Why is the window U value different to that of the glass?

The U value for a window is the combination of the U value of the glass and the U value of the frame. The frame is made of a different material to the glass so will have a different U value to the glass. The overall U values of window system using aluminium, timber or PVC frames for example will all be different even if the same glass is used in each case. Because aluminium frames have a higher U value than those of either timber or PVC, the overall U value of an aluminium system is higher and as such is less effective as a thermal insulator.