Volume 37, Issue 11, November 2002

Work in Progress

High Performance Glass--Continuously Improving the Energy Efficiency of Windows
by Frank Davey

Architects continually look for new types of reflective and low-E coatings to achieve a certain look as well as the best possible energy efficiency.

In the last ten years, the residential and commercial window markets have undergone a major technological revolution. Windows are no longer considered the weak link in energy-efficient building design. New and improved coating technologies help architects continue to expand glass applications without sacrificing energy efficiency. Some innovations include: new reflective and low-E coatings, low-conductance gas fills, warm-edge spacers, thermally improved sashes and frames, solar control glazings and coatings and improved weatherstripping products. 

Current Technology

For commercial buildings, the most important consideration in choosing the proper glazing is solar heat gain control. However, heat loss must also be evaluated, particularly in cold climates. Utilizing passive solar heat gain efficiently to reduce heating costs is an important consideration. This “free” heat can be used in either commercial or residential markets.

According to a recent study performed by Ducker Research Co. (May 2001--Study of the 2000 North American Fabricated Glass Market), the market for coated products has grown significantly over the past five years. This is due to the strong demand in construction markets and the increased demand for high-performance glazing, particularly low-E glass. Reflective glass shipments have also grown, due primarily to the increase in overall glazing demand. However, forecasts predict declining shipments over the next few years, based on a softening market (which we are now experiencing), and a decline in market penetration as multi-functional low-E glasses increase their share. We have also seen a dramatic increase in the use of silk-screened ceramic frit, combined with low-E coatings and the increased use of laminated glass. The trend in glass selection is changing quickly from approximately 70 percent reflective and 30 percent low-E, to 70 percent low-E and 30 percent reflective. This is due to the efforts on the part of architects to achieve more natural light in their building designs with less “mirror-like” appearance.  

Customers in different areas of the world are ordering glass systems featuring different color combinations, reflectance levels, solar heat gain coefficients and other performance characteristics. 

Thanks to new coating technologies and recently developed manufacturing processes, flat glass manufacturers are well-equipped to meet specialized demands. To satisfy customers’ energy-control requirements, a comprehensive line of low-E glass has been developed, tailored to different geographic regions and seasonal heating/cooling requirements. For example, AFGD Glass’ Comfort ES 72N is a solar control type low- E coating, with an excellent U-value and good solar heat gain control. Other types of low-E coatings may be designed for cold climates, still with excellent U-value, but will allow more heat from the sun to enter the house or building for more effective passive solar heating.

Performance Standards

Performance standards change from year to year and from region to region.  Various states and provinces have passed legislation geared to reducing energy consumption and controlling costs for both residential and commercial markets.  Requirements are driven mostly by local building codes and regulations. Texas recently adopted the energy efficient chapter of the International Residential Code (IRC), as it existed on May 1, 2001, for single-family residential construction. It adopted the International Energy Conservation Code (IECC) for all other residential, commercial and industrial construction. According to Senate Bill 5 (SB5), all counties in the State of Texas are included except for 16 panhandle counties that do not meet the 3,500 Heating Degree Days requirement. Under SB5, windows must have a solar heat gain coefficient of 0.40 or lower, or a shading coefficient of 0.48 or lower.  

Internationally, the city of Shanghai limits the use of glass to 40 percent of the exterior building facade for improved energy efficiency. The People’s Republic of China has mandated the use of tempered glass in its buildings. Currently in Singapore, the exterior reflectance of the glass must meet the maximum 10 percent.  Heat soaking of tempered glass is now mandatory in Hong Kong and in some parts of China and is being specified more and more--not only overseas, but also in North America. 

A building’s annual energy requirements, the impact of HVAC and interior lighting also have a bearing on performance requirements.

Architects worldwide have increased their focus on controlling energy costs for both heating and air conditioning. With the continued growth of low-E coatings in building envelopes, architects continue to look for new combinations of reflective and low-E coatings to comply with energy cost goals for specific individual buildings. In hot climates, such as Australia, the primary concern is minimizing solar heat gain. However, in Canada, and much of the United States, solar heat is an important source of “free energy” during the cold winter months, particularly for residential housing. Canada’s climate has dictated an increasing popularity of high light and solar energy transmitting glass, as well as growth of argon filling and triple glazing to minimize the use of both heating and cooling, and maximize year-round energy efficiencies.

Custom Specifications

Whatever the reasons--aesthetics, energy control or environmental factors, architects around the world are no longer satisfied with a “one product fits all” approach when they specify glass products. Today, more than ever, manufacturing facilities are receiving a wide variety of custom specifications.

As with any other product, high-performance glass is generally called for using one of three specifying methods: 

1. Descriptive. Detailed description of the physical properties, quality, and workmanship required for the proper installation of a product of material. (i.e., a solar-control type low-E coating that combines very high light transmittance with sufficient solar heat gain control, while maintaining a neutral appearance similar to uncoated glass).
   

2. Reference Standard. Uses nationally recognized product and material standards rather than individually written product or installation criteria.
   

3. Proprietary. Describes product and materials by manufacturer’s name, brand, product code, or other unique characteristic (i.e., AFGD Glass Comfort ES 72 N).

Customers today demand a high-quality product. To ensure that glass products meet certain minimum quality and performance standards, there are several certification agencies in place: IGMA and IGCC for sealed units and SGCC for tempered glass products. In addition, there are a number of national standards for most types of glass products, such as CAN/CGSB in Canada and ASTM in the United States.  Associations such as IGMA, SIGMA, GANA and AAMA, also publish very useful information pertaining to glass products and their uses.

High-performance glass also requires unique fabrication capabilities and quality control. Once on site, extreme care must be exercised when glazing and cleaning the glass to ensure that it is not damaged. Reflective coatings that are not insulated, laminated or otherwise protected can be scratched. Once glazed, consideration should be given to protecting the coated surface until construction and all sub-trade work is completed. 

The technology available today meets the unique specifications of architects around the world: combining low-E glass with a full offering of high performance/medium performance coatings, different color options, insulated glass configurations, and special processes such as tempering, heat strengthening and laminating, to name a few. Clearly, there has been great progress in improving window energy efficiency in the last decade, but even greater evolution lies ahead. Many more new innovations, such as self-cleaning glass and “smart windows,” which can be programmed to modulate the flow of light or solar heat across the window automatically, are being developed. High-performance glass products will continue to be the right choice for architects and designers worldwide.

Frank Davey is executive vice president for AFGD Glass Canada.

USG