Power of perception

Industry fights incorrect assumption that less glass is more effectual in energy-efficient building design
Katy Devlin
December 7, 2010

Photo by Andy Ryan Photography Inc., New York City, courtesy of W&W Glass, Nanuet, N.Y. 

The 55-story, $2 billion One Bryant Place, or Bank of America Tower (center), in New York City is the first commercial high-rise to earn Platinum certification from the U.S. Green Building Council's Leadership in Energy and Environmental Design program. The crystal-like tower, designed by Cook+Fox Architects, New York, is designed to maximize daylight, while minimizing glare and heat gain. The tower features silk-screened VE 13-2M glass, low-iron Starphire glass with a double silver low-emissivity coating from Viracon, Owatonna, Minn. "The floor-to-ceiling glass features a graduated silk screen pattern on the top and bottom portion of the glass," says Don McCann, architectural design manager, Viracon. "The silk screen starts with a tight dot pattern and essentially fades away as it moves toward the central viewing area of the glass."  

The green building movement presents an incredible opportunity for the glass industry, but only if code bodies recognize the energy-saving potential of glazing. While the architectural community is pushing for more glass in their designs, it won't make a difference if energy regulations limit glass usage in buildings, glass industry officials say. This scenario almost played out in the 2010 version of the American Society of Heating, Refrigeration and Air-Conditioning Engineers 90.1 standard, where the committee voted to greatly reduce the maximum glass area allowed in commercial buildings (See Editor's notes). While the vote was overturned, the near-blow to the industry demonstrates an incorrect assumption by many in the building community that windows are simply "a poor wall," says Tom Culp, owner, Birch Point Consulting LLC, La Crosse, Wis. It's up to the glass industry to educate the overall building and code community that glass-clad buildings featuring holistic daylighting design can offer the best performance solutions.

Looking ahead, energy regulations coming from the International Energy Conservation Code and ASHRAE will have the largest near-term effects on the industry, while activities at organizations like the Zero Energy Commercial Building Consortium will influence energy codes down the road.

"Building codes that encourage more efficient designs on a performance basis should ultimately also encourage better daylighting solutions," says Stephen Selkowitz, head of the building technologies department, Lawrence Berkeley National Laboratory, Berkeley, Calif. "We have believed for a long time that daylighting strategies are essential elements for buildings whose goal is to be highly energy-efficient, low-carbon solutions ... that contribute to desirable, comfortable and healthy environments (see Fig. 1). But some prescriptive approaches to energy efficiency tend to opt for the simplest, foolproof solutions," he says. "For side lighting with windows, these tend to involve reduced glass area, lower transmittance glazings and no assumptions about the use of lighting controls—all of which work against better daylight utilization."

Fig. 1

Sunlight with high performance glazing produces 175 lumens of visible light for each watt of heat gain that must be cooled by mechanical means. Source: Data for electric light efficacies from U.S. Department of Energy's Energy Efficiency and Renewable Energy program, 2007; data for daylighting efficacies from David Kaneda, president, IDeAs, San Jose, Calif., 2008. Diagram courtesy of Viracon, Owatonna, Minn. 
Fig. 2

Higher window-to-wall ratios, with daylighting controls and sunshades, require less energy than lower WWR without daylighting controls, according to a modeling study from the Lawrence Berkley National Laboratory, Berkeley, Calif. Source: LBNL, courtesy of Viracon, Owatonna, Minn. 

The 2010 version of ASHRAE 90.1 is the most notable recent example of the power of energy codes to dictate glazing design—and in a potentially negative way. ASHRAE's Standing Project Committee 90.1 had voted to reduce the amount of glass permissible in the envelope of commercial buildings using the prescriptive path by a full 25 percent—from a maximum window-to-wall ratio of 40 percent to a maximum of 30 percent. The committee also voted for more restrictive U-factors and solar heat gain coefficient values, and the addition of a new minimum for VT/SHGC (of 1.1) in ASHRAE 90.1's prescriptive path even when there are no associated lighting controls. All of these issues, including the lower WWR, were reversed by the ASHRAE appeals panel, on grounds that the Project Committee did not have adequate justification for the requirements. The reversal marks an important acknowledgement of the ability of glazing to provide energy savings, Culp says. "The ASHRAE appeal was huge. It counters the attitude that windows are a bad thing," he says (see Fig. 2).

The glass industry members involved in ASHRAE teamed up with the lighting subcommittee to argue the case for daylighting in terms of energy efficiency, says Helen Sanders, vice president, technical business development at Sage Electrochromics Inc., Faribault, Minn. "We solicited help from the lighting subcommittee to help support the position that it is the presence of sufficient glass in the right location, combined with lighting controls, that will make the most impact on energy," she says.

