Glazing for Daylight
by Gregg D. Ander, FAIA
Windows and fenestration systems have undergone a technological revolution in recent years. Glazing systems are now available that can dramatically cut energy consumption and the associated pollution sources, reduce peak demand, enhance daylighting performance, and improve occupant comfort.
But all these improvements have made windows more complex. A number of characteristics for glazing need to be understood to properly specify an appropriate fenestration system.
The solar heat gain coefficient (SHGC) is the fraction of incident solar energy transmitted through a window. Visible transmittance (VT) is the percentage of visible light that passes through a window.
The luminous efficacy constant (Ke), also referred to as the coolness index, indicates a window's relative performance in rejecting solar heat while transmitting daylight. A glazing system with a selective coating will allow visible light to pass through it while blocking all or some of the invisible near-infrared and ultraviolet components. These glazing systems are highly suitable for daylighting projects and will yield Ke values greater than 1. It is generally better to have a high Ke value for daylighting systems.
The performance properties of glazing can be easily altered. Combinations of modifications can further improve the effectiveness of the glazing units. Tinted glazing, also known as heat-absorbing glass, use materials dispersed throughout the glass material to reduce the amount of solar radiation and visible light transmitted.
Unfortunately, this also causes the glass temperature to rise, which will decrease comfort for occupants in the space due to the higher mean radiant temperature (MRT) of the surface of the glass. Common colors for tinted glass, such as bronze and gray, block light and near-infrared heat in equal proportions. >>>
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This article is excerpted from Daylighting Performance and Design by Gregg D. Ander, FAIA, with permission of the publisher, John Wiley & Sons.
Switchable optical glazing materials. Photochromic materials become cloudy under bright light. Thermochromic materials change properties as a function of temperature. Electrochromic materials respond to an applied voltage.
Image: Gregg D. Ander
Light transmission versus heat gain for selected glazings.
Image: Loisos + Ubbelohde
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