- Why Are Glass Buildings So Popular?
- The Problem With Highly Glazed Façades
- Making All-Glass Buildings Work
Rethinking the All-Glass Building
By Alex Wilson
Is it time to end our love affair with the all-glass building? A lot of proponents of high-performance, green design certainly think so—while other respected architects, including some leading green designers and energy experts, argue that all-glass can work well if done right.
From Shanghai to Las Vegas, Abu Dhabi to Frankfurt, highly glazed façades have been in the vanguard of high-rise, high-design buildings for the past half-century. Some of the world’s most prominent “green” skyscrapers, including New York City’s One Bryant Park (the LEED Platinum Bank of America skyscraper) and the New York Times Tower, wear the mantle of green with transparent façades. But there is a high environmental cost to all that glitter: increased energy consumption. Until new glazing technologies make technical solutions more affordable, many experts suggest that we should collectively end our infatuation with heavily glazed, all-glass buildings.
Why Are Glass Buildings So Popular?
There are a lot of reasons why all-glass buildings are appealing to architects and building owners. While design and aesthetics are clearly the drivers of heavily glazed façades, there are other reasons why we like glass so much.
Transparent skins provide access to daylight, and natural daylight is one of the leading drivers today of architectural design—green or otherwise.
Well-designed daylighting is very energy-efficient. According to Paul LaBerge, the green building strategy manager for Apogee Enterprises in Minneapolis (the parent company of Viracon, Wausau Window and Wall Systems, and other fenestration-related businesses), sunlight through spectrally selective low-emissivity (low-e) glazing has a good light-to-heat ratio of about 175 lumens per watt (lpw), compared with less than 100 lpw for the best fluorescent and LED lighting. If we incorporate features to distribute daylight deeply into a building and to block it when it isn’t wanted, highly glazed façades offer a tremendous amenity in large buildings. Some research suggests that good daylighting also improves productivity.
Connection to the outdoors
Closely related to daylighting is the visual connection to the outdoors that can be provided by a transparent façade. “For us to have contact with nature and access to nature is huge,” says Bob Fox, FAIA, of Cook + Fox. With his firm’s design of One Bryant Park, this was a top priority. “Everyone has access to the outside,” he says of the 2.1 million square-foot (195,000 m2), 52-story building owned by the Durst Corporation. “You can look out a window no matter where you are,” he told EBN. “To us that is very important.”
Phillip Mead, AIA, the architecture program coordinator at the University of Oregon, notes that glass façades are thought to “promote transparency—in particular, with government buildings.” Notes John Myers of Harmon, a leading glazing and curtainwall service company, many companies like the association of transparency with corporate image, as if it says, “See, we’re in here, doing something for you; we’re not hiding anything.”
Glass is durable
As long as it doesn’t break, glass is a highly durable material. Michael Utzinger, AIA, an associate professor of architecture at the University of Wisconsin–Milwaukee, notes that “glass, as a surface material, ages in urban environments better than many building skins.”
David Lee Smith, a professor of architecture at the University of Cincinnati, looks to the history of the Modernist movement in architecture for the popularity of glass. “The extensive use of glass was, in part, a response to the fact that such a novel thing was possible,” he said. Two factors contributed to this, according to Smith. First, a freestanding structural frame no longer required an exterior wall to serve as a major bearing support; second, glass became available in larger sheets. “Being able to do something is at times a motivation that might overwhelm any sense of logic,” Smith told EBN.
Outsourced engineering, manufacturing, and liability
Most, but not all, highly glazed façades are curtainwalls (as opposed to structural walls with inset windows), and with curtainwalls the entire glazing system is engineered and manufactured offsite. “You can get the curtainwall manufacturer to design it and take responsibility for it,” says Fiona Cousins, P.E., a principal of the engineering firm Arup, in the company’s New York city office. Her colleague, senior engineer Scott Bondi, Ph.D., P.E., added that “the quality and precision is generally a lot better [with curtainwalls], and they go up a lot faster.”
