This is the first in a three-part series about the most important light-managing medium, glass. Part one examines conventional building glass, particularly as it affects daylight and daylighting; part two will look into glass as a medium for lighting and lighting effects; and part three will explore the artistic opportunities of some new and exciting glass types.

Energy gurus and passive solar geeks aside, most of us have historically paid little attention to glazing other than how it looks. If anything, most want it to be as transparent as possible. However, since the Leadership in Energy and Environmental Design (LEED) program and energy efficiency demand more efficient use of natural light, many architects, lighting designers, and engineers are now scrambling to master the technical side of this material as well.

Glass Terms

Most practical uses of glass in buildings are for windows, clerestories, and skylights. Windows are defined as glass that is mounted more or less in the vertical plane with the intent to be looked at or through. Clerestories are defined as windows mounted above eye level whose primary purpose is to introduce daylight into a space. Skylights are generally defined as glass installed in a more or less horizontal plane. Glazing is a generic term that can include glass, plastic, or other light-transmitting materials as well as combinations of them. Within the last two decades, coatings have been developed to change the optical properties of glass; today, they are essential in achieving desirable performance values. Films can be applied to glazing to change properties as well.

Glazing For Geeks

While single-pane glazing is sometimes used in very benign climates, in order to provide insulation and protect special coatings most buildings use multilayer assemblies of glazing to create windows and skylights. For practical purposes, the following principal characteristics are used to evaluate alternatives.

  • Visible Light Transmission (VLT or Tvis)
  • The percentage of visible light that is transmitted through the glazing assembly. This is the essential characteristic for daylighting calculations. A perfectly clear window would have a VLT of 100 percent. Most practical assemblies for architectural use are between 35 and 80 percent.

  • Solar Heat Gain Coefficient (SHGC)
  • The percentage of total solar radiant energy that is transmitted through the assembly. This is the essential characteristic for solar gain calculations. For ordinary windows without special coatings, the SHGC and the VLT are the same and sometimes called the shading coefficient (SC). However, with modern coated windows, the SHGC is almost always lower than the VLT. Such window systems are generically referred to as low-emissivity or lowE and are used in most commercial construction.

  • U-factor
  • U-factor is the rate of heat loss through the window and its frame. This is the essential characteristic for heat gain and loss other than solar. The lowest practical U-factor is preferred when the building is exposed to extreme cold or heat.

    While these characteristics are usually applied to clear glazing, they can also be applied to glazing that is diffuse or translucent (like white acrylic) or refracting (like obscure glass or prismatic acrylic), or that has any combination of refractive and diffusing effects.

    In choosing a glazing system, a primary objective is usually to employ a window glass system that allows the transmittance of the most possible visible light (maximum VLT) while minimizing the solar gain (minimum SHGC). The ratio of VLT to SHGC is sometimes called the light-to-solar-gain ratio or LSG. Most typical window glass systems have an LSG between 1.2 and 1.4. In other words, to have high visible light transmission, it is necessary to allow in quite a bit of solar gain. However, a “dream glass” is now available using a special combination of low-lead glass and state-of-the-art coatings that achieve an LSG as high as about 2.4. When designing ultra-efficient buildings, having relatively clear windows (VLT .7 or more) with an SHGC less than .30 can be the magic combination, especially in cooling-dominated climates.

    The Interaction of Glazing and Light

    While the big numbers above (VLT, SHGC, and U) are principal factors in building design, there are several other issues to consider. These include: