Glass and Glazing

Windows, curtain wall systems, skylights, and other daylighting openings usually incorporate glass to allow the passage of light while providing protection against the elements. Of course, many different types of glass are available, offering a broad range of characteristics. The glass can be clear or tinted, of various thicknesses, and be produced with a wide range of coatings in order to manage energy performance and facilitate many different aesthetic objectives. A quick summary of some of the fundamental terms and common choices in glass options that are suitable for daylighting follows:

• Float Glass:The term “float” refers to the manufacturing process in which molten glass is floated atop a pool of liquid tin in order to establish its surface flatness. Float glass is available as clear, low-iron (in which the trace green tint of clear glass is reduced) or a range of tint colors including a fairly new series of lighter colors (light gray and light blue). Different thicknesses of float glass are available associated with the structural capacity and deflection-control requirements of a broad range of glazing applications.
• Annealed Glass:All float glass is initially produced as annealed, meaning that the glass is cooled to room temperature with a minimal final level of residual stress. Annealed glass can be readily cut during the fabrication process.
• Heat-Strengthened Glass: Heat-strengthened glass is produced by a heat-treatment process within which the temperature of the glass is gradually elevated to around 1,300 degrees Fahrenheit, and then the surfaces of the glass are rapidly cooled in order to develop permanent compressive stresses at the glass surfaces. When heat-treated glass is necessary to resist the thermal loads on a project (and tempered glass is not otherwise necessary), heat-strengthened glass is often the optimal solution. Heat-strengthened glass is approximately twice as strong as annealed glass.
• Tempered Glass: Tempered glass is heat-treated in the same manner as heat-strengthened glass, except that the quenching process is intensified in order to develop higher residual compressive stresses. Tempered glass will break into small, dice-like pieces with relatively dull edges, and tempered glass qualifies as safety glazing. While tempered glass is approximately four times as strong as annealed glass, heat-strengthened glass is less likely to escape from its frame in the event of breakage.
• Laminated Glass:Laminated glass consists of two or more plies of glass bonded with an interlayer material, most commonly polyvinyl butyral (PVB). Because the interlayer serves to retain shards in the event of breakage of the glass, laminated glass can constitute safety glazing. It can also provide significant acoustic performance and forced-entry resistance benefits.
• Insulated Glass Units (IGU):Insulating glass units, consisting of two or more panes of glass separated by a sealed gaseous space, are widely necessary in order to meet energy codes. A low-E coating is commonly implemented on the #2 surface of the unit (the inner surface of the outermost lite of glass) in order to support energy performance.

The optimal glazing selection will depend upon matching the particular project requirements for performance and aesthetics with the combination of features provided by each glazing type. That includes understanding any effects of different glass types on the final color and appearance of the glass. It also includes determining the need for a clear line of sight, as in vision glass, or if diffuse light and privacy are needed, as in translucent glass. If a project design team prefers an all-glass exterior aesthetic, spandrel (fully opacified) glazing may be strategically placed on the building skin in order to conceal the structural and mechanical features of the building. There are numerous other options available to design teams that affect aesthetics and performance, including a range of tint colors, a palette of coatings, acid-etched glass, patterned glass, frit of many colors and in many configurations, colored interlayers, and digitally printed glass and interlayers.

For daylighting, glazing should be selected recognizing that a strategic level of visible light transmittance (VLT) is required while still needing to control solar heat gain. (See sidebar for performance term definitions.) IGUs are typically required by energy codes to improve U-Factor or R-value performance of exterior walls. All of these will impact the quality and color of the natural daylight, so they should be reviewed and selected with care in each daylighting situation.

Photo courtesy of Guardian; Photo credit: Ara Howrani

A mix of vision glass and dark spandrel glass is used on the Guardian Science & Technology Center addition and takes advantage of the multiple visual and performance attributes of available glass products.

Trends in Glass and Glazing

Architects have been using glass in innovative ways for decades, drawing upon an expansive array of glazing performance and design options. Computerized energy modeling and daylight simulations have facilitated the refinement of building designs for appearance, performance, and benefit to occupants. The outcomes have been increasingly advanced and efficient solutions.

In recent years, glass manufacturers have been asked to respond to the needs of owners and architects to provide new and higher-performing glass and glazing products to suit a range of design trends. For example, there has been a growing interest in large-sized IGUs to facilitate unobstructed vision. That can be done, but architects and owners are of course still seeking good glass flatness and optical clarity, so it is important that manufacturers have the capability to achieve all of those criteria. Larger glass sizes also mean that thickened glass may be necessary in order to maintain appropriate deflection control. As a result, the fabrication, handling, and installation of these glazing units may necessitate special provisions to accommodate the increased sizes and weights of the units. Special requirements should be carefully coordinated early in the design process.

There has also been a sustained trend in northern climates to control heat loss better in glazing. In commercial buildings, argon fill and triple glazing are among the features that are being increasingly specified to support intensified insulating performance. In addition, interior surface coatings are being used to reflect heat back inside a building, lowering the u-factor and improving the performance of IGUs.

For climates where cooling is the dominant control mode, intensified screening of solar energy is often a key objective while still achieving good daylight. The light-to-solar gain ratio (LSG) is a useful metric in this case and is calculated as the VLT divided by the SHGC. High LSG values mean strong levels of visible light transmission occur simultaneously with significant solar heat shielding. In some cases, low-E coatings may incorporate double or triple layers of silver in order to attain powerful LSG ratios.

Finally, the use of bent glass has been increasing to help to deliver architectural objectives. Testing has demonstrated that, in fact, there is little or no effect on VLT with bent glass, although it can certainly create some pleasant and varied lighting effects depending on the conditions. Robust energy performance remains available through the use of low-E coatings that have been proven to fully accommodate the glass-bending process. As with architecture involving large units, coordination on the prospective use of bent glazing units should begin early in the design process.