When heat or light energy is absorbed by glass, it is either convected away by moving air or reradiated by the glass surface. The ability of a material to radiate energy is called its emissivity. All materials, including windows, emit (or radiate) heat in the form of long-wave, far-infrared energy depending on their temperature. This emission of radiant heat is one of the important components of heat transfer for a window. Thus reducing the window’s emittance can greatly improve its insulating properties.
The solar reflectance of low-E coatings can be manipulated to include specific parts of the visible and infrared spectrum. This is the origin of the term spectrally selective coatings, which selects specific portions of the energy spectrum, so that desirable wavelengths of energy are transmitted and others specifically reflected. A glazing material can then be designed to optimize energy flows for solar heating, daylighting, and cooling.
A glazing designed to minimize summer heat gains, but allow for some daylighting, would allow most visible light through, but would block all other portions of the solar spectrum, including ultraviolet and near-infrared radiation, as well as long-wave heat radiated from outside objects, such as pavement and adjacent buildings. These low-E coatings still maintain a low U-factor, but are designed to reflect the solar near-infrared radiation, thus reducing the total SHGC while providing high levels of daylight transmission (Figure 3-19).
Figure 3-20 illustrates a low-solar-gain low-E coating on a bronze-tinted, double-glazed unit. Figure 3-21 shows the same coating on spectrally selective tinted glass. Low-E coatings can be formulated to have a broad range of solar control characteristics while maintaining a low U-factor.