The last, and potentially most important, item that determines a window’s relative energy efficiency is that of the windowpane itself. That is not to say that the panes alone determine all of a window’s energy efficiency. As shown previously, it is how all parts interact as a whole that dictates just how energy efficient a window setup actually is.
Low Emissivity Glass – commonly referred to as “Low-E,” this is glass that has been treated with an invisible metal oxide coating that reflects both infrared and ultraviolet light, lowering the window’s solar heat gain coefficient while generally not affecting visible light transmittance. This is important because if a window limits the amount of visible light coming in, any savings due to reduced heating or cooling is offset, at least in part, by the increase in energy expenditure for lighting.
While older houses generally feature windows with only a single pane of glass, modern windows usually come with double- and even triple-pane options. The idea behind this is that the air between the multiple panes of glass forms an insulating layer in the window, reducing heat loss through the window, and thus your home. Naturally, triple pane windows will have a lower U-value than double-pane windows, but are proportionally more expensive.
Not content to rely solely on “normal” air to act as a heat loss buffer between windowpanes, glass manufacturers have found that inserting certain clear, inert gases in between the glass panes has a positive effect on a window’s relative energy efficiency. The two most common gases used for this purpose are krypton and argon. Argon is, by far, the more common of the two, and the less expensive. Krypton, for all its expense, is significantly more effective than argon in reducing the amount of heat lost through the windowpane.
No single feature, ultimately, determines a window’s particular energy efficiency rating. Rather, it is the different combinations of features that make up the entire window structure that determines that. It’s the different interactions between these that make all the difference between a good window, energy-efficiency wise, and a truly great, energy-efficient window.