Architects like glass, and a lot of it. Juliet Landler, a senior instructor in the architecture department at the University of Technology in Sydney, Australia, and previously a façade engineer for Arup in Sydney, Hong Kong, and London, suggests that there are parallels between architects’ infatuation with all-glass façades and the appeal of women’s clothing. “I have worked with architects who believe that façades should be like women’s lingerie—sleek, smooth, sexy, shimmering, simple—and simultaneously transparent and mirroring,” she told EBN. “Isn’t that worth a little stupidity?” she asks.
Stephen Selkowitz, head of the Building Technologies Department at Lawrence Berkeley National Laboratory and a leading expert in glazing technology, notes that architects are often caught between the desire to produce an efficient façade design and owners who believe they need the all-glass image to meet “Class A office lease competition.”
The Problem With Highly Glazed Façades
So what’s the problem? In a word: energy. In general, heavily glazed buildings consume more energy than buildings with more moderate levels of glass. With a higher glazing fraction, solar heat gain as well as heat loss in cold weather are both greater. Glass does introduce daylighting, of course, and well-executed daylighting can reduce both electric lighting and mechanical cooling costs but the ideal percentage of glazing is far below that of many of today’s prominent all-glass buildings.
To get a better handle on the impact of higher glazing areas on building performance, Fiona Cousins, Scott Bondi, and Cameron Talbot-Stern at Arup’s New York office carried out a detailed energy-modeling exercise for EBN. They looked at the effects of five different variables—building footprint, location, glazing type, orientation, and percentage of glazing—on overall energy consumption of a ten-story building with 10,000ft2 (930 m2) per floor.
The footprint, or shape, of a building affects the “façade zone” (or perimeter zone), which Arup defined as the outer 15 feet (5 m) of the building (see Building Footprints figure). Also modeled was the impact of orientation on elongated buildings. It is assumed that lights are dimmed based on available daylight in the façade zone.
We selected three cities for energy modeling to show how the energy impact of glazing percentage varies by climate. New York was picked as a fairly cold climate, Miami a hot climate, and San Francisco as a very moderate climate (low heating and cooling loads).
For glass type, Arup looked at single glazing, double glazing with no low-e coating, high-performance double glazing with low-e, and triple glazing with low-e (see Glazing Types figure). In new com-mercial buildings, single glazing is almost never used except in the mildest of climates, and triple glazing is rare except in very cold climates, applications where condensation control is highly important (such as hospitals and art museums), and in the highest-performance green buildings. The analysis assumes neither exterior nor interior shading.
Finally, Arup modeled four different glazing percentages: 20%, 40%, 60%, and 80% (see Glass Percentage figure and graph). It is assumed that wall area not glazed consists of spandrel panels and floor plate.
The impact on annual energy consumption on the modeled 100,000 ft2 (9,300 m2) building, assuming a square building footprint (Building Type I), with increasing glazing area shown in the three graphs below. Energy consumption is shown in million Btus per year (mmBtu/yr) for the building, including cooling, heating, ventilation, lighting (1.2 watts/ft2), and miscellaneous loads (1.5 W/ft2).
We can draw two conclusions from this analysis. First, the penalty of increasing the glazing area is greater with single glazing and clear double glazing than with low-e clear double glazing and low-e clear triple glazing. But with all glazing types, increasing the glazing percentage increases annual energy consumption. Second, the impact of increasing the glazing area is greater in more extreme climates.
Impact of more glazing on HVAC loads
Again, assuming a square building footprint, the graphs below show the impact of a higher glazing percentage on cooling capacities (in tons) and heating capacities (in million Btus per hour, MBH). As with the last graphs, the four different glazing types are compared (from singleglazing to triple with low-e).
These graphs at right show that the impact on peak cooling and peak heating demand from increasing the glazing is greater than the impact on annual energy consumption (previous graphs). In New York, for example, going from 40% to 60% clear double glazing increases the cooling capacity required for a square-footprint building by 21% (from 280 to 340 tons), while the impact on heating capacity is 24% (from 2,500 to 3,100 MBH).
