To save energy and the environment, jurisdictions are rewarding better-insulated buildings with lower total U-values (U-value is the inverse of R-value’s thermal resistance and a measure of how well a building element conducts heat). From Massachusetts, with its “reach” energy code, to California and its Title 24, Building Teams report a difficult balancing act between code compliance and the glass-heavy envelope designs that many building owners desire.
Some argue that glass technology and fenestration systems need to catch up to today’s codes and performance requirements. Others call for more creativity, and possibly a return to punched windows and more opaque walls, to meet these design challenges.
After reading this article, you should be able to:
+ DISCUSS the chief requirements of building energy codes and the impact on glazing variables.
+ DESCRIBE up to three enclosure technology and whole-building design solutions that help buildings meet or exceed energy-use targets.
+ LIST techniques for improving glazing performance that are applied to glass surfaces or fenestration production.
+ COMPARE the qualitative performance of buildings in terms of energy usage or other operations concerns before and after the use of any advanced glazing methods or materials.
Additional reading is required for this course. To earn 1.0 AIA CES HSW learning units, study the article carefully and take the 10-question exam posted at: www.BDCnetwork.com/GlazingSystems.
But new, stricter energy codes are here to stay, and Building Teams need to respond. As a result, many of the features that give glass such strong appeal for buildings can, in the wrong hands, make it a less-than-optimal energy performer.
“The impact of low-performing elements is much greater than many building professionals realize,” says Richard Keleher, AIA, CSI, LEED AP, a noted façade consultant and founding chair of the Building Enclosure Council Boston. As much as 40% of a building’s energy use can be attributed to thermal losses and other effects of glass enclosures, windows, and doors, says the DOE’s Building Technologies Office.
As jurisdictions and standard-setting organizations continue to introduce mandates for increased thermal performance and reduced energy consumption, tougher requirements are being placed on glazing and fenestration. Among the most important new codes and standards: the 2012 implementation of the International Energy Conservation Code (IECC), the 2013 updating of ASHRAE standard 90.1, and California’s Title 24-2013.
All of these rules tend to reduce key figures like the window-to-wall-ratio (WWR)—from 40% maximum allowable fenestration area on the façade, to 30%, in many climate zones. Many also specify default U-values and limits for solar heat-gain coefficient (SHGC), the measure of solar radiation directly transmitted or absorbed by a translucent building system. Then there are movements like LEED certification, the Living Building Challenge, and the AIA 2030 Challenge, as well as growing interest in net-zero-energy buildings.
There are two ways to meet these new requirements: prescriptive- or performance-based approaches. In the prescriptive method, each component of a building needs to meet given minimum energy requirements and related measures. For glass openings, these rules are usually quantified through rated or calculated variables such as U-value and SHGC. In California’s Title 24, the prescriptive minimum for window performance is 0.40 U and 0.35 SHGC.
Verifying conformance should be as easy as checking for product ratings or labels from the National Fenestration Rating Council. The NFRC recently released a new version of its commercial ratings tool, known as CMAST.
Because the specific prescription may require every window to perform at a very high level, in practice this can limit the overall number of windows and doors or glazed area allowed in a climate zone. “For sustainable projects, we don’t recommend using curtain wall on all façades, as this would not take into consideration heating and cooling load conditions that vary based on solar orientation,” says Gita Nandan, RA, LEED AP, Principal of design firm thread collective. “We reconcile this by providing glazing in effective locations based on energy and daylighting analysis.”
The performance-based path looks at the energy performance of the building as a whole rather than as individual components, but it has drawbacks, too. First, it necessitates a detailed account of the proposed building design’s performance, usually carried out through a specialized software program, which adds an additional layer of complexity to the design process. Michael Knopoff, LEED AP, Principal at Montalba Architects, notes, “The newest codes specify use of a particular NFRC simulation program that many custom manufacturers have not yet adopted, which makes acquiring the necessary NFRC label more challenging.”
Furthermore, by taking some of the pressure off the glazing’s performance, other building systems have to offset any inefficiencies of the fenestration. This can be expensive in the short run and leave buildings at risk of being unable to meet the next round of energy code requirements.
The picture is clear: Building with glass now requires more careful strategizing and product choice.
For a mixed-use contextual building in Brooklyn, N.Y., thread collective (designer, developer, and contractor) stipulated high-performance insulated glazing units to achieve the kinds of energy-efficiency levels typical of Passive House construction. Photo: Fran Parente, courtesy Thread Collective. Click photo to enlarge.
