Low E.  The biggest contributor to the U Factor of a window is the Low E coatings applied to the glass.  Low E is a microscopic layer or layers of metal on the glass which reflects radiant energy.  In the summer it keeps some of the sun’s radiant energy from entering your house, and in the winter time it keeps some of the radiant energy you pay for (a.k.a., heat) from radiating through your windows.  Just how much energy your Low E coatings reflect is determined by how many layers of Low E are applied.  Currently, the majority of the market has one coat of Low E, as this is the bare minimum to qualify as energy efficient.  But there are a growing number of customers upgrading to three coats of Low E, and it does an amazing job of reflecting heat.