrofitting, no repair capabilities comparable to the “plug-and-play” systems found in other product categories such as information technology or audio systems, and a perverse set of economic incentives that disregards energy and emissions.
Citing remarks by Antony Wood of the Council on Tall Buildings and Urban Habitat (CTBUH), Patterson says, buildings should not be designed for just a few decades; instead, “they should last until we’re done with them…We’re making a massive commitment of resources” in constructing high-rise buildings, Patterson observes, “and to not define a service life for them─60 years, or maybe a stretch to 100 years─is really shortsighted. I think we should be talking about hundreds of years, if not thousands, and even better, perpetual service life until we’re done with them. That calls for an entirely different kind of thinking…Embodied carbon is telling us that our systems that we’re designing aren’t nearly adaptable enough.”
During power outages, heavy HVAC dependence can compromise more than performance and comfort, adds Stéphane P. Hoffman, M.Arch., M.Eng., PE, principal, senior building science specialist, and vice-president at Morrison Hershfield, the Toronto-based engineering firm that served as principal investigator for a 2011 American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) report providing thermal-transmittance data on envelope details (Lawton et al., 2011). “Some of these highly glazed buildings are fine as long as the lights are on, but if you lose that power, they rapidly become uninhabitable,” even hazardous to life during disasters or severe weather.
Progress in the thermal efficiency of glazing has been steady, though it still has far to go to help the nation reach its environmental goals. Proportions of glazing types, driven by technological improvements, code evolution, and EnergyStar incentives, have changed dramatically over the past five decades, particularly in the residential sector (Figure 1), where double-pane windows rapidly overtook single-pane during the 1970s and 1980s, followed by low-E double-pane windows replacing clear double-pane. The energy savings are a component of broader nationwide reductions in energy wastage due to improved efficiency during the same decade (relative to how much energy would have been squandered if 1975 standards for efficiency and structure had remained in effect), which outstrip the effects of growth in renewable energy sources (Figure 2).
Image courtesy of Stephen Selkowitz
Figure 2. Reduced energy intensity has had 30 times the impact of growth in renewables in the U.S. from 1965 to 2018. Source: Selkowitz S. New IGU Technologies for Carbon Reduction and Zero Net Energy. National Glass Association conference, Jan. 26, 2022.
There is arguably considerably more low-hanging fruit in the commercial sector, where an estimated 41 percent of the existing building stock still has single-pane glass (Guidehouse 2021). If thermal efficiency continues to improve, it is conceivable that windows can approach zero net energy or even theoretically become net positive. Passive House standards and practices, common in northern Europe but not integrated into code in the U.S., offer models that specialists find worth emulating, though caveats exist about embodied carbon in materials often used in Passive House projects, offsetting gains in operational carbon (Bernstein 2022). The British Columbia Step Code, for example, a performance-based system using a Thermal Energy Demand Intensity [TEDI] metric, approximates Passive House-level rigor. There are reasons to view the glass, as it were, as either half empty or half full.
The Optimists' Case For Nudging The Market
Stephen Selkowitz, retired senior advisor for building science at Lawrence Berkeley National Laboratory (LBNL), is a leading advocate of better-performing glass facades, noting that the necessary technologies for thermal improvement do not require technological leaps, just wider acceptance of known approaches whose long-range energy-savings payback outweighs their up-front costs.
The imminent paradigm shift is not awaiting disruptive technology. Selkowitz generally views cutting-edge approaches such as vacuum insulated glass (VIG), aerogel, dynamic “smart glass,” and krypton filling as more futuristic than economically or operationally realistic at the moment—yet his decades of work in developing low-E glass and demonstrating its feasibility, originally impelled by the recognition that coatings could control heat transmission without seriously impairing light transmission, have convinced him that continued progress is feasible, given appropriate incentives. Double-glazed windows with argon filling dominate today's market in northern climates; Selkowitz currently envisions triple glazing, sometimes with an internal layer of extra-thin glass (0.5–1 mm) to help manage weight and IGU dimensions, eventually becoming the norm in the U.S. as it already is in northern Europe, where governments are more proactive and performance standards are higher. Triple-glazed windows, according to LBNL's calculations of U factors and the reciprocal R values, can achieve R5-7 metrics, comparable to levels found in the Passive House system. Quad glazing is even better, provided designs can accommodate an inch-wide IGU; with two internal lites of thin glass and tight seals, he says, performance can reach as high as R14.