The use of automatic lighting controls provides more energy savings for all window-to-wall ratio scenarios. Source: Information from a modeling study by S. Treado, R. Mistrick et. al. at Penn State University, sponsored by: AGC Flat Glass North America Inc. , Alpharetta, Ga.; Guardian Industries Corp., Auburn Hills, Mich.; Lutron Electronics, Coopersburg, Pa.; Pilkington North America, Toledo, Ohio; PPG, Pittsburgh; Sage Electrochromics Inc., Fairbault, Minn.; WattStopper, Santa Clara, Calif. Diagram courtesy of Sage Electrochromics.

In late 2009, to evaluate the proposed changes to ASHRAE 90.1, several industry companies commissioned a modeling study to measure efficiency improvements with WWR reductions, with improved VT/SHGC ratios, and automatic lighting controls. The study found that automatic lighting controls had a much greater impact on energy efficiency than WWR, offering 7 percent to 25 percent savings over buildings with no lighting controls, Sanders says (see Fig. 3) "When you look at the data ... the use of dimmable lighting controls saves a lot more energy than making incremental energy improvements in the fenestration itself," she says.

"The energy savings simply from [using lighting controls] dwarf the small changes in VT or window-to-wall ratio," Culp adds. "There will always be groups arguing for less glazing, but this ASHRAE reversal will relieve some pressure."

The activity at ASHRAE indicates the need for developing a cross-functional code structure that takes into account the relationship of energy savings across trades. "For example, how much reduction in air-conditioning tonnage can be achieved if sunshades are applied to the south facing elevation?" asks Jot Chahal, product manager, curtain walls, sun control and BIPV, Kawneer North America, Norcross, Ga. "Allowing natural light to enter the building space is a key component in creating an occupant-friendly environment. Decreasing the glazed area will limit the ability of the architects to use this key natural ingredient. [The industry] will need to continue to drive daylighting and transparency, with product technology and innovation stepping in to ensure the energy footprint for buildings continues to shrink," Chahal says.

The industry also needs to be aware of the activities of the IECC to increase stringency. "At IECC, the New Building Institute, American Institute of Architects and the Department of Energy have worked together on joint proposals to advance energy codes, but they are doing it with broader consensus. They're increasing energy efficiency, but in a little more realistic way," Culp says.

What do the changes at ASHRAE and IECC mean for the industry? This year, IECC is coming out with its 2012 version, and ASHRAE its 2010 version, now without the limited window-to-wall ratio, and the industry needs to be prepared for more prompt adoption of both. "A lot of money from the stimulus went to states, but only if they commit to adopting the most recent energy codes and put a plan in place for enforcement," Culp says. "In the past, some states would adopt the new codes right away, while some states would wait 10 years. Now that's going to change. States will be adopting the new codes right away, with enforcement. This is going to mean more thermal breaks, low-E everywhere, increased use of warm-edge spacers and gas fill. In the North, especially where buildings are following green codes, we're going to see more triple glazing—something we've seen in residential is now being specified in commercial glazing."

Looking further into the future, particularly as the zero energy building movement progresses, the industry should be prepared for the possibility of more performance-based codes in lieu of prescriptive codes. "In the Zero Energy Commercial Building Consortium, we are discussing whether prescriptive codes are relevant anymore," Sanders says. "Can you have prescriptive codes and have zero energy buildings? We're beginning to see that the one-size-fits all approach doesn't work when you're developing a zero energy building. A holistic systems-based approach is needed to maximize energy efficiency, and it's difficult to see how prescriptive codes can support that without mandating specific designs [or] limiting design freedom."

The best thing the industry can do to make sure glass and glazing—and daylighting—remain a critical aspect of energy-efficient design is to be engaged, says Chris Dolan, director of commercial glass markets, Guardian Industries, Auburn Hills, Mich. "As major stakeholders, the glazing industry needs to be part of the process," he says.

Selkowitz adds, "Beyond that, the industry should work toward demonstrating how and when (and with what other requirements) a glazed façade will actually deliver better measured performance. Restrictions on glass area are partly a response to the perception and reality that highly glazed facades without appropriate solar and glare control are not efficient designs. Glass companies and curtain-wall suppliers may want to actively but constructively challenge architects to rethink designs that they know will create later problems for occupants after occupancy."

A "glaring" problem

One problem area of particular concern is glare. In the name of daylighting, architects want glass with high visible light transmission, and they want large expanses of it. However, in order for daylighting to provide improved energy savings, occupant comfort and aesthetics—the triumvirate of good design—architects need to do more than specify high VLT glass.

