Is There Such a Thing As Too Much Glass?

High-performance curtain wall design
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Sponsored by The Ornamental Metal Institute of New York
By William B. Millard, PhD
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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.


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Originally published in Architectural Record
Originally published in December 2022