WINDOWS 101

Low-E Coatings

In recent years, remarkably advanced technologies have been developed which allow us to change the characteristics of glass to achieve the different energy performance ratings we desire. Manipulating the different performance ratings is very convenient since it allows windows to be designed and manufactured specifically for cold, hot, or moderate climates. Furthermore, it allows homeowners the ability to choose different window characteristics for different parts of the home. For example, if there is a room in the home that gets very hot, or is very dark, a windows' characteristics can be selected to specifically address the problem.

The most important technology which allows us to manipulate the performance ratings is the Low-Emmisivity coating (Low-E). The following section will explain what Low-E is, and the different types available on the market.

Low-E: In Multilayer glazing (i.e. dual or triple pane), the principal mechanism of heat transfer is thermal radiation from the warmer pane to the cooler pane of glass. Adding a low emmisivity coating (Low-E) to the inside surface of one of the panes significantly blocks the amount of radiant heat transfer, which lowers the total heat flow through the window.

Low-E coatings are colorless and clear coatings. The coatings solar reflectance characteristics can be manipulated to reflect or emmit specific parts of the visible and infrared spectrum. This ability is called "spectrally selective glazing."

There are two ways of manufacturing Low-E coatings. "Soft Coats," or sputtered coating, is a multilayer coating that is deposited on glass in a vaccum chamber. The coating is an ultra thin layer of silver. By increasing the thickness of the coating layer, or by adding additional layers, the glazing is able to reflect more of the sun's near infrared and long wave infrared while still maintaining a high visible transmittance.

The other type of Low-E coating is a "hard coat," or pyrolytic coating. A pyrolytic Low-E coating is a metallic oxide, usually tin, which is applied directly onto the glass while it is still hot. This results in a hard, baked-on surface layer.

Sputtered coats are more spectrally selective than pyrolytic because of the multiple layers. They maintain a higher visible transmittance, while lowering the SHGC, than do pyrolytic coats. Sputtered coats are, thus, more energy efficient than pyrolytic coats. Pyrolytic coats, on the other hand, require less manufacturing time since they are applied during the manufacturing of the glass, and also cost less as a result. Pyrolytic Low-E can also be used to coat single pane units, where sputtered coats can only be applied to dual or triple pane units because the soft coat needs to be protected from contact by being in between the two or three panes of glass.

Now that you know what each of the energy performance ratings on the NFRC label mean, and what Low-E coatings can achieve, how do you go about picking the right window? A good start would be to consider what the ENERGY STAR program recommends, and certifies, for the specific region you live in.