U-value measures how readily a window conducts heat. The lower the U-value, the greater a window’s resistance to conductive heat flow and the better its insulating value.
Note that it’s a ‘whole-window’ measure: all window performance measures in Australia include the effect of the frame and edge of the glass as well as the main central glass area.
Real-world U-values range from about 8 (worst case, single-pane glass) down to 1 (best case). In almost all cases, a low window U-value is better in all climates.
U-value is the inverse of R-value, the window’s resistance to heat flows. So, for the range mentioned above of U-values from 8 down to 1, the equivalent R-value is 0.125 to 1. However, most windows never achieve a U-value much lower than 2, which equates to an R-value of 0.5.