The challenge is to expand the early-adopter market segment into a national norm. “There's certainly a big chunk of the building industry that is very price-sensitive, and so they're going to go for lowest first cost,” Selkowitz observes. “If you look at life-cycle cost, that's another story. That's not yet common practice.” Different business models create different incentives to replace double-glazed (or even single-glazed) panes with triple. “It just depends on whether you philosophically are looking to maximize or optimize long-term value and benefit for the occupants, for the owner, and for the country—or the world, maybe, if you want to look at climate change—or you're simply saying, 'How can I get the cheapest building out on the market as soon as possible so that my rental income starts flowing?'”
This position leads to a form of market failure, Selkowitz contends: weak demand for technologies known to enhance performance, insufficient to induce window and IGU manufacturers to rethink production lines and supply chains. “If I walk in and I say, 'Oh, why don't you make triple-glazed windows instead of double-glazed windows?' they may say, 'Conceptually, we agree that should be done, but it's going to take us a year to buy the new equipment we need; we need to go to a new supplier that will make this or make that.' So these things are all doable; it's just that they take some extra time and effort.”
While codes continually become more rigorous, Selkowitz finds many decision makers viewing code requirements as a level for short-term aspiration rather than a baseline to outperform. “Some people think philosophically that if you meet the code, you're doing really well,” he says. “My standard line is you're building the building which is the least efficient that the building code allows you to build, right? If it were less than that, it wouldn't be legal to do it.” Other voluntary standards such as residential EnergyStar and the commercial systems LEED and WELL, he finds, create incentives for longer-range, broader-scope thinking, addressing occupants' health, comfort, productivity, and quality of life as well as energy use.
The appeal of curtain walls extends beyond metrics. From his Oakland office, Selkowitz says, “If you ask someone, do they want to work in an office with a porthole or no window at all, or an office with a beautiful view of San Francisco Bay, it's a no-brainer what they're going to say.” Instead of limiting window-to-wall ratios and forgoing the benefits of daylight, he advocates investing in windows with performance closer to that of nonglazed elements. “What we've argued for 30 or 40 years is, if you take the time, trouble, and a little bit of extra money, you can make the windows—we call them Net Zero energy windows, but essentially a net-negative or a net-neutral contributor to the energy balance of the house.”
LBNL, the Department of Energy (DOE), utilities, and other groups, he says, have been working “to change the market forces so that the market is actually asking for those products, and then the manufacturers say, 'Okay, yeah, I can invest and do that.' That's happening now on the residential side with EnergyStar: they're going to announce, probably in the next month, a new set of requirements for EnergyStar windows, and it's going to force or encourage a number of manufacturers to switch to triple glazing. The values are set; the U values are set low enough that the best way to do it is to go to from double to triple glazing...The current market share of EnergyStar nationally is up around 85 percent. So EnergyStar on the residential site has become a de facto standard; the building codes are a step or two behind that, but usually they follow.”
EnergyStar designation (EnergyStar 2022) applies only to residential windows, Selkowitz notes; “there has been periodic discussion about extending it to the commercial side, but no action to date.” The new Version 7 specification has been circulating for comments and is not yet finalized. Architects and clients may benefit, he suggests, from exploring both the new specs and federal and state tax credits in the commercial sector. Although “upgrading windows in an older building can be complex and costly,” he points to prominent successful replacement projects, including triple glazing replacing double in the Empire State Building's over 6,500 windows (Koch 2013), using a lightweight suspended coated film internally rather than a third lite of glass, with minimal operational disruption.
Code Evolution Vs. The Superhighway Of Heat
A contrasting view holds that the all-glass aesthetic and the imperatives of energy performance and occupant well-being are simply hard to reconcile. “Daylighting is great,” says Helen Sanders, PhD, a general manager at Technoform North America and founding president of the Facade Tectonics Institute (FTI), “but your toes and knees don't need daylight. You can certainly get enough daylight by having less glass than from floor to ceiling; maybe from desk height to ceiling is sufficient...All-glass buildings can give glass a bad name if they're not designed with comfort and energy efficiency in mind,” she comments, noting problems with temperature, condensation, and glare. “You can end up with three, four, five, six feet of perimeter floor space that's uninhabitable next to your curtain wall, if it does not have sufficient thermal performance.”
