Issues in Window Selection: Energy-Related
Heat Transfer Mechanisms | Measuring Properties | Overview of Energy Use | Codes and Standards
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 2-3 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.
Transmittance
Transmittance refers to the percentage of radiation that can pass through glazing. Transmittance can be defined for different types of light or energy, e.g., visible transmittance, UV transmittance, or total solar energy transmittance.
Transmission of visible light determines the effectiveness of a type of glass in providing daylight and a clear view through the window. For example, tinted glass has a lower visible transmittance than clear glass. While the human eye is sensitive to light at wavelengths from about 0.4 to 0.7 micrometers, its peak sensitivity is at 0.55, with lower sensitivity at the red and blue ends of the spectrum. This is referred to as the photopic sensitivity of the eye.
More than half of the sun's energy is invisible to the eye and reaches us as either ultraviolet (UV) or, predominantly, as near-infrared. Thus, total solar energy transmittance describes how the glazing responds to a much broader part of the spectrum and is more useful in characterizing the quantity of solar energy transmitted by the glazing.
With the recent advances in glazing technology, manufacturers can control how glazing materials behave in these different areas of the spectrum. The basic properties of the substrate material (glass or plastic) can be altered, and coatings can be added to the surfaces of the substrates. For example, a window optimized for daylighting and for reducing heat gains should transmit an adequate amount of light in the visible portion of the spectrum, while excluding unnecessary heat gain from the near-infrared part of the electromagnetic spectrum.
On the other hand, a window optimized for collecting solar heat gain in winter should transmit the maximum amount of visible light as well as the heat from the near-infrared wavelengths in the solar spectrum, while blocking the lower-energy radiant heat in the far-infrared range that is an important heat loss component. These are the strategies of various types of low-emittance coatings.
Reflectance
Just as some light reflects off of the surface of water, some light will always be reflected at every glass surface. A specular reflection from a smooth glass surface is a mirrorlike reflection similar to when you see an image of yourself in a store window. The natural reflectivity of glass is dependent on the quality of the glass surface, the presence of coatings, and the angle of incidence of the light. Today, virtually all glass manufactured in the United States is float glass and has a very similar quality with respect to reflectance. The sharper the angle at which the light strikes, however, the more the light is reflected rather than transmitted or absorbed. Even clear glass reflects 50 percent or more of the sunlight striking it at incident angles greater than about 70 degrees. (The incident angle is formed with respect to a line perpendicular to the glass surface.)
The reflectivity of various glass types becomes especially apparent during low light conditions. The surface on the brighter side acts like a mirror because the amount of light passing through the window from the darker side is less than the amount of light being reflected from the lighter side. This effect can be noticed from the outside during the day and from the inside during the night. For special applications when these surface reflections are undesirable (i.e., viewing merchandise through a store window on a bright day), special coatings can virtually eliminate this reflective effect.
Most common coatings reflect in all regions of the spectrum. However, in the past twenty years, researchers have learned a great deal about the design of coatings that can be applied to glass and plastic to reflect only selected wavelengths of radiant energy. Varying the reflectance of far-infrared and near-infrared energy has formed the basis for high-solar-gain low-E coatings for cold climates, and for low-solar-gain low-E coatings for hot climates.
Absorptance
Energy that is not transmitted through the glass or reflected off of its surfaces is absorbed. Once glass has absorbed any radiant energy, the energy is transformed into heat, raising the temperature of the glass.
Typical 1/8-inch (3 mm) clear glass absorbs only about 8 percent of sunlight at a normal angle of incidence. The absorptance of glass is increased by glass additives that absorb solar energy. If they absorb visible light, the glass appears dark. If they absorb ultraviolet radiation or near-infrared, there will be little or no change in visual appearance. Clear glass absorbs very little visible light, while dark tinted glass absorbs a considerable amount. The absorbed energy is converted into heat, warming the glass. Thus, when these "heat-absorbing" glasses are in the sun, they feel much hotter to the touch than clear glass. They are generally gray, bronze, or blue-green and are used primarily to lower the solar heat gain coefficient and to control glare. Since they block some of the sun's energy, they reduce the cooling load placed on the building and its air-conditioning equipment. Absorption is not the most efficient way to reduce cooling loads.
All glass and most plastics, however, are generally very absorptive of far-infrared energy. This property led to the use of clear glass for greenhouses, where it allowed the transmission of intense solar energy but blocked the retransmission of the low-temperature heat energy generated inside the greenhouse and radiated back to the glass.
Emittance
When heat or light energy is absorbed by glass, it is either convected away by moving air or reradiated by the glass surface. This ability of a material to radiate energy is called its emissivity. Windows, along with all other objects, typically emit, or radiate, heat in the form of long-wave far-infrared energy. This emission of radiant heat is one of the important heat transfer pathways for a window. Thus, reducing the window's emission of heat can greatly improve its insulating properties.
Standard clear glass has an emittance of 0.84 over the long wavelength portion of the spectrum, meaning that it emits 84 percent of the energy possible for an object at its temperature. It also means that for long-wave radiation striking the surface of the glass, 84 percent is absorbed and only 16 percent is reflected. By comparison, low-E glass coatings have an emittance as low as 0.04. This glazing would emit only 4 percent of the energy possible at its temperature, and thus reflect 96 percent of the incident long-wave infrared radiation.