When looking at different models of windows you want to be cognizant of the rating that the manufacturer applies. That rating will either be a “U Value” which gives an indication of that windows tendency to transfer heat either in or out, or by an “R Value” which describes the windows resistance to transferring heat in or out.
U-Values are a measure of heat conductance
R-Values are a measure of heat flow resistance
If a window's R-value is high, it will lose less heat than one with a lower R-value. Conversely, if a window's U-value is low, it will lose less heat than one with a higher U-value. In other words, U-values are the reciprocals of R-values (U-values = 1/R-value). NOTE: Most window manufacturers use R-values in rating their windows.
Usually, window R-values range from 0.9 to 3.0 (U-values range from 1.1 to 0.3), but some highly energy-efficient exceptions also exist.
When comparing different windows, you should ensure that all U- or R-values listed by manufacturers: