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 black body, 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.
- Reflectivityis inversely related to emissivity and when added together their total should equal 1 for an opaque 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 aluminium 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 house wrap 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|
Pyrolytic vs. sputtered
Window glass is by nature highly thermally emissive, so to improve thermal efficiency (insulation properties), ultra thin, invisible film coatings are applied to the raw soda-lime glass. There are two primary methods in use:
|Pyrolitic (CVD):||Magnetron sputtering (MSVD):|
|Involves deposition of fluorinated tin oxide (SnO2) at high temperatures.||It involves depositing thin silver layers with antireflection layers.|
|Such coatings are usually applied at the float glass plant when the glass is manufactured.||It uses large vacuum chambers with multiple deposition chambers depositing 5 to 10 or more layers in succession.|
|Since the coating is covalently bonded to the glass; Pyrolitic low-E is extremely durable and can be handled, transported and stored just like clear glass.
|Silver-based films are environmentally unstable and must be enclosed in insulated glazing or an Insulated Glass Unit (IGU) to maintain their properties Specially designed coatings are applied to one or more surfaces of insulated glass over time.|
For years, MSVD coating technology and sputtered low-E have been the only widely available options. However, the inherent handling sensitivity and limited shelf-life of sputtered low-E render it ill-suited for transport and use in regions with developing infrastructures. Moreover, the dominant glazing in these regions is often a single lite, while sputtered low-E can only be used in a multiple lite IG unit assembly.
These low emissivity coatings reflect radiant infrared energy, thus tending to keep radiant heat on the side of the glass where 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.
The higher solar heat gain coefficient of Pyrolitic low-E coatings can result in significantly lower heating costs and provide a greener alternative, making Pyrolitic low-E coatings an ideal substitute.
“Naturally” low thermal emissivity is found in some formulations of borosillicate 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.
Setbacks of low-E windows:
Since energy-efficient windows 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.
Low-E windows may also block radio frequency signals. Buildings without distributed antenna systems may then suffer degraded cell phone reception.
Recent trends show that Pyrolitic low-E coatings are attracting more and more attention in both the commercial and residential glass coatings markets due to the strong focus on Energy Star and Green Building programs
Reflective insulation thermal:
Reflective thermal insulation is typically fabricated from aluminium foil with a variety of core materials such as low-density polyethylene foam, polyethylene bubbles, fibreglass, 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 aluminium foil is used as the facing material, reflective thermal insulation can stop 97% of radiant heat transfer.
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.
By façade experts