U-values are most commonly used when talking about heat loss or heat gain through a building assembly like windows, doors, and some other common building sections - walls, floors, roof. In the case of windows, U-values or U-factors are used to provide a representation of how much heat (in BTUs of energy) can pass through the window assembly or section. Windows are intended to be good conductors of light, but unfortunately they are great conductors of heat as well.
The higher the U-value the more heat is able to pass through. The lower the U-value the more efficient a window would be at keeping heat in or out of a home. A single pane window has a U-value of about 1.2 (or an R-value of .83) and a good, insulated, double pane window could have a U-value of around .53 (R1.9).
Care needs to be taken when comparing U-values, because they are not always standard. Some U-values refer to only the glass, and others refer to the entire window or door structure including the framing and glass. An R-value is a measurement of thermal resistance. The R-value is the inverse of the U-value. If you have the R-value of a material, you can determine the U-value with the formula U = 1/R. Likewise, if you have the U-value of a material you can determine the R-value with the formula R=1/U. For example, a window with an R-value of 0.5 would have a U-value of 2. Thus, R values can be thought of as the opposite of U- values. The higher the R value, the better the material is at resisting heat transfer.
