Balancing act

Solutions to meet architects' aesthetic and performance requirements
Don McCann
February 17, 2009
FABRICATION : COATINGS AND FILMS

The quest for natural light in buildings has become a delicate balancing act for architects and their glazing industry partners. Architects and their clients want clarity, light and transparency. The glass industry must balance these demands with challenges like glare, solar heat gain, fading of furniture and floor coverings, site orientation and tougher energy codes. The glazing industry has  solutions, but it takes creativity, research and the ability to compromise to achieve the best results.

Just ask homeowners living in trendy glass-clad condominiums in New York, Chicago and other major urban areas. They’ll tell stories of wearing sunglasses to read, excessive heat, birds crashing into the glass and lack of privacy. But they also will tell you they wouldn’t trade the view for anything.

Commercial buildings are no different. A crisp, neutral curtain wall is highly desirable and makes for a beautiful aesthetic. However, consider the plight of check-in staff at the Humphrey terminal at the Minneapolis airport. Beautiful natural light flowed into the new terminal, but the glare was so bright that staff had to create makeshift cardboard shades to read their computer screens. The glare problem wasn't solved until a new parking garage was built in front of the window wall several years later.

Fortunately, there are several solutions architects can use to control light without limiting views.

Coatings
Coatings added to glass can be one of the most cost-effective ways to achieve both design and specification objectives. Added to clear glass or used in conjunction with silk screening, tinted substrates or laminated interlayers, high-performance coatings can have a tremendous impact on glazing performance. With more options than before, coatings offer scores of choices for architects and glazing contractors looking to balance light and transparency with solar performance.

Reflective coatings offer superior solar-control characteristics. Glass with a reflective coating has a mirror-like appearance and reduces heat gain through high solar reflection while allowing lower levels of visible light to enter compared to other coating platforms. The higher visible light reflectivity creates a homogeneous look with limited contrast between vision and spandrel glass areas. Reflective coatings are available in silver, copper, gold and earth-tone appearances.

Traditional low-E coatings strike a balance between light transmission and solar energy control. The coatings offer high visible light transmission, low exterior reflectance and excellent U-Values. This makes the coatings ideal for situations requiring  two-way vision or maximum light transmittance. Applied to tinted glass, low-E coatings also can help reduce glare. Some low-E coatings may be anti-reflective with lower exterior reflectance than uncoated glass. In general, the coatings have a more flat appearance and the finished glass product looks natural or unaltered.

Built to recall the once-famous New York Crystal Palace, the new Bank of America skyscraper in New York City was conceived as a crystalline tower of sculptural glass facets. To achieve the shimmer and sheen desired while providing inhabitants with solar protection in the clear glass curtain wall, architects turned to a low-E coating. The combination of a low-iron glass with VE-2M gives the building a clear, crystalline appearance with low reflectivity.

The addition of a silk-screened ceramic frit pattern on the same surface as the low-E coating helped the building meet qualifications for a Leadership in Energy and Environmental Design Platinum rating. At eye level of the floor-to-ceiling glass, the glass has no pattern, providing stunning views of the New York skyline. A graduated silk-screen pattern then extends below eye level to the floor and above eye level to the ceiling, adding another element of solar control.

Hybrid low-E coatings offer architects even more choices by combining the low solar heat gain offered by reflective coatings with the high light transmittance of traditional low-E coatings. The hybrid products, also known as radiant low-E, provide significant performance benefits over traditional coatings.

Neutral low-E coatings represent the next advance in technology, blending low solar heat gain, low reflectance and neutral reflected color. The coating is applied to the second surface of an insulating glass unit, whether the outboard ply is clear or tinted glass, providing superior energy performance, solar comfort and high visible light transmittance.

Ultra low-E coatings, such as Viracon's VUE-50, also offer a clear improvement over traditional low-E coatings. Using tinted substrates with this type of coating can  improve both reflective and UV transmission performance.

Silk screening
One of the most targeted ways to control light transmission is with the use of a silk-screen pattern. Silk screening glass is the process of applying a ceramic-based lead-free paint, known as ceramic frit, to the surface of the glass. The painted glass is then run through a heat-treating furnace at approximately 593.3 degrees Celsius or 1100 degrees Fahrenheit. When the firing is complete, the ceramic paint is essentially part of the glass. The silk-screened pattern can improve solar performance, create energy savings and reduce glare, all while offering a unique opportunity for artistic expression on building façades.

The extent of glass performance enhancement gained by adding a silk-screen pattern depends on a number of variables. The amount of coverage is the most easily recognizable variable. In most cases, more coverage equals better solar performance. A silk-screen pattern of 50 percent coverage in white ceramic frit will block more light, reduce glare to a greater extent and provide greater energy savings than a silk-screen pattern covering 25 percent of the glass. Graduated patterns at the top and bottom of the glass can be used to keep the vision area clear.

