WINDOW TECHNOLOGIES: Glass

Reflective Coatings Center-of-glass values of double pane units with and without reflective coating.

Reflective Coatings

As the SHGC falls in single-pane tinted glazings, the daylight transmission (VT) drops even faster, and there are practical limits on how low the SHGC can be made using tints. If larger reductions are desired, a reflective coating can be used to lower the solar heat gain coefficient by increasing the surface reflectivity of the material. These coatings usually consist of thin metallic or metal oxide layers. The reflective coatings come in various metallic colors—silver, gold, bronze—and they can be applied to clear or tinted glazing. The solar heat gain coefficient can be reduced by varying degrees, depending on the thickness and reflectivity of the coating, and its location in the glazing system. Some reflective coatings are durable and can be applied to exposed surfaces; others must be protected in sealed insulating glass units. The emittance of the coating creates modest changes in the U-factor (see figure to the right).

Similar to tinted films in retrofit situations, reflective coatings may be applied to the inner glass surface of an existing window by means of an adhesive-bonded, metallic-coated plastic film. The applied films are effective at reducing solar gains but are not as durable as some types of coated glass. As with tinted glazing, the visible transmittance of a reflective glazing usually declines more than the solar heat gain coefficient. Reflective glazings are usually used in commercial buildings for large windows, for hot climates, or for windows with substantial solar heat gains. Reflective glazing is also used by many architects because of its glare control and uniform, exterior appearance.

Special consideration should always be given to the effect of the reflective glazing on the outside. Acting like a mirror, the reflective glass intensifies the sun's effects, and should be avoided (or is not permitted by local zoning regulations) in some locations because of its impact on adjacent buildings. It is also important to remember that reflective glass acts like a mirror on the side facing the light. Thus, a reflective window that acts like a mirror to the outside during the day will look like a mirror on the inside during the night. These coatings will not provide visual privacy at night if interior lights are on.

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