Again, the impact of increasing the glazing percentage is reduced when better glazing is used, but the increases are still significant. With cooling in particular, those increases in capacity (peak demand) result in fairly proportionate increases in first costs—doubling the capacity of a chiller roughly doubles the cost—while boiler costs are not nearly as tied to capacity.
Another way to look at this issue, notes Selkowitz, is to fix the size of the cooling system, say at 220 tons in the New York example, and look at how much glazing area that chiller will serve. With standard double glazing, you can have only 20% glazing area, but by boosting the glazing performance to double glazing with low-e you can double the glazing area to 40%, and with triple glazing and low-e you can jump to 60% glazing area. “So my conclusion from your data is that you can buy an enormous amount of design flexibility—i.e., a 300% change in glass area—by adding a sheet of glass and coating,” Selkowitz told EBN. He also notes that this modeling assumed no shading (exterior or interior), and effective use of either (see “Making All-Glass Buildings Work”) can change the results significantly.
Arup examined the impact of increasing the glazing percentage in New York when the shape of the building footprint is changed from square (53% façade zone) to moderately elongated (73% façade zone), to highly elongated (100% façade zone). In the results shown in the graph below, the building orientation was averaged, and only the two most advanced glazing options were modeled: Type 3 (double glazed, low-e,) and Type 4 (triple glazed, low-e).
These results show that the impact of increasing the glazing percentage is greater when a building footprint is more elongated—which puts more of the floor area in the façade zone. The differences aren’t huge: in a 60% glazed building with double-glazed low-e glass, for example, changing from a square footprint (Type 1 building) to a moderately elongated footprint (Type 2 building) increases the modeled energy consumption 3.9%, while changing from square to a highly elongated footprint (Type 3 building) increases energy consumption 9.7%.
Effect of orientation
Finally, because the orientation of an elongated building footprint would be expected to have an impact on energy consumption, Arup modeled the differences between moderately elongated (Type 2) buildings that are aligned east-west with those that are aligned north-south for the New York climate. These results are presented in the chart below.
The effects of orientation are more pronounced with lower-performance glazing, but with all glazing types, having the longer façades face east and west results in higher annual energy consumption than when the longer façades face north and south. With double-glazed, low-e glass and 40% glazing area, for example, that energy penalty is 5.9%. At 80% glazing, the penalty jumps to 9.4%.
Beyond the concerns of the glazing itself are concerns about the framing. “The thermal bridging that takes place in most framing elements,” says Cousins, “exacerbates [the energy] problem—the frame almost always performs worse than the glass.”
Engineer Marc Rosenbaum, P.E., also raises concerns about how curtainwall systems are detailed at junctions with other building assemblies, such as concrete slab edges. “The idea that the curtainwall building is R-8 is unlikely when the entire façade is actually modeled,” he says.
Building codes and percent of glazing
Commercial building energy codes already address the amount of glazing that can be installed in buildings. In the ASHRAE 90.1-2007 energy code, the prescriptive performance path sets a maximum glazing area at 40%. To exceed that glazing percent, according to Cousins, you have to show that a building will use less energy than it would with that 40% glazed façade. That’s pretty hard to do, she says. In fact, it’s not clear that some of the very prominent commercial green buildings that were built in New York before the current energy code went into effect would even meet today’s 90.1-2007 code.
For many reasons, all-glass buildings use more energy than buildings with punched window assemblies. From an energy standpoint, the arguments are pretty clear that we should move away from using all glass. According to Sean Scott, AIA, a Portland, Oregon architect and instructor on high-performance envelopes at the University of Oregon, “The bottom line is that the data does not support more than about 25% glazing on most building types and climates.” Beyond that percentage (“give or take 5%”), says Scott, glazing does not contribute more daylight, and it starts to create a net energy loss.
The smart thing to do would be to stop designing these highly glazed buildings. Because such a wholesale change in design aesthetic is not likely in the near future, how can we lessen the energy impact of lots of glazing?
