Heat flows through a window assembly in three ways: conduction,
convection, and radiation. Conduction is heat traveling through a solid,
liquid or gas. 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 energy through space without relying on conduction
through the air or by movement of the air, the way you feel the heat of
There are two distinct types of radiation or radiation heat transfer:
- Long-wave radiation heat transfer refers to radiant heat transfer between objects at room or outdoor environmental temperatures. These temperatures emit radiation in the range of 3–50 microns.
- Short-wave radiation heat transfer refers to radiation from the sun (which is at a temperature of 6000K) and occurs in the 0.3–2.5 micron range. This range includes the ultraviolet, visible, and solar-infrared radiation.
Ideal spectral transmittance for glazings in different climates (Source: McCluney, 1996).
- Idealized transmittance of a glazing with a low-E coating designed for low solar heat gain. Visible light is transmitted and solar-infrared radiation is reflected. Long-wave infrared radiation is reflected back in to the interior. This approach is to reduce solar heat gain and is suitable in almost all climates.
- Idealized transmittance of a glazing with a low-E coating designed for high solar heat gain. Visible light and solar-infrared radiation are transmitted. Long-wave infrared radiation is reflected back in the interior. This approach is more commonly used in cold climates where solar gain is wanted.
Even though the physical process is the same, there is no overlap
between these two wavelength ranges. Coatings that control the passage
of long-wave or solar radiation in these ranges, through transmission
and/or reflection, can contribute significantly to energy savings and
have been the subject of significant innovations in recent years.
Glazing types vary in their transparency to different parts of the
visible spectrum. For example, a glass that appears tinted green as you
look through it toward the outdoors transmits more sunlight from the
green portion of the visible spectrum and absorbs or reflects more of
the other colors. Similarly, a bronze-tinted glass absorbs or reflects
the blues and greens and transmits the warmer colors. Neutral gray tints
absorb or 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
long-wave infrared radiation but generally transparent to solar-infrared
radiation. Strategic utilization of these variations has made for some
high-performance glazing products.
The four basic properties of glazing
that affect radiant energy transfer: transmittance, reflectance,
absorptance, and emittance.
There are four properties of windows that are the basis for quantifying energy performance:
- U-factor. When there is a temperature difference between inside and outside, heat is lost or gained through the window frame and glazing by the combined effects of conduction, convection, and long-wave radiation. The U-factor of a window assembly represents its overall heat transfer rate or insulating value.
- Solar Heat Gain Coefficient. Regardless of outside temperature, heat can be gained through windows by direct or indirect solar radiation. The ability to control this heat gain through windows is characterized in terms of the solar heat gain coefficient (SHGC) or shading coefficient (SC) of the window.
- Visible Transmittance. Visible transmittance (VT), also referred to as visible light transmittance (VLT), is an optical property that indicates the amount of visible light transmitted through the glass. It affects energy by providing daylight that creates the opportunity to reduce electric lighting and its associated cooling loads.
- Air Leakage. Heat loss and gain also occur by air leakage through cracks around sashes and frames of the window assembly. This effect is often quantified in terms of the amount of air (cubic feet or cubic meters per minute) passing through a unit area of window (square foot or square meter) under given pressure conditions.
These four concepts—as well as Light-to-Solar-Gain ratio, a ratio of VT/SHGC—have been standardized within the glazing industry, and allow accurate comparison of windows.