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.
Solar Heat Gain Coefficient (SHGC) - SHGC indicates how much of the sun's energy striking the window is transmitted through the window as heat. As the SHGC increases, the solar gain potential through a given window increases. In temperate and cool temperate climates such as Sydney, Perth, Melbourne, Adelaide and Hobart, northerly glazing should have a high SHGC. This is standard passive solar practice for temperate climates.
SHGC is expressed as a number between 0 and 1. Colder climates need a high SHGC.