Flat glass products may be coated to enhance the thermal and optical performance characteristics of products used in residential and commercial glazing, and transportation applications. There are two basic types of coated glass: solar control (reflective) and low-emissivity (low-e). The major differences are visible light transmission, 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 light with wavelengths ranging from 300-390 nm, visible light (390-770 nm) and infrared light (770-2100 nm). The distribution of energy within the solar spectrum is approximately 2% ultraviolet (UV), 46% visible and 52% infrared (IR).
Solar-control glass may have a variety of metal coating layers that are highly reflective of solar energy, i.e., those energy wavelengths from 300-2100 nanometers (nm) that constitute the solar spectrum.
The major benefits of reflective solar control glass include the following:
Low-emissivity (low-e) coated 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 (IR) part of the solar spectrum and all low-e coatings reflect long wave IR energy. Long wave IR 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.
While some low-e coatings can be used in monolithic or laminated glass constructions, the coatings provide maximum performance when sealed within an insulating glass unit. The location of the low-e coating within a unit affects the product performance. A low-e coating on the second (#2) surface of an insulating glass unit is more effective at reducing 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 and residential applications in warm climate regions, this is generally the most practical way to maintain comfort levels.
In cold climate regions where building owners and occupants want to maximize solar heat gain from the sun while minimizing radiant heat loss, insulating glass units commonly incorporate clear glass with a low-e on the third (#3) surface. The low-e coating reduces heat loss through the glass in winter by reflecting interior long wave IR back into the home or office.
Center of glass U-values in the range of 0.25 - 0.36 can be achieved with low-e coatings on the second or third surface of insulating glass units. Low-e coatings can be combined in an insulating unit with a solar-control / reflective coating and gas filling to create an insulating unit having lower U-values and a lower shading coefficient. Since 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. A generic listing of U-values of various glazing products is provided in the GANA Glazing Manual.
The major benefits of low-e coated glass are:
Optical and aesthetic quality requirements for coatings applied to glass are addressed in ASTM C 1376 Standard Specification for Pyrolytic and Vacuum Deposition Coatings on Flat Glass.
Note: The GANA Glazing Manual should be consulted for additional detailed information on Coating Methods, Specifications and Coating Imperfections (Section II - Fabricated Architectural Glass Products, pages 14 & 15) prior to specifying and using coated glass.
The above information is from the GANA Glazing Manual, 2004 Edition - the most frequently referenced resource in the architectural glass and glazing industry. The Glazing Manual is an excellent addition to any technical library. Go here to order a copy of the manual or CD-ROM. For further information on this and other GANA reference documents visit the PUBLICATIONS section of the GANA website.
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