Before innovations in glass, films, and coatings in the past decade, a typical residential window with one or two layers of glazing allowed roughly 75-85% of the solar energy to enter a building, which has a negative impact on summertime comfort and cooling bills, especially in hot climates.
External window shading devices such as awnings, roof overhangs, shutters, and solar screens, and internal shading devices such as curtains and blinds, can control the entry of solar heat. However, shutters, solar screens, curtains, and blinds make rooms dark. Curtains and blinds also let in some of the undesirable heat. While exterior shading devices are about 50% more effective than internal devices at blocking solar heat, they may create problems with the building's aesthetics and are sometimes expensive to build. It is also impractical to construct roof overhangs to effectively shade east and west facing windows.
The following are the percentages of the radiant energy that different types of internal shading devices transmit, reflect, or absorb:
Roller Shades: up to 25%, 15-80%, 20-65%
Vertical Blinds: 0%, 23%, 77%
Venetian Blinds: 5%, 40-60%, 35-55%
The weak thermal characteristics of windows became a prime target for research and development in the attempt to control indoor temperatures of buildings. This led to the development of low-emissivity or "low-e," glass and films that control heat gain and loss, reduce glare, minimize fabric fading, provide privacy, and occasionally provide added security in wind, seismic, and other high-hazard zones. New construction and window replacement applications commonly use glazing with these coatings.
Some low-e coatings and solar control films reduce solar heat gain without impairing visible light transmission excessively. These include tinted glass and spectrally selective coatings, which transmit visible light while reflecting the long-wave infrared portion of sunlight. Many spectrally selective coatings also have some low-e properties as well. Modern window glazing falls into three categories: chemically or physically altered glass, coated glass or films, and multiple-layered assemblies with or without either of the first two items.
Chemically or Physically Altered Glass Tinting is the oldest of all the modern window technologies and, under favorable conditions, can reduce solar heat gain during the cooling season by 25% to 55%. Tinted glass is made by alteration of the chemical properties of the glass. Both glass and plastic laminate may be tinted. The tints absorb a portion of the sunlight and solar heat before it can pass all the way through the window to the room. Tinted glazings reduce the latter by 25-55%. "Heat absorbing" tinted glass maximizes its absorption across some, or all, of the solar spectrum. Unfortunately, the absorbed energy often transfers by radiation and convection to the inside.
Spectrally selective tints reduce infrared light (heat) transmission while allowing relatively more visible light to pass through (compared to bronze- or gray-tinted glass). For buildings that use daylight for lighting, a spectrally selective window is a good choice. Spectrally selective glass also absorbs much of the ultraviolet (UV) portion of the solar spectrum. In a multi-paned window, they function best as the outermost sheet of glazing. Thermal performance is increased when combined with a low-e coating. Spectrally selective coatings often have a light blue or green tint.
Coatings and Films Low-e and reflective coatings usually consist of a layer of metal a few molecules thick. The thickness and reflectivity of the metal layer (low-e coating) and the location of the glass it is attached to directly affects the amount of solar heat gain in the room. Most window manufacturers now use one or more layers of low-e coatings in their product lines.
Any low-e coating is roughly equivalent to adding an additional pane of glass to a window. Low-e coatings reduce long-wave radiation heat transfer by 5 to 10 times. The lower the emissivity value (a measure of the amount of heat transmission through the glazing), the better the material reduces the heat transfer from the inside to the outside. Most low-e coatings also slightly reduce the amount of visible light transmitted through the glazing relative to clear glass. Here are representative emissivity values for different types of glass:
Clear glass, uncoated: 0.84
Glass with single hard coat low-e: 0.15
Glass with single soft coat low-e: 0.10
A pyrolytic coating baked on at a high temperature constitutes a "hard coat" low-e coating. These are often made of a metallic oxide. One layer is about 1/10,000 the diameter of a human hair.