Glazing Physical Properties

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Substrate

The selection of substrates determines many of the base visual and thermal properties of an Insulated Glazing Unit (IGU). Low-iron substrates provide the highest light transmittance with least spectral distortion. These substrates are often called “water white” due their reduced green tint when compared to clear float glass. Trade names for low-iron glass include Starphire®, Optiwhite™ and UltraWhite™. Tinted substrates tend to absorb light differently across the visible spectrum, imparting transmitted light with a cool or warm tonality, or coloring it bronze, green or blue. Tinted substrates that absorb light evenly across the visible spectrum are called neutral grays. Similar to low-iron substrates, they provide the least amount of spectral distortion, but at a lower visible transmittance. Trade names for neutral gray substrates include CrystalGray™ and Graylite®. Tinted substrates, especially those characterized as “heat absorbing”, can absorb a considerable amount of light energy, which converts into heat and warms the glass.

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Lead Author(s): Matthew Tanteri

Strength

Various processes exist to afford glass substrates increasing levels of strength. These include annealing, heat strengthening, tempering and laminating. In the annealing process glass is cooled in a controlled manner to minimize residual internal stress. After annealing the glass sheet is cut to size, reheated to near its softening point and quickly cooled. This “tempering” process creates compression in its outer surface and tension in its interior, making it three to five times stronger than annealed glass. Tempered glass meets federal, state, and local building codes that require “safety glass” in fenestration where human impact is possible. However, when Tempered glass breaks, which can happen spontaneously, it breaks into small pieces and falls out of its opening. For this reason it is more appropriate for doors as sidelights and not recommended for use in overhead glazing above occupied areas, e.g., skylights or other toplighting device. Heat-strengthening is a similar process to tempering, resulting in glass that is about twice as strong as annealing alone. It involves quicker than normal cooling of glass after annealing, but occurs slower than tempering. Heat-strengthened glass is used where thermal stresses are high, and in overhead glazing. When heat-strengthened breaks, the pieces are larger than tempered glass and usually remain in the frame. However, building codes do not consider heat-strengthened glass as safety glazing, therefore it should not be used where human impact is a concern, or where building codes require safety glazing. For added strength, and enhanced UV filtration, two glass lites may also be laminated together to form a single, unified construction using an interlayer of Polyvinyl Butyrate (PVB). PVB interlayers may be colored, translucent, or printed with patterns or imagery to change the appearance of the glazing. Almost all glass can be laminated, including heat-strengthened glass. Typical applications of laminated glass include overhead glazing and safety glazing when approved by building codes.

Section Key Resources
  • Tanteri, M. (2010). Part 6. A Glazing Primer, Daylighting IESNA Seminar.
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Lead Author(s): Matthew Tanteri

Coatings and Surface Treatments

There are a wide variety of coatings and surface treatments that may be applied to the individual surfaces of an IGU to alter the visual appearance of the glazing, modify its visual or thermal performance properties, or impart it with a desirable attribute.

Ceramic frit may be silkscreened onto the glass surface in a variety of standard patterns (dots, holes and lines) or as custom images. The glass surface may also be acid-etched, sandblasted or patterned to achieve varying levels of light transmittance, optical effect and obscuration. Other coatings provide a variety of desired enhancements such as low emissivity (low-E), low reflectance and self-cleaning features. Self-cleaning glass uses a hydrophilic coating and photocatalytic process has a titanium dioxide coating that when struck by UV breaks down organic compounds such as dirt that allows rain to wash it away.

Section Key Resources
  • Berson, D.M., Dunn, F.A., Takao, M. (2002). Phototransduction by retinal ganglion cell that set the circadian clock. Science, 295: 1070-1073.
  • Boyce, P., Hunter, C., Howlett, O. (2003). The benefits of daylight through windows. Troy (NY): Lighting Research Center.
  • Carmody, J., Selkowitz, S.E., Lee, E.S., Arasteh, D., Willmert, T. (2004). Window Systems for High-Performance Buildings. New York: Norton & Company.

Lead Author(s): Matthew Tanteri

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