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.