Specifying the right high-performance glass

Recent developments in glass technology deliver energy efficiency, daylighting and visual appeal.
Alissa Schmidt,  Viracon architectural design manager
June 12, 2013
By Alissa Schmidt, Viracon architectural design manager
Architects and glass professionals have long understood the benefits of incorporating high-performance glass into building design to maximize daylight, comfort, energy efficiency and esthetics. With the right glass design and glass fabricator, these building advantages can be achieved.

ritz-carlton-  
The triple silver low-E coating on the Ritz-Carlton Toronto achieves one of the lowest ratios of solar gain to light transmittance possible today. New coating technologies make it possible for glaziers to selectively tailor the light spectra that enter the building.  Photo credit: Shai Gil Photography

 

High-performance glass includes heat-absorbing, reflecting and spectrally selective properties, which can contribute to significant energy savings if the glass is selected carefully and in accordance with a whole building design concept. Glass elements should be selected based on many factors, including the building’s climate zone, usage and orientation. If optimized based on these features, the glass selection will result in a remarkable difference in the performance and effectiveness of the completed building – a result of quality assurance and effective integration of materials.

Glass substrates
There are many more glass colours than just clear glass available on the market today, with the most popular being shades of green, grey and blue. Tinted glass substrates are used to lower the SHGC of a glass product – effectively reducing the amount of solar heat entering the building.

Low-iron glass is another popular glass substrate due to its clearer, less green appearance than traditional clear glass. When opting for low-iron glass, it is important to note that low-iron glass is not a performance-improving product, but rather can provide an appearance enhancement where reducing a green hue is desired. Low-iron glass was used to provide the architect’s desired appearance for Telus Tower at 25 York St. in Toronto. 

Low-E coatings
Coatings are thin layers of metal applied to glass to improve solar performance. The first architectural glass coatings were reflective coatings with a mirror-like appearance that reduced solar heat gain by reflecting the sun’s energy away from the building.

Due to changing esthetic trends along with building codes requiring high-performance glazing to meet more strict energy requirements, the most popular coatings applied to glass have become low-E, or low-emissivity coatings. Coatings with low-emissivity properties are spectrally selective, have low heat-transfer properties and offer higher light transmission than traditional reflective coatings.

With the profusion of low-E coating options available, selecting a specific coating may seem overwhelming.  It will help to understand that each coating has unique appearance and performance characteristics.  When selecting glass it is best to start by with a general understanding of the project requirements, either appearance or performance, or both. This will quickly narrow the low-E coating options to a more manageable number.

Telus Tower, referenced earlier, used a radiant low-E coating on low-iron glass. The appearance of this coating and substrate combination is silver – blue with dynamic reflectivity. This reflectivity, combined with higher light transmittance than older generations of coatings, allows the building to look different as weather and lighting conditions change from day to day even though the glass isn’t changing. The 38 per cent VLT and 0.24 SHGC of this glass provides an excellent balance between heat and light.  

Ritz-Carlton Toronto used glass with a triple silver low-E coating. As the name implies, this coating has three layers of silver, which provides an LSG of 2.14, one of the highest achievable with glass products available today. Triple silver low-E coatings have a slight green hue combined with high light transmittance and low reflectivity. 

Silk-screened glass
The process of silk-screen printing on architectural glass has been around for more than two decades, and is implemented for two primary reasons: to increase the solar performance and to create the specific visual effect desired by an architect.  The paint used is ceramic-based and must be fired to the glass. The glass is then run through a heat-treating furnace and the paint, also known as ceramic frit, essentially becomes part of the glass.

Silk-screened glass fabricators typically offer standard line, dot or hole patterns, in a variety of standard colours including white, black, and multiple shades of grey. Most also have the capabilities to accept custom patterns and develop custom colours.

Adding silk-screen patterns improves solar performance, again leading to a potential savings in energy costs compared to a similar glass type without a silk-screen. For example, a one-inch insulating glass unit with two plies of clear glass has an SHGC of 0.70. Adding a 40 per cent-coverage white silk-screen pattern drops the SHGC to 0.54. 

Combining silk-screen patterns with low-E coatings allows a designer to tailor the performance and appearance to the specific needs of their project. Silk-screen patterns are applied to the glass first and then the low-E coating is applied. This combination allows both the silk-screen pattern and low-E coating to aid in blocking heat from entering the building.

Silk-screened glass is also a potential solution to reduce bird collisions with glass. While ongoing testing is necessary to understand the effect of various glass products, including specific silk-screen patterns and coatings – such as those with low, medium and high levels of reflectivity – it is evident based on the testing done thus far that silk-screen patterns can be an effective way to reduce bird collisions with glass in building design.

Laminated glass
Laminated glass features an interlayer bonded between two or more glass plies using heat and pressure. It is typically used in overhead glazing and areas where codes dictate safety glazing. Laminated glass also provides a durable, high-performance glazing product, designed to provide protection against threats such as hurricanes, bomb blasts or forcible entry. 

Laminated glass improves specific performance characteristics. Ultraviolet light is the portion of the spectrum that causes interior damage such as fading fabric and deteriorating plastic. The interlayers customarily used in laminated glass will help reduce the amount of UV light allowed to pass through the glass to less than one per cent. While one per cent can still cause some damage, it is important to understand that even the highest-performing insulating low-E coated glass products have five to 10 per cent UV transmission.

Another performance characteristic improved by laminated glass is acoustics. A good way to improve acoustic performance is by selecting an insulating laminated make-up. A standard one-inch insulating unit with two plies of quarter-inch monolithic glass has a sound transmission class rating of 35. If an insulating laminated unit is created by substituting the interior quarter-inch monolithic ply with quarter-inch laminated glass, the STC increases to 39. Increasing the thickness of the laminate will further improve the STC rating. 

The future
Glass manufacturers and fabricators are continually evolving to further advance glass and glazing technologies. New products and techniques will continue to bring new alternatives to market that will meet an extensive variety of performance standards, further solidifying the importance of glass in high-performing, functional and visually appealing buildings.

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