"In some cases with high VLT, you might have someone sitting in an office who finds there's too much light and uses blinds," says Don McCann, architectural design manager for Viracon Inc., Owatonna, Minn. In these situations, the benefits of the daylighting are lost, because the blinds are down, and the heat gain in the building is high because the heat is still entering the building. The blinds also take away from the clean, all-glass aesthetic, he says.

"In the simplest form, glare is excessive brightness," Selkowitz says. "It can come from direct view of the sun or sky or surrounding surfaces. Glare is context-dependent and dynamic. Glare sources can be so bright that they create physical discomfort in the eye and brain, but at much lower levels can reduce productivity by making it more difficult to effectively view a task."

An accurate measurement of glare is difficult to identify and model, as glare depends heavily on surroundings. "If there are large buildings around, and you've been looking to optimize light transmission, you now have buildings blocking the view," McCann says. Highly reflective adjacent buildings can also affect glare, as can interior design elements, such as wall colors and material types. Other contributing factors might include the appearance of the sky, layout of furniture, and even small factors like type of computer screen. While the building community had developed precise definitions and measurements for the variables of daylight, solar heat gain and U-value, glare continues to lag, due to these challenges.

"The building community has not spent enough time addressing glare in designs, and some high profile problems have raised the issue over and over again," Selkowitz says. "At the same time, the tools for glare evaluation are not as well developed as for more traditional functions like calculating light levels. ... The biggest challenges are that glare is a context-dependent problem. It's not solely a function of the properties of glass, and it's a highly variable, time-dependent solution. There may be no glare on a dark overcast day, but significant problems from a hazy sunlit sky. ... Glare also involves human reactions and behavior."

Some glare modeling is available. Design tools, such as COMFEN from Lawrence Berkley National Laboratory, allow users to evaluate the performance of various fenestra- tion systems, including in regards to glare, for project-specific commercial building applications. LBNL has also started to develop glare indices.

COMFEN measures glare based on foot candles. "Windows allow a range of 30 to 70 foot candles, with the optimal being 50," McCann says. The lower numbers are too dark, requiring artificial lighting, and the higher numbers create too much glare for occupants.

Viracon, along with LBNL and a group at the University of Minnesota, developed a design tool—an offshoot of COMFEN—for its Web site that allows users to compare performance of various Viracon products for their project. "Our tool gives architects a cursory glance at the impact of window-to-wall ratio, with various products, on energy transfer," McCann says.

With improvements in understanding of glare, and the growing ability to evaluate glare in building models, designers can begin to develop effective daylighting solutions that account for the four critical factors of glazing design: VLT, SHGC, U-factor and glare. By avoiding glare, designers will begin to see the intended energy benefits of the daylit environment: a reduction in artificial lighting and potential reductions in heating and cooling costs, even with higher WWR.

Better understanding and modeling of glare also allow the industry to counter notions that less glass is better, including the idea that glare only occurs when there is too much glass or too high VLT. In fact, glare can occur in interiors with small windows and very low VLT glass. "Even when you look at offices from the 1970s, with highly reflective glass and perhaps 8 percent VLT, you still end up with blinds on those buildings," Sanders says. "It's all a matter of degree. You need to get down to below 3 to 4 percent VT to really control direct sunlight glare."

Additionally, low window-to-wall ratios aren't the answer, she says. Darker spaces with low WWR can actually produce more glare than lighter spaces with larger WWR because of the illumination contrast, Sanders says, citing a recent study commissioned by the California Energy Commission and SoCal Edison, based on information from California's Heschong Mahone Group. Buildings with small punched-opening windows can create glare from excessive contrast between the bright window and adjacent dark wall. Buildings with high VT glass and large curtain-wall expanses are susceptible to glare in the form of excessive brightness, resulting in the need for interior shades or blinds, and then artificial lighting.

A holistic approach

Holistic design offers the best strategy to effectively control glare and provide daylighting solutions to meet ever-tightening energy codes, says Keith Boswell, technical architecture director of Skidmore Owings & Merrill LLP's San Francisco office. As documented by the ASHRAE 90.1 modeling study, the glass industry alone can't provide the most effective daylighting solutions—involvement of other players on the job, from lighting to HVAC, is critical.

"With more requirements for performance-oriented codes going forward, there will be a greater need to share information and to collaborate earlier in the design process," Boswell says. "It's not going to be a linear collaboration. All of the players need to be in the room together, looking at all aspects of the design, as they are all integral. You can't look at one [aspect] in isolation; you have to look at the building holistically."

