The U-Factor of the window accounts for all the non-solar heat transfer of the window.

At night and when the window is shaded, it is the U-factor which defines its rate of heat flow.

This chart shows that that there are significant benefits from an improved frame. The U-factor is lower with a thermally broken aluminum frame than with solid aluminum. A vinyl or wood frame window has an even lower U-factor. Adding low-E glass reduces (improves) the U-factor even more, and this is true for windows of all frame types.