STRATEGIES FOR MAXIMIZING EFFICIENCY
In specifying the amount and types of glass in a project, designers must consider such factors as building type, function of interior spaces, room geometries, glazing location, glare control, daylighting, project location, site characteristics, and climate. “In California, the main issue is solar heat control, a totally different problem than dealing with a winter climate,” says Ben Tranel, AIA, LEED AP, a Principal at Gensler.
In colder climates, most decisions are guided by insulating strategies to reduce heat loss; in warmer climates, shading and glare control techniques to reduce heat gain are the key. In practice, designers must balance between insulation and shading in all projects, since siting and orientation can complicate the energy-use equation no matter the climate. “These aren’t new strategies,” says Tranel. “One of the concepts new energy codes have encouraged is a relearning of what we’ve forgotten from vernacular architecture: simple things like orientation and the opportunity for natural ventilation that can make a big difference in building performance.”
According to the DOE Building Technologies Office, these factors are (in priority order): 1) highly insulating windows; 2) building envelope material; 3) air-sealing system technologies; 4) dynamic windows and window films; 5) visible light redirection; and 6) highly insulating roofs.
ARCHITECTURAL GLAZING SOLUTIONS
One way to meet the new energy code requirements is to focus on siting and architectural and materials solutions. Possibilities include using less glass, incorporating shading and solar control elements, varying enclosure materials, and using passive solar concepts like seasonal protections from deciduous trees.
It’s important to recognize the need for balance and diversity of expression in façade design. “Highly efficient buildings can still have a lot of glass,” says Nandan, an expert in Passive House design. “It is just that the glass must be designed appropriately in the right locations for a building to function, while also providing beautiful interiors.”
Choosing punched walls offers a certain kind of aesthetic, and opaque surfaces like precast concrete or brick veneer don’t have to deal with solar heat gain and heat loss as much as a glass façade does. Masonry also has a high heat capacity and provides good thermal mass to help keep interiors cool, according to “Passive Solar Heating,” an entry in the DOE-reviewed Whole Building Design Guide.
The most common form of windows used with punched walls are preglazed manufactured units, which offer consistent quality and easily customizable performance characteristics, says Peter J. Arsenault, FAIA, NCARB, LEED AP. Preglazed units also offer design flexibility. They can be used as a series of single installations or grouped together to form larger window walls. As for cost, Arsenault says preglazed windows deliver “high performance potential associated with pre-manufactured products” at unit costs that are well below those for equivalent applications of curtain walls.
Another option is to employ a double façade or double-skin enclosure. Popular in Europe since being introduced by the Swiss-French architect Le Corbusier as the so-called “neutralizing wall” in 1916, a double façade consists of two skins in which air flows through the intermediate cavity. The air movement helps improve building efficiency through natural ventilation and mitigation of solar gain. This was the strategy employed by Gensler for the recent construction of PNC Financial Services Group’s new corporate headquarters in Pittsburgh.
GLAZING TECHNOLOGY STRATEGIES
Technological solutions remain the most popular way to address the squeeze between energy codes and the desire for large glass areas, while also offering significant improvements in performance. Building enclosure specialist Keleher gives the example of a subject building with 50% fenestration at R-2 and 50% opaque wall at R-14. “If the opaque wall R-value is increased to R-18, the overall R-value only goes up to R-3.6,” he explains. “If the fenestration R-value is increased to R-4 by using triple glazing, the overall R-value goes up to R-6.2.”
In other words, using advanced technology like triple low-emissivity (low-e) glazing can double system performance if all else remains equal—and if the budget allows for its use. Triple low-e glazing with a special gas fill may not be financially feasible in every project. Do other improvements or trends in advanced glazing technology offer similar bonuses for overall R-value? Which have the greatest efficiency benefits? When it comes to choosing among the most effective glazing technologies, what do building professionals need to keep in mind?
At a new 27-story mixed-use and multifamily tower in Boston, The Kensington, a solarium is offered as a common-area amenity with floor-to-ceiling insulated glass and thermal breaks, illustrating that highly efficient buildings can still have a lot of glass. Peter Vanderwarker, courtesy The Architectural Team. Click photo to enlarge.
BETTER FORMS OF GLASS UNITS
Insulated glass units (IGUs) are one of the most common advanced glazing technologies. In the early 20th century, most windows were constructed with a single-pane design and had poor U-values, above 1.0. In the mid-1940s, the first commercially viable double-glazed IGUs were introduced, featuring two individual glass lites connected by a perimeter spacer of steel, aluminum, or plastic and separated by an air gap. This space between the lites brings the U-values down from 1.0 or more to about 0.5. Adding an inert gas fill (usually argon) to the space between lites offers an even greater improvement in efficiency.
