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Glass & Windows 101

Heat Transfer Mechanisim

Heat Transfer Mechanisms and Glazing Properties Related to Radiant Energy Transfer
Most window assemblies consist of glazing and frame components. Glazing may be a single layer of glass (or plastic) or multiple layers with air spaces in between. These multiple layer units, referred to as insulated glazing units (IGU), include spacers around the edge and sometimes lower conductance gases in the spaces between glazings. Coatings and tints affect the performance of the glazing. The IGU then is placed within a frame of aluminum, steel, wood, plastic, or some hybrid or composite material. Some advanced curtain wall systems do not have frames in the conventional sense.

Heat flows through a window assembly in three ways: conduction, convection, and radiation. Conduction is heat traveling through a solid material, the way a frying pan warms up. Convection is the transfer of heat by the movement of gases or liquids, like warm air rising from a candle flame. Radiation is the movement of heat energy through space without relying on conduction through the air or by movement of the air, the way you feel the heat of a fire.

Conduction through glass and solid frame materials and convection within air spaces are discussed in the section on insulating value (U-factor). Heat transfer through radiation deserves special attention because it has been the source of much recent innovation in window energy performance. Three things happen to solar radiation as it passes through a glazing material. Some is transmitted, some is reflected, and the rest is absorbed. Figure below shows the solar and thermal parts of the electromagnetic spectrum that relate to windows. These include the ultraviolet, visible, near-infrared, and far-infrared ranges.

Glazing types vary in their transparency to different parts of the visible spectrum. For example, a glass that appears to be tinted green as you look through it toward the outside will transmit more sunlight from the green portion of the visible spectrum, and absorb/reflect more of the other colors. Similarly, a bronze-tinted glass will absorb/reflect the blues and greens and transmit the warmer colors. Neutral gray tints absorb/reflect most colors equally.

This same principle applies outside the visible spectrum. Most glass is partially transparent to at least some ultraviolet radiation, while plastics are commonly more opaque to ultraviolet. Glass is opaque to far-infrared radiation but generally transparent to near-infrared. Strategic utilization of these variations has made for some very useful glazing products. The four basic properties of glazing that affect radiant energy transfer--transmittance, reflectance, absorptance, and emittance--are described below

Glass Transmittance
Glass Reflectance
Glass Absorptance
Glass Emittance
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