Low-emissivity


Low-emissivity

Low-emissivity (Low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from warm surfaces to cooler surfaces. Coating a glass surface with a low-emittance material reflects a significant amount of this radiant heat, thus lowering the total heat flow through the window. Low-E coatings are transparent to visible light, and opaque to infrared radiation. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.

To make Low-E glass, certain properties such as the iron content may be controlled. Also, some types of glass have natural Low-e properties, such as borosilicate or "pyrex" (tm). Specially designed coatings, often based on metallic oxides, 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. This often results in more efficient windows because: radiant heat originating from indoors is reflected back inside, thus keeping heat inside in the winter, and infrared radiation from the sun is reflected away, keeping it cooler inside in the summer.

In typical insulated glazing, the low-e coating is found on one of the interior faces of the glass. A simple low-e coating helps to reduce heat loss but allows the room to be warmed by any sunshine. If solar control is required then the outside pane of glass would have either a film or a body tint to reflect solar radiation. The principle of operation is similar to the way selective wavelength transmission of glass or other glazing helps warm a greenhouse, popularly known as the greenhouse effect [Note that the the most important function of glass in a greenhouse is actually slowing convective heat loss rather than slowing radiative heat loss; only once most of the convective heat loss is stopped by a barrier such as glazing do differences in radiative transmission matter.] , in which short wavelength radiation is transmitted through the pane, but longer wavelength radiation absorbed rather than transmitted. However, low-e glass reflects the radiation rather than absorbing it, improving performance compared to the glass in a simple greenhouse.

Footnotes


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • emissivity — The giving off of heat rays; a perfect “black body” has an e. of 1, a highly polished metallic surface may have an e. as low as 0.02. * * * emis·siv·i·ty .em ə siv ət ē, .ē .mis iv n, pl ties the relative power of a surface to emit heat by… …   Medical dictionary

  • Low-e —   A special coating that reduces the emissivity of a window assembly, thereby reducing the heat transfer through the assembly.   California Energy Comission. Dictionary of Energy Terms …   Energy terms

  • Radiant barrier — Radiant barriers or reflective barriers work by reducing heat transfer by thermal radiation. All materials give off, or emit, energy by thermal radiation as a result of their temperature. The amount of energy emitted depends on the surface… …   Wikipedia

  • Building insulation — refers broadly to any object in a building used as insulation for any purpose. Whilst the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation. Often an… …   Wikipedia

  • Solar collector — A solar collector is a device for extracting the energy of the sun not indirectly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short)… …   Wikipedia

  • Architectural glass — is glass that is used as a building material. It is most typically used as transparent glazing material in the building envelope, including windows in the external walls. Glass is also used for internal partitions and as an architectural feature …   Wikipedia

  • Building insulation materials — A selection of insulation materials can aid in building insulation. All of these are based on standard principles of thermal insulation. Materials used to reduce heat transfer by conduction, radiation or convection are employed in varying… …   Wikipedia

  • Heat transfer — is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase change …   Wikipedia

  • R-value (insulation) — The R value or R value is a measure of thermal resistance (K·m²/W) [Oak Ridge National Laboratory, [http://www.ornl.gov/sci/roofs+walls/insulation/ins 02.html Which Kind Of Insulation Is Best?] , retrieved 2008 07 13.] used in the building and… …   Wikipedia

  • Thermal insulation — The term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer.Heat energy can be transferred by conduction, convection, radiation or when undergoing a phase… …   Wikipedia