Sanders has concerns that excessive daylight (glare) and heat are detrimental to comfort and productivity, yet she favors improving glass facades rather than returning wholesale to opaque walls with punched windows because of the essential role of access to daylight and views in human health and well-being. “Glass facades can be done well, but you just have to invest in the technology, and the technology's out there,” she comments; the key is to motivate demand to create economies of scale. With advanced proprietary technologies, organizations (particularly government) may require three alternatives and a ten-year track record before they can be specified, so an innovator may have to wait for competitors to catch up. Moreover, “the codes will not require that level of performance until it's cost-effective, and you can't get to cost-effectiveness without driving demand.” This chicken-and-egg impasse means “we can end up with these big, highly glazed buildings that are probably not good poster children for high-performance curtain wall.”
In energy analyses, Sanders says, “the weak point in the curtain wall is the spandrel area, primarily because in codes the spandrel area is opaque, and it's treated as an opaque area of the building...but its performance tends to be a lot worse than other opaque elements.” Thermal bridging between the spandrel and the vision glass is common through the mullions that connect them, since typically there are no thermal breaks vertically up each mullion; “if they're getting really hot or really cold in one area, there's nothing to stop all that heat loss going all the way up the building.” Inadequate modeling also leads to suboptimal construction: two-dimensional modeling typically overestimates performance, and three-dimensional modeling correlates more robustly with real testing but is less common. Also, the center-of-glass U-factor is often confused with the assembly U-factor, which causes an overestimation of fenestration performance. Thermal conduction through the frame and edge of glass increases the assembly U-factor compared with the center-of-glass U-factor. Double- or triple-silver low-E argon-filled units, or even VIG, will underperform in a fenestration assembly when the edge and frame are thermally unbroken because much of the heat flow is through the edges of the fenestration. Consequently, Sanders and her colleagues stress the need to “spec the edge.”
Code evolution is driving envelope design toward more aggressive thermal control. ASHRAE 90.1's prescriptive path calls for 40 percent window-to-wall ratios, Sanders observes; the International Energy Conservation Code (IECC) equivalent is 30 percent, plus an allowance to 40 percent with certain daylight provisions. A common tradeoff when meeting codes on the performance-based path, she notes, is to compensate for neglected facade insulation with highly efficient HVAC or lighting, each of which is cheaper than investing in a superior envelope. Some of the most advanced codes (Washington State and the city of Seattle; Massachusetts; New York; and the next iteration of ASHRAE 90.1) thus include an envelope backstop: a minimum U factor that the envelope performance must meet, regardless of HVAC or lighting performance.
Although ASHRAE 90.1-2019 lacks an envelope backstop, the 2022 version will incorporate Addendum CR (2020), which describes that provision in the form of proportional margins between proposed and base envelope performance factors (15 percent for multifamily residential, hotels/motels, and dormitories; 7 percent for other building types), as media reports have indicated (Sanders “Envelope...” 2022, Melton 2022). With the 90.1-2022 standard scheduled for release this fall (an ASHRAE official reports that the Standards staff was reviewing it at press time), architects and engineers may infer that allowing extensive thermally bridged spandrel area reflects short-term thinking, and that decisions should lean toward longer-range investment in facade efficiency.
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.”
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.
Lou Podbelski, AIA, senior vice president at Hayward, Cal.-based smart-glass firm Halio, describes a market on the verge of expansion, as perceived drawbacks yield to R&D advances. Visual light transmission through older smart glass began around 60 percent, dropping by stages to 20 percent, 6 percent, then a very dark 1-2 percent, he notes; the full range of adjustment could take 20 to 30 minutes, as algorithms waited to ensure that exterior changes from clouds to full sun were not temporary anomalies —making the transition “a day late and a dollar short,” not optimizing energy before reaching the desired tint. Today's advanced smart glass can make the full change from clarity to darkness as a smooth three-minute transition that occupants can stop anywhere during the process. “The one thing preventing electrochromic technology from getting adopted,” Podbelski comments, “even though it saves energy and provides wellness and productivity gains, is the price.” With the IRA credit lowering that obstacle through 2024, now is the moment to test this industry's transformative potential.