The position of the silk-screen pattern within an insulating glass unit also is important. The highest performance is achieved through putting both a low-E and silk-screen pattern on the second surface. When the coating and silk screen are on the same surface, the silk screen is applied first, followed by the coating. The silk screen may also be placed on the third surface; however, this blocks less heat, decreasing the performance of the unit.

The color of ceramic frit can affect solar performance. A medium gray frit, for example, may block more heat than the traditional white frit. Using the medium gray frit would require a lower percentage of coverage than a white frit to achieve the same level of performance.

While standard patterns often consist of dots or lines, ceramic frit can be applied in an infinite number of ways to not only customize solar control but create unique aesthetic appearances. Consider the Minneapolis Central Library. Architect Cesar Pelli designed each of the building’s four facades with images such as birch trees and prairie grass to provide unique aesthetics and solar control appropriate for the façade. A snow pattern on the south façade provides the highest percentage of coverage, while a water pattern on the north façade, where direct sunlight is not a concern, provides the lowest percentage of coverage. The north façade allows more natural light, while the south façade works to minimize solar heat gain with the higher percentage silk-screen coverage.

The unusual detail of the silk screen provides an additional benefit: it prevents birds from mistakenly hitting the glass, an increasingly common problem in urban areas. The library was recently recognized by the New York City Audubon Society for its bird-safe characteristics.

Silk screening can help reduce interior glare that creates challenges for living and working in light-filled environments. When glazing allows 50 percent or more of the visible portion of the light spectrum into a building, the potential for glare increases. Adding a silk-screened pattern can significantly reduce the potential for glare.

Tinted glass substrates
One of the easiest ways to enhance glass performance is through the use of tinted glass substrates. Common colors include variations of green, gray, bronze and blue. While clear glass does little to block solar energy, tinted glass absorbs energy. The level of visible light transmitted depends on both the color and thickness of the glass.

The Glass House development located in the heart of Denver offers residents floor-to-ceiling views of the city and surrounding mountains. To provide residents with some protection against the 300 or more days of bright sun Denver gets every year, architects specified a blue-green glass substrate with a radiant low-E coating.

The two glass and coating combinations used on the project have 45 percent and 37 percent light transmission values, which worked especially well for a residential application. From the inside looking out, residents feel they are looking through clear glass. The blue-green substrate, combined with the radiant low-E coating, however, provides protection against solar heat gain and makes it more difficult to see inside from the outside.

Laminated glass
Adding a laminated glass component to a glazing configuration helps solve two common problems: interior fading and irritating outside noise. The plastic interlayers produced by most major manufacturers block up to 99 percent of incoming ultraviolet rays, between 300 nanometers and 280 nm. These UV rays cause fading and damage interior fabric, paint, furniture and artwork.

Laminated interlayers also improve the acoustical performance of glass and reduce unwanted noise by up to 50 percent without compromising aesthetic appeal or impairing the visual clarity. 

To enhance the solar performance of laminated glass, consider options such as a colored interlayer. Translucent interlayers allow light to flow into a structure but block a great deal of solar heat. The performance of an interlayer can be further improved by adding colored substrates, silk-screen patterns or high-performance coatings.

Adding laminated glass to a glazing configuration also provides benefits beyond UV screening and acoustical control. The laminated interlayer can provide enhanced security, blast protection, as well as doubling as safety glazing.

For example, in Washington, D.C., architects used insulating laminated glass to create the appearance of a building shingled in glass at 1099 New York Ave. Glass spans each of the building’s 11 floors, and the 12-foot tall pieces of glass cantilevers out in two dimensions, which creates a shingled appearance to the building’s exterior.

With the amount of custom metal curtain wall exposed by the cantilevered glass, enhancing the thermal performance of the glass itself was critical. The laminated interior lite, coupled with a low-E coating, significantly improves the thermal performance. The laminated glass adds an extra layer of safety to the floor-to-ceiling glass configuration with no interior handrail. It also provides enhanced acoustical performance, creating a peaceful interior oasis in the midst of the Capitol city’s hustle and bustle.

The perfect mix
As these case studies illustrate, the best solution to today’s glass dilemma is a mix of silk screening, tinted glass substrates, laminated glass and high-performance coatings. Many of the buildings being built in the country today contain at least two or more of these products. Used together, they solve common challenges associated with glass buildings and offer architects additional means of artistic expression. This versatility is part of what makes glass the powerful building tool it has become.

 

Don McCann is manager of the Architectural Design Team at Viracon, Owatonna, Minn., 800/533-0482 ext. 3137, dmccann@viracon.com.