Lighting consultants, mechanical engineers and interior designers can play major roles in daylighting design, and the earlier glass companies can get involved in projects, the better. Unfortunately, that might be easier said than done, McCann says, as often glass companies aren't brought to the table until the later phases of design. But, meeting regularly with architects and educating them about product developments and potential glazing solutions can help.

Dolan agrees. "At Guardian, we work very closely with architects to ensure we understand their needs, and to educate them about the latest products and their specifications. We offer technical support to firms and have set up a broad architectural program," he says. "We work to make sure architects receive accurate information. One thing you don't want to have is people making decisions on incomplete or inaccurate information. We have a number of AIA accredited courses and provide 'lunch and learns' in the architects' offices to help further educate and engage them on the characteristics of glass and LEED [Leadership in Energy and Environmental Design]."

The best solutions for daylighting performance "start with a careful assessment of occupant needs for the space and building type ... and provide appropriate glazing area and transmittance (properly located in the façade to reduce contrast across the interior spaces), to introduce enough light on average days while controlling glare and sunlight on sunny days, and finally have dimmable lighting to smoothly add or balance the overall light levels as conditions change outdoors or needs change indoors," Selkowitz says.

Some potential solutions for effective daylighting design include: light shelves to push light back into the building; fritted or patterned glass, or exterior sun shades to block certain angles of the sun; automatic lighting controls; switchable glazing products, such as electrochromic or thermochromic glasses; clerestory windows and skylights to allow for increased daylight without glare concerns; open floor plans to capitalize on daylight; and designs based on building orientation, maximizing daylighting on exposures with more indirect sunlight.

However, no matter what solutions the glass industry develops for better daylighting design, owners and architects need to commit to making the investments in time and money. "We have made better light transmittance products, better solar-performing products, but it comes down to how the architect is designing the building," McCann says. "There are things you can do, step the windows back into the building, adding sunshades, light shelves, lighting controls. We're seeing more of this. But, you have to have a committed owner or architect who wants to invest in energy efficiency, because it is going to cost more."

The investment in additional daylighting controls—whether automatic lighting controls or exterior and internal shades and shelves—goes beyond upfront costs. Owners and architects also have to consider long-term maintenance costs. "Some building elements can seem counterintuitive, cost-wise. If you put in huge amounts of vision glass, and then to counteract heat gain and glare put in exterior screen shades, you have to balance the energy savings with the upfront hard dollar construction costs, the lifecycle costing and the building maintenance," Boswell says.

Photo by the Los Angeles Police Department. 

The Los Angeles Police Department headquarters, gold certified from the U.S. Green Building Council’s Leadership in Energy and Environmental Design program, features glass silk-screened with a custom dot pattern that varies by elevation, depending on the solar and optical needs. The glass is VRE 1-67, a radiant low-emissivity product, and VE1-2M, a Solarscreen low-E, from Viracon, Owatonna, Minn. Silk screen patterns and other decorative glass enhancements can serve as effective daylighting controls, contributing to a building’s overall energy efficiency.

Looking ahead

The future of the building industry lies in sustainable and energy-efficient design. If the architectural community leads sustainable design trends, high VT and large expanses of glass will continue to play a major role. However,

the codes—as evidenced by the recent ASHRAE 90.1 proposals—could push it in the opposite direction if the glass

industry does not continue to promote effective daylighting design, the energy saving potential of glass and glazing, and the advanced glass technology it offers.

"We need to listen to what the architects want, follow what the codes are pushing and find a happy medium. Above all, we need to continue to push the envelope with our product development," McCann says. 

Katy Devlin is editor for Glass Magazine. E-mail Katy at kdevlin@glass.org.

  • Sidebars and resources


    Product development
    Effective daylighting solutions


    Keith Boswell, technical architecture director, Skidmore Owings & Merrill LLP, San Francisco, 415/981-1555

    Jot Chahal, product manager, curtain walls, sun control and BIPV, Kawneer North America, Norcross, Ga., 770/449-5555

    Tom Culp, owner, Birch Point Consulting LLC, La Crosse, Wis., 608/788-8415

    Chris Dolan, director of commercial glass markets, Guardian Industries, Auburn Hills, Mich., 248/340-1800

    Mike Krasula, senior commercial marketing manager, NSG Pilkington, Toledo, Ohio, 800/221-0444

    Don McCann, architectural design manager, Viracon Inc., Owatonna, Minn., 800/533-2080

    Helen Sanders, vice president, technical business development, Sage Electrochromics Inc., Faribault, Minn., 507/331-4848

    Stephen Selkowitz, head of the building technologies department, Lawrence Berkeley National Laboratory, Berkeley, Calif., 510/486-4000