All glass but not all glazing
The look of “all glass” can be achieved with significant spandrel panel area. Indeed, most of the buildings we think of as “all glass” have at least 20% of the wall in opaque area—often a lot more. Steve Fronek of Wausau admits that we’re overusing vision glass today. “If downward view is not important, the glass below sill height is nothing but an energy waster,” he told EBN. Vision glass at floor level also tends to expose the backs of desks, power-strips, and other items that add visual clutter when looking in.
Paul LaBerge of Apogee points out that curtainwalls can be designed to have anywhere between 20% and 80% glazing area (or window-to-wall ratio). “When you look at an all-glass building, you have to dig to understand its thermal performance,” he points out. “You can’t simply say, ‘Oh, that’s bad.’”
Spandrel panels can be relatively well insulated (compared to glass and framing). Some companies offer spandrel panels insulated with polyisocyanurate, polystyrene, or rigid mineral wool in thicknesses up to 4½ inches (115 mm) with insulating values in excess of R-25, though spandrel panels an inch to an inch-and-a-half thick (to match the insulated glass units, IGUs) are far more common.
Where the vision glass and spandrel panels belong is governed by common sense. Norbert Lechner, author of the text Heating, Cooling, Lighting: Sustainable Design Methods for Architects, is a master of common sense in energy design: “On any one floor, the higher glass produces the best daylighting, because the light travels further inside and the window causes less glare. The middle glass is best for views and only fair for daylighting, while the lower glass has almost no daylighting benefits and questionable view benefits.” Thus, Lechner argues that it’s most efficient to have insulated spandrel panels for the lower part of the window wall.
When a lot of glazing is called for in a design, the most common way to reduce the heat gain penalty is to lower the solar heat gain coefficient (SHGC) of the glass. Tinted glass is the traditional solution for this, but it blocks most of the visible light as well, so spectrally selective clear glazing is more popular today. This type of glazing provides high visible light transmittance but selectively blocks out a lot of the solar spectrum outside of the visible band.
Another option is to apply frit to the glazing panels. This is a screen-printing pattern that serves to block heat gain while still allowing fairly good visibility out through the glass. There are both translucent and opaque frits, and these behave differently, according to Selkowitz.
“Frit can be used to reduce effective aperture,” says lighting designer and daylighting expert James Benya, P.E., “but it cannot address the poor insulation of glass walls.” Benya says that the fritted clerestories and skylights used at PDX Airport in Portland, Oregon, work pretty well because it’s a temperate climate. “The same technique at O’Hare [in Chicago] works comparatively poorly, because there’s too much glazing, and the climate is too extreme.”
Glazing tuned by orientation
“Since the east and west exposures get the most summer sun, they should have the least glazing,” says Lechner. The shading, too, should vary by orientation. “Each façade would look a little different in order to respond to the varying sun angles.”
Unfortunately, providing different types of glazing on different orientations is rarely done—because of aesthetic concerns, according to Myer. It can be done, though. With the Minneapolis Public Library, low-iron glass is used for all façades, but each orientation has a different fritting pattern to regulate heat gain, according to Don McCann of Viracon.
The modeled energy impact of glazing percentages presented in this article assumes no shading. With effective exterior shading strategies, tuned by orientation, it’s possible to significantly reduce the energy penalty of a high glazing fraction. Exterior shading helps control solar gain, so it can reduce both total annual energy consumption and peak cooling capacity. Exterior shading is more effective in this capacity than interior shading (with adjustable blinds) because the sunlight is blocked before entering the building; once sunlight is transmitted through glazing, most of that solar energy will be trapped in the building.
Exterior shading can be either fixed or adjustable. Adjustable shading is attractive as it provides a greater level of control, but the mechanisms to adjust exterior louvers are prone to failure. According to Steve Fronek of Wausau, “A lot of climates in the U.S., with ice and dust and snow, make these a bit impractical.” Fixed shades are far more common. Note that fixed exterior shading has no effect on heating energy use or demand; the benefit relates solely to reduced solar heat gain.
