The perennial debate over how much glazing to incorporate into a building’s façade rages on. These days, the energy-conscious advocates of punched openings seem to be gaining favor; proponents of glass-box minimalism are viewed paradoxically as promoting something of a luxury. Yet the two sides are likely to reach a compromise soon and into the coming decade, as super-high-performance products—some in R&D and others market-ready—are helping boost the case for more glass. In every functional and aesthetic category from security and fire protection to energy efficiency and green building, glass is overcoming its perceived limitations.
Still, the discussion is far from over, says Stephen Selkowitz, head of the building technologies department at Lawrence Berkeley National Laboratory (btech.lbl.gov/btd.html), Berkeley, Calif. “Highly glazed, ‘transparent’ buildings are put forth by some as the iconic image of a green building, and derided by others as a trend driven to wasteful excess,” he says. “At the extremes, both are probably correct, but stay tuned.”
In the meantime, even as demand for commodity glazing materials has eased considerably, the industry is slowly but surely turning its attention toward more advanced glazing materials and the emerging technologies collectively described as “smart glass.” Moreover, costs are coming down as performance trends upward.
In fact, it is now possible to provide glazed curtain wall and storefront systems for buildings with an overall U-value of 0.30 or less, according to the nonprofit National Fenestration Rating Council standard NFRC 100. (According to the NFRC [www.nfrc.org], “U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor [U-value] of a window assembly. U-factor ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value.”) Just a few years ago, the same investment would have procured units with a lower performing overall U-value of about 0.45, according to A. Michael Voigt, RA, CCS, a senior associate with RNL Design (www.rnldesign.com), Denver.
Ultimately, many sustainability-minded Building Teams are aiming for an ideal glass spec that balances cost with performance; for example, a low-emissivity (low-e), untinted glass capable of achieving a low exterior reflectivity of less than 15% with a high visible light transmittance (VLT) of at least 60%, a solar heat gain coefficient (SHGC) below 0.28, and a shading coefficient of less than 0.33. (This hypothetical example courtesy of Kevin Day, AIA, LEED AP, a senior associate with San Francisco-based Stantec [www.stantec.com]).
However, the optimal way to achieve these ideal levels is not always so clear-cut.
FROM DOUBLE-GLAZED TO TRIPLE-GLAZED
There’s another debate animating the building industry: whether triple glazing’s improved performance outweighs its additional cost and weight.
Manufacturers claim that the most advanced triple-pane windows can bring U-values down by 60% to 70%, while also improving condensation resistance and thermal comfort, as compared to a conventional double-glazed window. Currently, triple-glazed products currently account for just 1% of the commercial windows marketplace. Yet they are finding applications in very cold climates, in addition to the residential market, where new building codes require more rigorous U-values. “Some manufacturers are considering retooling [their plants] to triple glazing as they feel it is the future of the industry and would prefer to get ready for the change now,” says Rich Walker, president and CEO, American Architectural Manufacturers Association (www.aamanet.org), Schaumburg, Ill.
Along similar lines, the U.S. Department of Energy recently launched a volume-purchase program for “high-insulating and low-e storm windows.” For the purpose of this initiative, high-insulating has been defined as R-5, which in general only triple-pane windows are capable of achieving. With 30 participating manufacturers or brands, the DOE program is trying to propel triple glazing beyond its niche markets and the mainstream.
AAMA’s Walker warns that “the advantages of triple over double glazing are considered by some to be marginal and subject to the law of diminishing returns, meaning that the additional cost of triple glazing does not yield cost-effective benefit.” For example, a triple-glazed, low-e vinyl or fiberglass window with an air gap will deliver the same U-value as a vinyl or fiberglass double-glazed unit with warm-edged spacers and gas infill, according to Walker.
To get a fully functional triple-glazed window with the lowest U-value, the designer must be sure to “better insulate the frame itself, reinforce the framing to bear the additional weight, and use more expensive materials, such as krypton gas infill instead of argon,” he says.
RNL’s Voigt contends that triple glazing only makes economic sense with the most thermally efficient storefront and curtain-wall framing systems. Others counter that the technology has value in many applications. Huston Dawson, PE, a senior associate with Weidlinger Associates (www.wai.com), New York, N.Y., sees triple-pane windows as playing an important role not just in cold climates but also in very hot climates and in locales that endure significant fluctuations between hot and cold weather. Dawson also remains optimistic about the role triple glazing may eventually play in the blast-resistant glazing market, although more research and testing still needs to be done, he says.