Others may be further from prime time. VIG is theoretically promising, Selkowitz says, since “heat doesn't transfer in a vacuum,” even one so tiny that nearly invisible microspheres or cylinders are used to hold the panes about a third of a millimeter apart. Maintaining the vacuum is a problem: “You have to make this super-tight seal, even better than a normal insulating glass unit, because if you get any crack or defect at the edge, the vacuum is gone,” and performance drops from R10-12 back to R2. Fabrication requires high-temperature heat sealing; “the bottom line is, it's complicated and expensive,” though VIG is finding applications in historic preservation, where older buildings' frames require “a glass element that's 8 or 12 mm thick, and you can't get that in an IGU.”
Aerogel is further back in the R&D process, Selkowitz adds, though recent reports describe advances in clarity and aesthetics (Carroll et al., 2022). Krypton gas (replacing argon) is useful in ¾-inch IGUs but carries pricing vulnerabilities, he says: “Most of the krypton comes, it turns out, from Ukraine and Russia, and it's also used in satellites for thrusters; [Elon] Musk uses it in all his little satellites up there to keep them on track. So the cost went up by a factor of five or 10…It's added two or three dollars a square foot to the price of the window.” Embedded photovoltaics offer the possibility of harvesting energy to power building systems, shades or louver systems, or an electrochromic window itself, but create tradeoffs with visibility and raise questions of scale. With photovoltaics among multi-system energy-sparing strategies, “if you really wanted to do a net-positive building today, you could,” Hoffman suggests, “as long as the energy use intensity inside the building was manageable...There are several Net Zero buildings, but I can't point to one that's actually proven to be net positive.”
Along with software tools widely used in the glass industry (see https://windows.lbl.gov/software), LBNL helped create the National Fenestration Rating Council, which maintains a standardized, nonproprietary database of glass products' properties. A second DOE-supported group, the Attachments Energy Rating Council, provides similar ratings for shades, blinds, and associated products. Selkowitz also cites a new collaborative, the Partnership for Advanced Window Solutions, dedicated to accelerating the adoption of high-performance windows and attachments, beginning in the residential sector and later broadening to commercial. These resources are indispensable for designers and specifiers aiming to keep up with this fast-changing field.
Conclusion
As Shannon points out, “We've gotten very skilled at creating glass buildings that can appear to be monolithically glass and hide the stuff you don't want to see: the spandrel, some of the column construction, and so on. And we're also doing that because we have to meet energy code, and no matter how good the glass is, we need a certain percentage of opaque area.” The concept of a wholly transparent building is a limit that can never be reached.
Some future-watchers look to technology transfer from the information-technology and automotive fields for breakthroughs. “The automotive industry in glass is actually larger than the architectural glass industry,” Shannon says. “One time I was challenging somebody on why we aren't doing these things, and they said, 'Because we sell more glass to General Motors and Ford than we do to you.'” The super-thin glass used in triple- and quad-pane products has become far cheaper, Selkowitz observes, with the smartphone industry driving demand. True paradigm shifts are likely to cross industry-sector borders.
“We innovate; we don't experiment,” Shannon summarizes. “Buildings are not a great place for an experiment, because [with] experiments, some go well and some don't; there's a hypothesis, and a good scientist is as much informed by the failure of his hypothesis than he is by its confirmation. A building owner is not going to be happy if we disprove our hypothesis by building a building that doesn't work.”
“The root problem,” Patterson comments, “is that the industry does not yet well understand these issues of resilience and sustainability.” Citing Bjarke Ingels's dictum that “sustainability is a design problem,” he reminds the profession that its urgency mandates dramatic rethinking: “If we were a rational society, we would have a moratorium on building until we figure out how to do it without negative consequences to the environment. Of course, that is a total non-starter”—yet the time to start is immediately.
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Bill Millard, is a New York-based journalist who has contributed to Architectural Record, The Architect's Newspaper, Oculus, Architect, Annals of Emergency Medicine, OMA's Content, and other publications.
