Insulated Glass Units

The multi-layered assemblies that are used instead of single panes of window glass are called insulated glass units, or IGUs. IGUs consist of layers of glass, or glass and  transparent plastic, separated by layers of air, or other gases. The sheets of glass and  plastic are held apart and joined together with perimeter spacers and seals. Better IGUs also have extremely thin, practically invisible metallic layers called low emissivity, or low e, coatings adhered to the glass or clear plastic.

By itself, glass is a poor insulator. It is the air between layers of glass that slows heat conduction through an IGU. Argon, a gas that insulates better than regular air, is used for better IGUs. Better still, but more costly, are the gases krypton and xenon. To reduce heat loss further, very high performance IGUs have three or more layers of glass, or glass and plastic, separated by layers of gas fill. The edge spacers used to separate glass layers also significantly influence the amount of conductive heat loss through an IGU. Aluminum, a material commonly used for spacers, is highly conductive; stainless steel spacers are much less conductive; silicone foam spacers are less conductive still.

In addition to conductive heat flow, a significant portion of heat loss through an IGU occurs through radiative transfer between layers of glass. Unlike conduction, which is the movement of thermal energy through a material, radiative heat transfer occurs through space itself. As analogies, I think of electricity conducting through wires, and radio waves, even in a vacuum, radiating through space.   Glass, like any building material, absorbs and stores thermal energy – a  lot of us would call stored thermal energy heat , but, to physicists, heat has a more specific meaning.  Materials also radiate, or emit, that energy to their surroundings. Each material, depending on the character of its surface, has a particular ability to emit. Emissivity is measured using a number from 1, which describes a hypothetical, perfectly – highly – emissive surface, to 0 at the bottom of the low emissivity end of the scale. Glass, with an emissivity of around 0.94, is highly emissive, and both radiates and absorbs heat well. Therefore, normal glass does a poor job of blocking radiative heat loss. In order to lower the emissivity of glass, low emissivity coatings can be added. These low e coatings slow radiative heat flow from indoors to out during the winter, and from outdoors to in during the summer.