Incorporation of Low Emissivity Coating

Brewsters Fringes

Brewsters Fringes 2

Brewsters Fringes 2

With one of the glass surfaces having a coating with emissivity less than 0.2 (compared with 0.84 for the uncoated glass surface), the radiation exchange is reduced by approxiamately 75% and consequently the U value is reduced. The term "Low E" is now generally taken to refer to coatings with an emissivity less than 0.2. The change of U value with coating emissivity is illustrated for for air filled double glazed units in Figure A. At ambient temperatures, the long wave radiation lies between 5,000 - 50,000nm where the reflection of Low E coating is high - extending beyond the wavelength coordinate of Figure B.

The higher insulating effect (lower U value) provide by a Low E coating in a double glazed unit is due to the high reflectance of long wavelength radiation at ambient temperatures (Figure C).

Emissivity and reflectance in the far infrared are related by Kirchhoff's law (Figure D).

The development of hard low emissivity coatings widens the possibility of including coated monolithic glazing in secondary frames applied to existing windows; the earlier, softer low emissivity coatings were restricted to protected use in sealed glazed units only. Metallic surfaces, including some metallic oxides, provide low emissivity.

Water on the coated surface of the glass, perhaps as a result of condensation, will cancel out the effect of the Low E coating because of the high emissivity of water.

The present technology of on-line, hard Low E coatings can provide slightly higher solar heat transmission than that exhibited by soft coatings, giving improvements to passive solar gain applications. In cold climates the higher temperature of the inner glass surface of double glazed units using Low E coatings diminishes the effect of colder long wave radiation causing discomfort near the window.