Spectrally selective glass. According to a recent Ducker Worldwide study (http://www.aamanet.org/news/2/10/0/all/341/aama-wdma-industry-review-and-forecast-available-predicts-industry-trends-), low-e glazing now accounts for more than 50% of the commercial market, much of it from spectrally selective glass, a highly energy-efficient material.
Spectrally selective glazing is able to discriminate between ultraviolet, daylight, and infrared, and only allow the desired rays to penetrate through the lite. Already a mature technology, spectrally selective glass is available from almost all major distributors and manufacturers. The newest products offer visible transmittance at almost twice the solar heat gain coefficient value.
Gas-filled glass panels. Another low-e variant is an insulated glass unit (IGU) filled with inert gases that lower the U-factor. More popular in the residential markets with strict building codes, argon is the most common offering and specification, while krypton offers greater energy efficiency with a higher price tag, says A. William Lingnell, PE, Lingnell Consulting Services, Rockwall, Texas.
Another option is air-filled panels, which also offer some benefit and cost less than those with rare elements. “Argon and other gases can eventually seep out of insulated glazing units,” says Voigt. “Because of this we require that our energy models be calculated using air-filled units.”
Double-glass walls. Although fairly popular in Europe, where energy prices are higher than in the U.S., paybacks are shorter, and financing and state incentives seem to work better, double walls have yet to catch on in the United States. As Voigt points out, “Their high cost tends to limit them to very selective applications.”
One such application, says Dawson, a board member of the Protective Glazing Council International, is a blast-resistant project where the exterior wall is meant to be sacrificial. In this application, the exterior glass layer is designed to dissipate the energy of a blast wave, reducing the energy load that the internal protective surface must resist.
In general, double walls can do a good job of keeping solar heat gain out of the building. “With the two separate panes of glass, the solar gain gets trapped and dealt with in the cavity,” says Lingnell, a technical consultant to the Sealed Insulating Glass Manufacturers Association, Chicago.
Electrochromic glass. Another one of the more promising areas of smart glass development is in the arena of electrochromic materials. These glass products have a dynamic film coating that either reflects or absorbs light depending on the application by means of a low-voltage charge. By reversing the charge, the glass can switch its behavior from allowing light through to blocking its passage.
“There is tremendous market interest in this technology held back, to date, by availability, cost, and the need to integrate these solutions into building controls to capture the best value,” says Selkowitz, who manages LBNL’s R&D and deployment program for windows and daylighting. “By 2012, we expect that at least two companies will be offering products with good thermal and optical properties in larger architectural sizes and at costs that are substantially reduced from the first-generation products now on the market.”
Traditionally, electrochromic windows have been made with tungsten oxide, which produces a relatively high amount of heat without blocking infrared light very effectively. However, a prominent electrochromic manufacturer has recently licensed some new technology that uses alternatives to tungsten oxide from LBNL’s Advanced Technologies Department, with the hope of developing a new generation of products that can better reflect and control light.
In terms of cost, electrochromic windows may be considered a novelty item these days at around $100/sf. But with newer technologies and an anticipated growing market demand, that price point is expected to drop considerably. At the same time, the window’s electricity requirements, which involves the necessary wiring and power output, must also be factored into the total cost.
EXTENSIVE R&D IN THE WORKS
The smart glass industry’s overarching goal is advancing toward the point where glazing becomes a net-zero energy element of the façade. “Conceptually, this is achieved with a solution where the thermal losses are very low in winter and the window admits solar gain to offset them; and the thermal gains are very low in summer, and the window admits daylight, which reduces electric lighting use,” explains LBNL’s Selkowitz.
To achieve this level of performance, the following parameters must be met, according to the LBNL researchers:
• Windows with thermal losses of between a 0.1 and 0.2 U-factor, and useful winter heat gain expressed as an SHGC of greater than 0.3.
• Windows with highly spectrally selective glass and dynamic control over light intensity to control solar gain and glare, while letting in daylight when desired.
• Smart controls or design that switches glass properties as needed.
• Lighting controls that reduce electric power and capture the energy benefits.
With regard to developing the technologies capable of meeting these performance levels, R&D within the glass industry is currently focusing on lower-cost, gas-filled, low-e coated triple-glazed units with thermally improved framing, as well as vacuum and aerogel glazings. “There are other types of active coatings under development and new passive thermochromic coatings on the market, and some emerging hybrid solutions,” says Selkowitz.
Another research focus at LBNL is in the area of highly insulated windows and window frames. One research project in particular is targeting a center-of-glass value of R-10 in a practical window assembly that is relatively affordable, durable, and lightweight. The approach is developing thin, nonstructural central glazing layers as an alternative to current industry approaches to achieving high insulation levels.