Triple glazing can result in U-values of less than 0.4, thus providing a simple means of improving performance. Nandan says that some of these products have U-values in as low as 0.15. A 2013 DOE study found that switching from double- to triple-glazing could reduce energy use in a building by >12%. But triple-glazed units can cost up to 25% more than double-glazed windows. This is partly due to a lack of market demand in the U.S. as compared to Europe. According to Mic Patterson, Vice President of Strategic Development with façade and curtain wall contractor Enclos, “Triple-glazing adoption has lagged in the U.S. marketplace, largely due to lax building code requirements for energy consumption.” This is likely to change in the near future.
IGU technology also offers a range of visual configurations. In a recent Passive House renovation of a historic multifamily building in Brooklyn, N.Y., Nandan’s firm, thread collective, used a custom, triple-glazed window with the appearance of a traditional double-hung system. This configuration allowed the firm to meet both performance goals and the local Landmark Preservation requirements.
Translucent insulation. Manufacturers have also been experimenting with new fill technologies like aerogel insulation—synthetic solids that consist almost entirely of air and exhibit extremely low thermal conductivity. Another new IGU technology comes in the form of vacuum-insulated glass units. By creating a vacuum instead of using a gas or fill, VIGs virtually eliminate convection and conduction, allowing for thinner double-glazed units with U-values that far exceed those of standard triple-glazed units.
MORE BUILT-IN SOLAR? MAYBE, MAYBE NOT
Building-integrated photovoltaic or BIPV systems, which integrate solar power-producing capacity into the building envelope and other structures, have shown much promise. Yet BIPVs have been slow to catch on.
BIPV glazing products emerged on the commercial market nearly two decades ago, but only in the past several years have begun to show signs of true market readiness. Improvements in the thin-film PV products necessary for vision glazing have increased performance to levels near those possible with more traditional solid solar cells, offering the potential for substantial energy savings and allowing more of a project’s façade area to be covered, since transparency or translucence do not have to be sacrificed.
Last summer, global market research firm RnR Market Research concluded in a report that BIPV will become one of the fastest-growing sectors of the energy market over the next six years. The consensus in the design and construction industry is that BIPVs still have significant hurdles to overcome, says Gensler’s Ben Tranel.
As with any PV system, collector orientation and inclination angle are crucial for successful integration and performance, which constrains the façade design or limits PV efficiency. Recent experience shows that proper inclination angle is hard to achieve on a typical nonresidential façade. Moreover, if PVs are integrated into the building’s exterior panels, updating and improving the systems as technology improves becomes prohibitively expensive.
However, “VIG production is a complicated process,” says Stephen Selkowitz, PhD, Group Leader of the Windows and Envelope Materials Group at Lawrence Berkeley National Laboratory. “Making small quantities of vacuum glass in a research and development facility is one thing, but you don’t get economies of scale unless you commission a dedicated facility,” which is not likely to happen soon, he says.
Fritted glazing. Fritting—the application of diffuse patterns to a glazing substrate during production—is having a minor rebirth. Fritted glazing has always been popular for adding visual interest, reducing glare, and cutting solar heat gain, although the SHGC of fritted glazing is affected by the frit color and its location in the window assembly. Usually composed of ceramic or glass batch, the frit is melted or bonded to the substrate.
A novel technique, sometimes called “living glass” (and trademarked as Adaptive Fritting by manufacturer Adaptive Building Initiative) provides an IGU with custom movable graphic patterns: by shifting the fritted glass layers, the IGUs can modulate from transparency to a translucent state to an almost opaque appearance. This allows for exceptional control of transmitted light, solar gain, privacy, and views, but at high cost.
Conventional fritted glass offers broad design flexibility in terms of pattern and color, but many manufacturers also carry off-the-shelf patterns of dots, lines, and holes. Pattern coverage can be specified, often in the 40–60% range of glass area coverage.
Fritted glass was used in such projects as Aurora Place, Sydney, NSW, Australia. Built by Bovis Lend Lease and conceived by Renzo Piano Building Workshop, the curtain wall façade employs a milky-white fritted and laminated glass. The LEED Platinum Bank of America Tower (One Bryant Park) in New York City presents a tight pattern of dots to reduce daylight penetration and provide shading.
The drawback of fritting is that the glass can still absorb solar heat and radiate it, says Agnes Koltay, Director, Koltay Facades. Fritting is an efficient way to reduce glare but may not always be the best technique for managing energy-related properties.
Spectrally selective coatings. Coated glass is a widely available treatment in which the glass manufacturer applies a thin metallic low-e coating to the surface of the lites. There are two common types of low-e coatings used in the building industry, each of which offers advantages and disadvantages. The first, known as pyrolytic low-e, consists of a thin metallic oxide like tin permanently bonded to the glass while in a semi-molten state during manufacturing. The resulting surface layer is hard and durable, which is why it is often referred to as hard-coat low-e.
