Low-emissivity
Encyclopedia
Low emissivity - actually low thermal emissivity - is a quality of a surface that radiates, or emits, low levels of radiant thermal (heat) energy. All materials absorb, reflect and emit radiant energy
. This article is primarily about material properties within a special wavelength interval of radiant energy - namely thermal radiation
of materials with temperatures approximately in the interval -40..60°C.
Emissivity
is the value given to materials based on the ratio of heat emitted compared to a blackbody
, on a scale of 0 to 1. 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 an thermal emissivity value of 0.90 its thermal reflectance value would be 0.10. This means that it absorbs and emits 90% of radiant thermal energy and reflects only 10%. Conversely, a low-e material such as aluminum foil has an thermal emissivity value of 0.03 and a thermal reflectance value of 0.97, meaning it reflects 97% of radiant thermal energy and emits only 3%. 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
.
. 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 coating
s 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 NIR/thermal radiation, instead this type of glass has higher NIR transmission leading to undesirable heat loss (or gain) in a building window.
, 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.
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
. This article is primarily about material properties within a special wavelength interval of radiant energy - namely thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
of materials with temperatures approximately in the interval -40..60°C.
Emissivity
Emissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
is the value given to materials based on the ratio of heat emitted compared to a blackbody
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
, on a scale of 0 to 1. A blackbody would have an emissivity of 1 and a perfect reflector would have a value of 0.
Reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
is inversely related to emissivity and when added together their total should equal 1 for an opaque
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
material. Therefore, if asphalt has an thermal emissivity value of 0.90 its thermal reflectance value would be 0.10. This means that it absorbs and emits 90% of radiant thermal energy and reflects only 10%. Conversely, a low-e material such as aluminum foil has an thermal emissivity value of 0.03 and a thermal reflectance value of 0.97, meaning it reflects 97% of radiant thermal energy and emits only 3%. 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 emittance |
|---|---|
| Asphalt | 0.90-0.98 |
| Aluminum foil | 0.03-0.05 |
| Brick | 0.93 |
| Concrete | 0.85-0.95 |
| Glass (unglazed) | 0.95 |
| Fiberglass/cellulose | 0.80-0.90 |
| Limestone | 0.36-0.90 |
| Marble | 0.93 |
| Paper | 0.92 |
| Plaster | 0.91 |
| Silver | 0.02 |
| Steel (mild) | 0.12 |
| Wood | 0.90 |
Low-emissivity windows
Window glass is by nature highly thermal emissive as indicated in the table above. To improve thermal efficiency (insulation properties) thin film coatings are applied to the raw soda-lime glassSoda-lime glass
Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers for beverages, food, and some commodity items...
. 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
Tin dioxide
Tin dioxide is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry...
uses) at high temperatures. Pyrolytic coating
Pyrolytic coating
Pyrolytic coating is a thin film coating applied at high temperatures and sprayed onto the glass surface during the float glass process.-Advantages:*Relatively durable coating.* Can be tempered after coating application....
s are usually applied at the Float glass
Float glass
Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made from 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
Insulated glazing
Insulated glazing also known as double glazing are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope....
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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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
Pyrex
Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
. Naturally low-e glass does not have the property of reflecting NIR/thermal radiation, instead this type of glass has higher NIR transmission leading to undesirable heat loss (or gain) in a building window.
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, fiberglassFiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...
, 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
R-value (insulation)
The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it or R = \Delta T/\dot Q_A.The R-value being discussed is the unit thermal resistance...
are required.