Moveable blinds on the interior provide an easily adjustable level of control for solar heat gain. With white or reflective exterior surfaces, a portion of the sunlight striking the lowered blinds will be reflected back through the glazing. Automated blinds can be programmed to open and close on an entire façade to optimize energy performance, or blinds can be operated manually. According to Selkowitz, manual shades or blinds in one-person perimeter offices may be used, but in an open space such blinds are rarely used to effectively control solar gain—which gets at the issue of behavior in buildings. “In my view,” says Selkowitz, “an automated system is preferred because it can more reliably deliver comfort and energy performance.”
Moveable blinds that are fitted into edge-tracks or that are sized to provide very small openings at the edges can help to control heat loss as well as heat gain. With relatively poor glazing, such blinds can also help to improve comfort by raising the mean radiant temperature.
Some advanced buildings, such as the highly glazed New York Times Tower, have both fixed exterior shading and automated, motorized interior blinds triggered by daylight sensors to provide a maximum level of control—which is very important with floor-to-ceiling, low-iron vision glass. The New York Times Tower also uses an addressable, dimmable daylight control system that provides lighting energy savings up to 40 feet (12 m) from the façade, according to Selkowitz, who was involved in the shading and daylighting specifications for the project.
Another design strategy with heavily glazed façades is to distribute the resultant daylight as deeply as possible into the interior of the floorplate—essentially extending the façade zone. Lightshelves can help to accomplish this, while reducing heat gain and glare at the perimeter. Combining exterior shading with lightshelf design illustrates the importance of integrated design, according to Benya: “A lightshelf, for example, that protects the view glazing from direct sun during the cooling season is a superior design. But the length of the overhang is a cost and structural problem, especially with the threat of snow or ice accumulation.” He adds that the higher the sill, the shorter the projection, so limiting floor-level glazing has an added benefit.
Most curtainwall systems today use double glazing—often with tinted glass or spectrally selective coatings to control solar gain. Triple glazing can also be used, and this is becoming more common with the highest-performance buildings.
“There’s interest in it,” says John Myers of Harmon on triple glazing, but he is quick to point out the challenges. “You’re doubling the risk of seal failure, and you’re limiting some of your glass choices and the ability of the framing to manage that because you’re talking about much heavier glass.” He told EBN that with residential applications, triple glazing is easier because you have relatively thin glass and small apertures, but with commercial buildings, you’re adding 50% to the weight and increasing the glazing thickness.
When suspended low-e films are used to provide triple glazing, as is being done currently with the window retrofit of the Empire State Building, the added weight can be avoided.
Additional layers of glass can also be achieved with double-envelope or double-façade designs, a strategy that is more common in Europe than in the U.S. The basic approach is to provide two glass skins, separated by a fairly deep air space (typically several feet) that is usually ventilated to the outside. Typically, the outer lite is single-glazed and the inner lite double-glazed, but this is occasionally reversed, according to Selkowitz, who believes most of the interest in double-façade design in Europe is driven by desire for improved solar control in buildings that often do not have air conditioning. He notes that with a double façade you can provide an “external shading equivalent” with a less costly system that doesn’t have to withstand wind and weather.
While Wausau fabricated the first double wall in the U.S. on the Hooker Chemical building in Niagara Falls, New York, in 1983, the Apogee companies are generally not enthusiastic about this option. “Unless you’re effectively harvesting the air inside that cavity, you’re not gaining a whole lot of benefit from it,” said Myers. “Maintenance is a nightmare and, quite frankly, if we’re talking about effective payback, someone who is leasing a space [is] losing footprint.”
Fiona Cousins at Arup said that the first work she did as an engineer in the early 1990s was modeling about 40 double-façade options for a project in Germany—but none of those worked better than a standard curtainwall design. That’s still the case. “There are ways to do it without doing harm,” she said, “but we can’t make it pay for itself.” Cousins notes that with a double-envelope building, it’s almost always warmer in the cavity than it is outside. “In Antarctica that’s probably a good idea,” she says. “In Austin it isn’t.”
