How Glazing gets its Properties

Low-e coatings were developed in the 1970’s and came to the market in 1980. Emissivity is the ability of a surface to emit radiant energy with the glass absorbing part of the energy by radiating it away from the surface.

Low-e coatings are thin, virtually invisible metal or metallic oxide layers deposited on a glass surface. Typically, they are used to impede heat flow through the glass and improves the u-value of the unit. They also can have a dramatic effect on the solar heat gain coefficient (SHGC) of the window depending on where the coating is located.

Low-emittance coatings can be applied in two different ways; as a hard coat, called a pyrolitic or a soft coat, known as a sputter coat.

Hard coat coatings (pyolytic) have a thin layer of tin oxide incorporated into the surface of the glass as it's made... hence the fire reference. It is a very durable coating and is typically used on outside surfaces, but it is not nearly as good at insulating as the soft coat and lets heat in more readily. This may be okay for a solar building, but not good if heat gain is a problem.

Soft coats, otherwise known as sputter coats, use thin layers of silver and anti-reflective components that are applied to the surface through a vacuum deposition process. Since the coating is delicate it stays on the inside of a sealed unit. These coatings can also be applied to suspended films as well.

Special versions of these coatings have even been designed to target specific wavelengths of light, usually to let in the visible spectrum (short wave) but to block the heat gain component (long wave)... this is called spectrally selective glass.

Spectrally selective glass might have a high visible light transmittance (vlt) and a low SHGC for example. With normal coatings, as the SHGC drops so does the vlt.

As briefly mentioned, a wide variety of characteristics can be achieved depending on where certain coatings appear. Surfaces are typically numbered as in the diagram below.
	That's an important point, Sidney. Coatings do affect the way the glass looks through some type of coloration, although more and more coatings are being developed that are ‘clear’ and don't change aesthetics a great deal.

Common colors for ‘tints’ are grey, green, blue and bronze. Grey glass typically transmits approximately equal visible light and infrared light. Bronze tends to let in less visible and more infrared than grey. Blue and green glass transmit more visible light and less infrared than grey.

And with that, we are finished with our discussion of how glazing gets its properties!