Window Facts
Aluminum framed windows
Cheap to install, but expensive to keep. Aluminum frames conduct heat and cold two-thousand times faster than vinyl. In the summer, the frames and thin glass actually work like a heat-exchanger, sucking cold air out of your home while piping hot air in. In the winter, this process is reversed, and in the long run, those cheap windows can cost you big energy bucks. Good for Enron, bad for you.
Argon
Some windows have a layer of heavy Argon gas between the panes for extra insulation. It is a great idea ... at least in theory. Argon is an extremely heavy gas that insulates well. The problem is, Argon gas is odorless and invisible. There is no way of telling if it has dissipated out of your windows over time.
"U-Factor" of 0.4 or less
The lower the U-Factor, the better the window. It tells you how well your window stops heat from escaping. Most windows rate between 0.20 and 1.20.
"SHGC" of 0.4 or less
The SHGC, or Solar Heat Gain Coefficient, tells you how well the window blocks out the hot summer sun. The best possible rating is zero, and the worst is 1.0.
Low-e & Low-e2 Coatings
Low-e" is industry jargon for "low emittance of energy." These coatings, which are on the inside of the glass between the panes, keep hot air out and cool air in (and vice-versa). Most utility rebates require a Low-e or Low-e2 coating.