The one situation where Cousins is encouraging of double façades is in wholesale re-skinning of older buildings. In such a retrofit application it’s possible to dramatically improve performance (because the base case is very poor) while allowing the building to be occupied during construction.
While a combination of exterior and interior shading can be fairly effective at controlling heat gain, another option is to use glazing that does the same thing. The most promising technology here is electrochromic glazing, in which the glazing is tinted on demand by supplying a low-voltage current.
One manufacturer, Sage Electrochromics (a Building-Green Top-10 product in 2006; see EBN June 2006), has been making significant headway in bringing the cost of electrochromic glazings down. Two years ago, according to marketing director Jim Wilson, costs were in the range of $100–$120/ft2 ($1,000–$1,300/m2). Today, those costs are about $65/ft2 ($700/m2), and Wilson expects the cost to continue dropping.
“It’s a common conception in the market that it’s just too bloody expensive,” Wilson told EBN, but he says that the product is already cost-competitive when compared with other methods for glare and heat- gain management. High-quality interior blinds typically cost $39–$79/ft2 ($420–$850/m2), according to Wilson, while exterior shading systems cost a minimum of $32/ft2 ($340/m2) and sometimes over $100/ft2 ($1,100/m2). Indeed, John Myers of Harmon said that the wall system for the New York Times Tower (including curtainwall system, glazing, exterior shading, and interior blinds) was somewhere in the range of $700/ft2 ($7,500/m2), though EBN was not able to verify this number or determine exactly what it included.
The solution of last resort with all-glass buildings is to provide the most energy-efficient mechanical solutions available for satisfying that demand. “If it’s really non-negotiable,” says Cousins, “then you have to look at the systems and make them really low-energy to compensate for the energy waste—not a good place to find yourself.”
With their layered transparency, connection to the outdoors, and daylighting—maybe even higher productivity—all-glass buildings have their appeal. But the energy penalty of such buildings cannot be ignored. As lighting designer James Benya told EBN, “It is hard for me to imagine that an all-glass building is ever a good idea, but smart variations can probably be responsible if the nasty east and west façades are intelligently addressed and the south side is shaded, with climate tuning.”
Selkowitz agrees that “most highly glazed buildings today probably don’t perform as well as similar buildings with a smaller glazing area—but, if designed and executed properly, that need not be the case.” He thinks the idea that there’s a standard glazing solution, such as 30% vision glass, that is best for all designers is simplistic. He sees a clear pathway to producing environments that are more pleasant, lower-energy, and well daylit with glass areas ranging from 25% to 75%. “The reason this works,” Selkowitz told EBN, “is because heating and cooling are typically not the biggest energy loads in commercial buildings—it’s lighting, and a well-designed, thermally efficient façade can produce significant lighting savings, thus beating the overall energy use of an opaque façade.”
“Imagine a high-performance façade,” says Selkowitz, “with operable exterior shading for sun control and daylight redirection, a triple-glazed, low-e, gas-filled window, with interior automated roller shades for glare control with dimmable daylight controls. Sounds complicated?” He argues that the motorized shade and daylight sensors are much less complicated than the centrifugal chiller, fans, pumps, cooling towers, and power plant that they displace.
Despite the potential for technical fixes, a growing body of experts in sustainable design argues that our architectural aesthetic should evolve away from all-glass façades. “Transparency being so often a non-
negotiable design choice,” suggests Henry Siegel, FAIA, of Siegel & Strain Architects of Emeryville, California, “is a real failure in leadership and vision in the design community.” He argues that “broadening the definition of design excellence to include values other than aesthetics seems like one place to start changing some of these fixed ideas.”
For more information:
Lawrence Berkeley National Laboratory
Building Technologies Department
Window Systems for High-Performance Buidings
Univ. of Minnesota Center for Sustainable Building Research