Spring 2004

Where Do We Go From Here?
Looking at the Evolution of 
Energy Efficient Glass Coatings
by Scott Hoover

There are so many coated glass products available that architects and building developers have almost endless options. Low-E, reflective, anti-reflective and scratch-resistant are just a few choices; but in lieu of time and print space, this article will discuss energy-efficient coatings. 

Most everyone agrees that buildings should utilize materials that reduce the amount of energy needed for their operation. Good building practices and legislation continue to direct and mandate the usage of glass that reduces heating, cooling and lighting costs. But how have products developed to address energy efficiency and what products will satisfy the needs and trends of tomorrow?

How It Began
Glass has always been a material of choice for building designers. However, during the energy conscious 1960s and 1970s many designers felt limited in the amount of glass they could utilize due to the inefficient properties of clear glass. Clear glass allowed large amounts of direct solar heat to enter the building, resulting in extremely high cooling costs. In addition, standard installations utilized monolithic glass, which provided little insulation or thermal control, resulting in high heating and cooling costs. The advent of tinted glass provided a product that absorbed direct solar heat, reduced solar heat gain and reduced glare for building occupants. Insulating glass was developed with improved thermal control characteristics by virtue of a hermetically sealed airspace. Glass usage amounts and sizes increased as designers discovered these products. Tinted glass improved with the creation of spectrally selective products, which allowed for enhanced solar control characteristics (a lower solar heat gain coefficient [SHGC]) with high visible light transmittance.

How It Has improved
The initial glass coatings applied for energy efficiency were designed to assist in reducing cooling costs by reflecting direct solar heat from the building. These reflective-coated products often were used in conjunction with tinted glass known for its heat-absorbing characteristics. From these mirror-like products all-glass wall and all-glass building designs emerged (i.e. the heavy reflective box trend of the 1970s and 1980s). During the 1980s, low-E coatings were developed to provide enhanced thermal control. These low-E products were used with clear glass for thermal control or combined with tinted (heat-absorbing) or reflective (heat-absorbing and reflecting) glass in an insulating glass unit (IGU) to achieve improved thermal and solar control. A trend away from dark, reflective facades toward those tinted with low-E had begun. 

The 1990s brought a new breed of low-E glass, termed solar control low-E, that not only possessed thermal control characteristics, but also solar control performance characteristics. These products enabled the trend of transparency, utilizing clear glass with solar control low-E coatings to maximize natural daylight and open up the views of the building both from the interior and the exterior. 

In recent years, however, architects and building owners have found several issues and concerns regarding transparency or maximum visible light products. While those coatings maximized natural daylight, they also yielded significant amounts of glare. For instance, office-building occupants often have to close their window blinds to see computer screens in their workspaces. The high level of transparency also provides little to no occupant privacy. The view of the outside is excellent, but so is the view into the building. Many designers learned that the reality of these glasses was a building that often had many of its shades drawn, at different levels, resulting in an exterior checkerboard effect that was not a design choice or consideration. 

When viewing the building from the outside, the extreme levels of transparency provided by these products significantly reduced the masking characteristics of the old reflective products, allowing a clear view of desks, stacks of paper, furniture, file cabinets, etc., which can detract from the design. There are, however, building areas that are ideal for these products, such as lobbies, atriums and other open spaces. Utilizing large expanses of these high transparency products will often require an applied frit of some pattern to achieve necessary solar control. While these products are often specified and seen today, there is a current trend toward the new, next generation, hybrid coatings.

Where Is It Going?
Significant coating enhancements of the last couple years have been toward solar control, low-E products that provide subtle reflectivity. They have been developed to address the concerns architects and building owners have regarding maximum visible light or transparency coatings. These hybrid coatings still provide high levels of visible light transmittance, but have a light, crisp shine that provides a more uniform exterior appearance, improving occupant privacy and comfort while reducing interior glare. Many of these products possess excellent solar and thermal characteristics in a single coating and are applied to clear glass and various tints giving architects much more design flexibility to utilize color and/or coordinate with other building materials. In the past, many low-E products (within an IGU) required a tinted outboard lite with a low-E inboard lite. These new, single-lite coated hybrid products are often insulated with the coating on the number-two surface of the outboard lite, allowing fabricators to simply (and cost effectively) use a clear inboard lite, reducing the overall project cost. If there is a need to enhance the SHGC or U-factor properties further, a low-E product can be specified on surface number three of the inboard lite. These new products are currently gaining popularity in many 
commercial building types, especially offices, healthcare facilities, schools and museums. Designers are quick to appreciate the qualities of these new hybrid coatings that include:
• High visible light 
   transmittance;
• Excellent solar control;
• Excellent thermal control;
• Crisp, uniform exterior 
   appearance;
• Improved occupant comfort through privacy and glare 
   control;
• Design flexibility of coatings on clear and tinted glass; and 
• Reduced costs by virtue of coatings on a single lite.

We should all be excited about the variety of products within the glass industry. Each of the glass products mentioned has unique characteristics and purposes. In fact, many of these products such, as tint with low-E insulating units, will continue to be specified for many years. 

Most building projects have unique design, appearance and performance criteria, ensuring the use of a wide diversity of products. While this is true, we must recognize and prepare for evolving architectural trends in the marketplace. The glass industry must understand the ever-changing properties of new glass products and technologies and provide accurate and forthright consultation to the architectural community. We must be committed to continually discover and develop products that are energy efficient and fit the correct balance of appearance, performance and cost.

Architect's Guide to Glass & Metal