Others efforts under way seek to improve the structural properties of glazing and fenestration products, says John Jackson, MEng, AIA, LEED AP, with the building enclosures knowledge group at HOK (www.hok.com), Washington, D.C. He describes a focus on the development of “glass composites and hybrids” with increased tensile strength and reduced brittle breakage behavior.
As for building-integrated photovoltaic (BIPV) glass, which has garnered enthusiastic interest from many Building Teams, the economic feasibility of these energy-producing systems is not exactly immediate. Yet some observers see good news on the BIPV horizon. “Photovoltaics built into glass spandrel units is a very promising approach and should have a very bright future,” says Stantec’s Day. He alludes to a company attempting to develop a technology that would enable power generation on see-through glass by spraying solar cells onto existing glass surfaces. Several of these transparent PV products have been launched in the last few years.
The power-generating capacity of today’s BIPVs embedded in the façade is lower than engineers and owners would hope, making them too costly, says Linton Stables, chief of specifications at Perkins Eastman (www.perkinseastman.com), New York, N.Y. “However, we have seen a combination of aesthetic and practical use of embedded PV in some of our Middle East work, where the geometric patterns of the PV cells reflect other decorative motifs in the culture, and at the same time provide some shading—all while providing some electrical generation,” he says.
One practical example of solar glass in action is glassmaker Pilkington’s Northwood, Ohio, manufacturing facility. Using the manufacturer’s solar glass panels, a team led by environmental engineers at Hull & Associates (www.hullinc.com), Dublin, Ohio, created a one-acre array of solar panels on top of a former waste impoundment. “The mound is about 55 feet high and free of trees, so it is ideally placed to receive the maximum sunlight,” said Cliff Fleener, environmental manager for Pilkington and NSG Group. The project yields about 12% of the plant’s power requirements, and suggests new ways to use brownfields or unusable grounds as “project-integrated PV.” Says Fleener, “Depending on how successful this project is, the [solar panel] field could eventually be extended up to about 50 acres.”
BLAST-PROOF GLASS: SHINING BRIGHTLY
While practical energy-producing glass remains a goal for the industry, glass that protects building occupants from external threats is a reality.
Ever since the 1995 bombing of the Federal Building in Oklahoma City, blast-resistant windows have been a standard requirement for all U.S. General Services Administration (GSA) buildings. More recently, the Department of Homeland Security issued the Interagency Security Committee’s Security Criteria, which require that windows be designed to mitigate potential hazards from flying glass.
Weidlinger Associates’ Dawson says this new requirement is starting to show up in RFPs. “The glazing industry and many others are eager to see what it’s going to be like in practice,” he says. Says Lingnell, a 47-year industry veteran who serves on several glass-related committees for the standard-making body ASTM International, “It’s a real challenge for the engineers to design the façade to both withstand blasts and allow the glass to break, but to not allow it to penetrate into the area.”
Note: AAMA’s Voluntary Guide Specification for Blast Hazard Mitigation for Fenestration Systems provides specifiers, contractors, and building owners with a guide for evaluating blast-resistant systems, according to AAMA’s Walker.
Anchoring requirements. Of course, it’s not just the glass materials that must be blast proof: the entire window assembly must be designed for impact resistance. “This becomes a particularly difficult issue when designs contain units with a great deal of framing and small areas of glazing, as this typically causes more of the blast force to be transferred to the building structure, which in turn affects anchoring design,” says RNL’s Voigt.
If the anchor system fails, the window’s blast resistance likely will be compromised. “Anchors must be used that can resist specified equivalent static loads, which vary with the vision area,” explains Walker. Consequently, “engineers must calculate bending, shear, stress, bearing, and pull-out loads for the connectors, taking the size and geometry of the particular frame and connector configuration into account.”
Some common product solutions can include thick aluminum, steel angles, expansion or wedge anchors, or subframes. However, in order to select the anchor most appropriate for the job, says Walker, the building’s design team must consider the applied load, the framing material, the thickness of the frame at the anchor, the gap between the frame and the wall, the number and spacing of the anchors, and the wall material and construction type.
With so many variables and the need to run complex calculations, experts strongly recommend that the structural engineers review the blast anchorage design as early as possible in the process. Dawson emphasizes the importance of fully utilizing the full Building Team’s range of expertise to best design and construct a blast-resistant structure. “This means gathering the right professionals with the right experience, and having good, uninhibited two-way communication,” he says.
