I have installed double pane IG sliding glass doors in my own home, and over the summer did not note wide disparities in glass surface and frame temperature. I foamed the gap between the frame and the studs before installing exterior insulating foam sheathing and trim/siding. The key is not only to reduce heat transfer via conduction (insulation) but also via convection (air movement). The foam accomplishes both. Stuffing fiberglass batt insulation into the gap between window frame and stud faces is NOT the way to go, as fiberglass insulation does little to stop convective currents if the cavity it is installed in is not air tight.