WINDOW TECHNOLOGIES: Properties Primer
When solar 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. Window glass, along with all other objects, typically emit, or radiate, heat in the form of long-wave far-infrared energy. The wavelength of the long-wave far-infrared energy varies with the temperature of the surface. 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-wave infrared portion of the spectrum, meaning that it emits 84% of the energy possible for an object at room temperature. It also means that for long-wave radiation striking the surface of the glass, 84% is absorbed and only 16% is reflected. By comparison, low-E glass coatings have an emittance as low as 0.04. This glazing would emit only 4% of the energy possible at its temperature, and thus reflect 96% of the incident long-wave infrared radiation.