The second type, sputter coat low-e, is produced separately from the glass manufacturing process and contains multiple layers of various metals, metal oxides, and metal nitrides tailored to meet desired performance and appearance criteria. A softer and less durable form of coating, sputter coat applications are often referred to as soft-coat low-e, even though the durability of many sputtered coatings has been improved. Some glass façade designs position the soft-coat surfaces on a protected side of the glass panels, such as within an IGU, so their performance will remain unaffected.
Coated glass can offer significant benefits from an energy use standpoint, especially when used in conjunction with technologies like triple glazing, says Fiona Cousins, PE, a Principal with Arup. “Triple-glazing with low-e coatings is best for providing insulation, and can help to improve solar heat-gain coefficient,” she says. But improving performance in one area can reduce performance in another, says Koltay: “The better the SHGC performance, the darker the coating, meaning lower visible light transmission,” or VLT.
MAKING GLASS MORE DYNAMIC
Dynamic glazing refers to a fenestration product with the ability to change performance properties, such as SHGC or VLT, generally through the employment of variable tints. These dynamic abilities can significantly reduce lighting, HVAC power consumption, and peak loading, in some cases allowing for downsizing of the mechanical and electrical systems.
There are two main forms of dynamic glazing: electronically activated and non-electronically activated. On the non-electronic side, the most common technology is thermochromic glass, which is based around a laminate made of organic compounds whose tint changes depending on the amount of solar heat absorbed. Thermochromic glass systems can reduce solar heat gain by as much as 30%, according to TCG manufacturers.
The most common form of active system is electrochromic or EC glazing, also known as switchable glass or “smart glass.” Using a thin-film technology consisting of layers of inorganic ceramic and metallic films embedded on the glass surface, the glass panels appear to be clear in the “off” stage and appear tinted when activated with a low-voltage power supply. Recent installations have included either manually operated EC fenestration or façades integrated with the automated building management system.
EC systems can operate on a wide range of visible light transmission–from 2% to 60%. When used in combination with spectrally selective low-e coatings, the resulting glazing can reduce building-wide energy costs by as much as 28% compared to using coatings alone, according to a recent DOE study.
The use of electrochromic systems is growing, although it is not yet widespread. “Currently, these materials are economically viable for niche markets,” says Gregg D. Ander, FAIA, a building energy-efficiency expert with Southern California Edison. However, says Enclos’s Patterson, “The building industry has recently invested in the manufacturing infrastructure necessary to produce materials with the right combination of aesthetic and performance attributes at a tolerable price point.”
Dynamic glazing systems can help Building Teams meet the requirements of new energy codes and even allow for more exterior glass. Sharp Development Company used the technology in a major reconstruction of a former athletic club at 415 Mathilda, Sunnydale, Calif. Led by engineering firm Integral Group, Studio G Architects, and Hillhouse Construction, the Building Team used dynamic glass to increase the office building’s wall-to-window ratio from 5% to nearly 40% while achieving zero net energy and zero net carbon status.
MORE FENESTRATION BREAKTHROUGHS
The key to adopting tools and techniques for glazing systems is to keep it simple, not only in design concept but also in terms of constructability and building operations and maintenance. Perhaps that’s why building-integrated photovoltaics (BIPVs) have seen only occasional use in the U.S.
The brass ring for Building Teams is a dramatic reduction in energy use and carbon footprint. Total energy lost annually through windows has been calculated at about $35 billon. A technical analysis by the DOE’s Pacific Northwest National Laboratory sought to show whether buildings constructed to meet the requirements of the 2015 IECC code would result in energy-efficiency improvements over the 2012 IECC. Yes, savings can accrue. Yet, of the 77 code changes approved, only six were considered “beneficial.”
In fact, some experts contend that the changes to energy codes have been too aggressive and too swift, leaving Building Teams struggling to adopt new best practices. One architecture firm, Seattle’s Build LLC, offers this advice:
- Use the prescriptive methods where possible, as they still allow for plenty of opportunity for innovation and good design with lots of glass (depending on the climate zone).
- Use a large ratio of glass to frame, especially with glazing types where the glass area offers better insulation than the aluminum or other frame construction.
- Use double- and triple-pane glazing strategically.
- Focus on the overall building systems. Locate operable windows for egress and natural ventilation, and keep in mind that operable fenestration produces more window breaks and mullions within the overall composition of total glazed areas—which may negatively affect its total insulation value.