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.
Definition
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.
Low-emissivity windows
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 energy-efficient windows can
reflect much more sunlight than simple glass windows, when these windows are
somewhat concave they can focus sunlight and cause damage. Damage to the
sidings of homes and to automobiles has been reported in news stories.
In addition, low-E windows may
also block radio frequency signals such as are used by cellular telephone
systems. Buildings without distributed antenna systems will then
suffer degraded cell phone reception.
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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