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Technical Resources Coated Glass There are two basic types of coated glass: solar control (reflective) and low-emissivity (low-E). The major differences are visible light transmission, ultraviolet (UV), visible, and near infrared wavelengths of energy that are reflected, and the directions in which these wavelengths are usually reflected. The solar spectrum consists of ultraviolet (UV) light with wavelengths ranging from 300 to 390 nanometers (nm), visible light (390 to 770 nm), and infrared (IR) light (770 to 2,100 nm). The distribution of energy within the solar spectrum is approximately 2 percent ultraviolet, 46 percent visible, and 52 percent infrared. Solar-control glass may have a variety of metal coating layers that are highly reflective of solar energy, i.e., those energy wavelengths, from 300 to 2,100 nm, that constitute the solar spectrum. The major attributes of reflective solar-control glass include the following: · Aesthetic appeal. The various silver, blue, gold, and copper reflective coatings, when applied to clear and/or tinted float glass, allow the architect considerable flexibility with exterior design. · Energy savings. Due to its ability to reflect, absorb, and radiate solar energy, solar reflective glass will substantially reduce indoor solar heat gain. The added cost of the coating will generally be offset by the reduced size and operating cost of the HVAC system. · Occupant comfort. Occupant comfort is improved when heat gain/loss differentials between sunny and shaded elevations are substantially reduced. Indoor temperature differentials are less and thus easier to control. Low-E glass may have various combinations of metal, metal oxide, and metal nitride layers of coatings that are nearly invisible to the eye. Some low-E coatings are highly reflective for the infrared part of the solar spectrum, and all low-E coatings reflect long wave infrared energy. Long wave infrared can be described as the radiant heat given off by an electric coil-type heater, as well as the heat that comes from a hot air register. The re-radiated heat from room furnishings that have absorbed solar energy is still another form of radiant heat. A low-E coating on the second surface of an insulating glass unit is more effective at controlling solar heat gain, especially when used in conjunction with tinted glass. The low-E coating will reflect re-radiated heat (IR), while the tinted glass reduces the solar radiation through the glass, resulting in less glare and heat gain. When using low-E glass in commercial buildings, this is generally the most practical way to maintain comfort levels. When using clear glass in insulating glass units, the coating may be placed on either the number two or number three surface. The low-E coating reduces heat loss through the glass in winter by reflecting interior long wave IR back into the office or home. U-values in the range of 0.25 to 0.36 can be achieved with low-E coatings on the second and third surface of insulating glass units. Low-E coatings can be combined in an insulating glass unit with a solar reflective coating and gas filling to create an insulating unit having lower U-values and a lower shading coefficient. Because technology continues to advance, and because the combinations of substrates and coatings are too numerous to list, it is best to consult the coated glass manufacturers' published literature for comparisons. This material has been excerpted from the Glass Association of North America (GANA) Glazing Manual. Information about purchasing the Glazing Manual is available by calling GANA at 785/271-0208, or at its Web site www.glasswebsite.com/gana. The manual provides extensive industry reference information including sections on primary glass products, fabricated architectural glass, reference standards, glass design considerations, and extensive glazing guidelines. Source: The Glass Guide for Architects and Specifiers, Glass Magazine, June 2001. |
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