Not all glass is the same. From time to time, new technology in glass manufacture brings new types of glass into the market, and low-e glass coatings are one of these recent innovations.
Low-e glazing is embedded or coated with a microscopically thin layer of metal or other reflective compound. These coatings reflect the sun’s energy in the infrared spectrum of energy. Also known as long-wave radiation, this is the part of the sun’s energy we experience as heat.
The ‘E’ in Low-E Coatings
The ‘E’ stands for emissivity, which means how much radiant heat the surface of the glass emits. Low-e coatings on windows reduce how much heat can pass through the pane.
Low-e coatings are compounds added to the manufacture of a glass sheet during the time the glass is still hot or after it has cooled. These coatings can be added to the glass in as many as four layers.
Types of Low-E Glass Coatings
Two main types of e-coatings are used in window manufacture for improving energy efficiency.
- Hard Coat, or Passive Low-E Coating, is done while the glass sheet is still molten, creating a permanent fusion between the glass and the coating. This type of coating works best in colder environments, letting more warming, short-wave energy inside, reducing heating costs in winter.
- Soft Coat, or Solar Control Low-E Coating, is applied to the surface of the cooled sheet of glass in a vacuum chamber. After that, the glass is sealed inside a laminate case. This type of coating maintains either warm or cool conditions inside the house, and it also offers superior protection against UV radiation.
Measuring Performance in Low-E Windows
Emissivity, or E, is how much something absorbs or emits infrared radiation. An emissivity of 0 is completely shiny, and something rated a 1 is completely black. While plane glass has an emissivity of .02, low-e coatings are rated at approximately .84.
Other standards used in measuring the performance of low-e glass are:
- U-Value measures heat loss.
- Solar Heat Gain Coefficient measures solar heat transmission.
- Visible Light Transmittance measures how much visible light travels through a low-e coated window.