Low emissivity
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Low-Emittance (Low-E) is a term used for a surface that radiates, or emits, low levels of radiant energy. Emittance refers to the ability of a materials surface to emit radiant energy.[1] All materials absorb, reflect and emit radiant energy. 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. Therefore, if asphalt has an emissivity value of 0.90 its reflectance value would be 0.10. This means that it absorbs and emits 90% of radiant energy and reflects only 10%. Conversely, a low-e material such as aluminum foil has an emissivity value of 0.03 and a reflectance value of 0.97, meaning it reflects 97% of radiant energy and emits only 3%. Low-E building materials include window glass manufactured with metal-oxide coatings as well as housewrap materials, reflective insulation's and other forms of radiant barriers.
The emittance of various surfaces is listed in the following table[2]
.
| Materials Surface | 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-E Windows
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). 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[3], 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.
Reflective Insulations
Reflective insulations are typically fabricated from aluminum foils 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 fire safety. When aluminum foils are used as the facing material reflective insulation can stop 97% of radiant heat transfer. Recently, some reflective insulation manufacturers have switched to a metalized polyethylene facing. The long-term efficiency and durability of such facings are still undetermined.
Reflective insulation can be installed in a variety of applications and locations to include but not limited to; residential, agricultural, commercial, and industrial structures. Some common installs include house wraps, duct wraps, pipe wraps, under radiant floors, inside wall cavity’s, roof systems, attic systems and crawl spaces. Reflective insulations can be used as a stand-alone product in many applications but can also be used in combination systems with mass insulations where higher r-values are required.
Low e²
Low e² or Low-e squared coatings coated with two layers of metal oxide. [4]. This increases the insulating value even more and also significantly reduces the UV light transmission, but also increases the cost[5].
Footnotes
- ^ Bynum, Jr. Richard T. Insulation Handbook McGraw Hill 2001 New York p. 249 ISBN: 0-07-134872-7
- ^ 2009 ASHRAE Handbook Fundamentals - IP Edition, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA. ISBN: 978-1-933742-56-4
- ^ 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.
- ^ http://www.panoramafilm.com/Assets/PDFs/Pan_Low-E_TechBulletin_v2.pdf
- ^ http://www.sebastopolwindow.com/glossary2.html