The glass manufacturers’ technical staffs are a key resource, says Dawson. “They can help identify limitations with how a system is put together and how to get the project built,” he says. “To do this well, they need to have unfettered, direct access to the people making the decisions, so they can directly engage and ask questions of the responsible parties.”
FIRE-RATED GLASS: NEW POSSIBILITIES
Consistent with advances in high-performance glazing and blast-resistant products, newer glass technologies in the fire-rated market have dramatically opened up design possibilities for Building Teams. Most notably, the industry transition from traditional wired glass to ceramic fire-rated glass is now enabling the use of larger expanses of glass in windows and window walls, glass enclosed stairwells, and even fire-rated glass floors.
Jeff Griffiths, director of business development for SaftiFirst, a specialty glass producer in San Francisco, sees fire-rated float glass as having certain advantages over fire-rated ceramics. “Even with noticeable improvements to the appearance of architectural ceramics over the past few years, they still do not offer the clarity, neutral coloration, and versatility of float glass,” he says.
Fire-rated glass gives Building Teams the opportunity to bring daylight deep into the interior in environments where fire-rated materials are required. For example, with the recent design of University of California Davis Medical Center’s 472,000-sf Surgery and Emergency Services Pavilion, Stantec Architecture went with a very large, two-hour-rated glass skylight to create a long corridor of light running through the space.
“We debated the skylight early in the design process as there was additional cost associated with that feature, but fundamentally the atrium really changes the character of the spaces, making it feel more comfortable and bringing much more light filtering into the different departments,” recalls Mike Boyd, executive director of facility services for the UC Davis Health System.
Stantec’s Day concurs that ceramic fire-rated glazing offers exciting aesthetic benefit, but he expresses concern about achieving color compatibility and consistency in appearance between fire-rated glass and non-rated glazing across an entire façade. This was the exact issue before architectural firm AVRP Studios (www.avrpstudios.com), San Diego, in the design of the 32-story Sapphire Towers in San Diego. Not wanting to compromise on the use of glass, but required to make the south-facing elevation fire-rated, the glazing contractor worked closely with the fire-rated glass manufacturer to customize a blue-tinted fire-rated window assembly to match the façades on the other three sides of the high-rise condominium.
Fire-protective vs. fire-resistive. One thorny issue with respect to fire-rated designs is making sure that that the glass specification meets the requirements of the space or occupancy. For instance, if the goal is simply to keep flames on one side of the glass for a certain amount of time, then fire-protective glazing is usually sufficient. However, because all fire-rated glass does have a significant ability to tolerate high temperatures, designers may assume that all fire-rated glass can act as a barrier to heat transfer. Such is not always the case.
“Unlike fire-protective glass, fire-resistive glass can defend against radiant and conductive heat transfer and should be specified, as required by code, for areas such as stairwells, corridors, and elevator shafts where people may be trapped for extended time periods,” cautions Devin Bowman, with Technical Glass Products, Kirkland, Wash. Moreover, designers need to consider the framing as well. “Since the glazing and framing work together to provide adequate fire protection, it’s essential to verify the frames carry a fire rating equivalent to the glazing,” he adds.
Many of these glass materials are made with a clear, intumescent component layered between two or more glass panels. When exposed to fire, the interlayers foam up, thereby blocking heat. The technology is so effective that you can touch the glass while fire is raging on the other side and not get burned. Furthermore, with proven performance, the codes do not restrict fire-resistive glass to limited areas. Rather, up to an entire floor-to-ceiling window wall can be specified to meet fire-rated requirements.
Technical feats like this showcase the extent to which the fire-rated glass industry has developed over the years. “Every type of fire-rated glazing certainly has its place, but the technology that allows glass to provide the same level of protection as the solid structural wall surrounding it really has changed the face of architecture,” says Griffiths.
In the near future, the smart glass industry can be expected to produce even more highly performing products, raising the bar for building safety and energy efficiency. One incentive is the recently approved 2012 International Energy Conservation Code, which will soon require that commercial glazing specifications and glass wall designs achieve a U-factor down to 0.32. That means that Energy Star window products and more sustainable façade concepts will be in greater demand. Not only that, but net-zero energy building initiatives such as the DOE-supported Zero Energy Commercial Building Consortium can expect to gain even more traction.
As the collective design experience from industry and leading Building Teams continues to increase and improve along with advancing glass technology, the possibilities for tomorrow’s buildings—both new and retrofit—are more optimistic than ever.
For more on net-zero energy buildings, see "Zero and Net-Zero Energy Buildings and Homes."
