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Low emissivity (low e or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect and emit radiant energy, but here, the primary concern is a special wavelength interval of radiant energy, namely thermal radiation of materials with temperatures approximately between 40 to 60 degrees Celsius.
Emissivity is the value given to materials based on the ratio of heat emitted compared to a blackbody, on a scale from zero to one. A blackbody would have an emissivity of 1 and a perfect reflector would have a value of 0.
Reflectivity is inversely related to emissivity and when added together their total should equal 1 for an opaque material. Therefore, if asphalt has a thermal emissivity value of 0.90, its thermal reflectance value would be 0.10. This means that it absorbs and emits 90 percent of radiant thermal energy and reflects only 10 percent. Conversely, a low-e material such as aluminum foil has a thermal emissivity value of 0.03 and a thermal reflectance value of 0.97, meaning it reflects 97 percent of radiant thermal energy and emits only 3 percent. Low-emissivity building materials include window glass manufactured with metal-oxide coatings as well as housewrap materials, reflective thermal insulations and other forms of radiant thermal barriers.
The thermal emissivity of various surfaces is listed in the following table.
|Materials surface||Thermal emissivity|
|Glass, smooth (uncoated)||0.91|
|Marble, Polished or white||0.89 to 0.92|
|Paper, roofing or white||0.88 to 0.86|
Window glass is by nature highly thermally emissive, as indicated in the table above. To improve thermal efficiency (insulation properties) thin film coatings are applied to the raw soda-lime glass. There are two primary methods in use: Pyrolytic CVD and Magnetron Sputtering. The first involves deposition of fluorinated tin oxide (SnO2:F see Tin dioxide uses) at high temperatures. Pyrolytic coatings are usually applied at the Float glass plant when the glass is manufactured. The second involves depositing thin silver layer(s) with antireflection layers. Magnetron sputtering uses large vacuum chambers with multiple deposition chambers depositing 5 to 10 or more layers in succession. Silver based films are environmentally unstable and must be enclosed in an Insulated glazing or Insulated Glass Unit (IGU) to maintain their properties over time. Specially designed coatings, are applied to one or more surfaces of insulated glass. These coatings reflect radiant infrared energy, thus tending to keep radiant heat on the same side of the glass from which it originated, while letting visible light pass. This results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside, while infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.
Glass can be made with differing thermal emissivities, but this is not used for windows. Certain properties such as the iron content may be controlled, changing the thermal emissivity properties of glass. This is "naturally" low thermal emissivity, found in some formulations of borosilicate or Pyrex. Naturally low-e glass does not have the property of reflecting near infrared (NIR)/thermal radiation, instead this type of glass has higher NIR transmission, leading to undesirable heat loss (or gain) in a building window.
Criticism of low-E windows
Since Low-E glass reflects more sunlight, it has been observed that the extra reflectivity combined with any concavity in the glass would effectively turn the glass into a concave mirror, concentrating sunlight onto other objects such as cars, or the siding on adjacent houses, melting plastic or vinyl   This problem is exacerbated by dual pane windows filled with Argon gas. Over time the Argon permeates the window edge seal leading to low internal pressure. This in turn serves to force the center of the panes inward forming a concave exterior "mirror." Even uncoated glass panes can produce high temperatures at the focus of such windows.
Reflective thermal insulation
Reflective thermal insulation is typically fabricated from aluminum foil with a variety of core materials such as low-density polyethylene foam, polyethylene bubbles, fiberglass, or similar materials. Each core material presents its own set of benefits and drawbacks based on its ability to provide a thermal break, deaden sound, absorb moisture, and resist combustion during a fire. When aluminum foil is used as the facing material, reflective thermal insulation can stop 97% of radiant heat transfer. Recently, some reflective thermal insulation manufacturers have switched to a metalized polyethylene facing. The long-term efficiency and durability of such facings are still undetermined.
Reflective thermal insulation can be installed in a variety of applications and locations including residential, agricultural, commercial, and industrial structures. Some common installations include house wraps, duct wraps, pipe wraps, under radiant floors, inside wall cavities, roof systems, attic systems and crawl spaces. Reflective thermal insulation can be used as a stand-alone product in many applications but can also be used in combination systems with mass insulation where higher R-values are required.
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