Foil as a heat barrier is called radiant barrier. If applied right it will reflect a large part of radiant heat. To work well cut the drywall to almost the exact size of the window opening, then cover the outside face of the drywall with the foil shiney side out to the world place it in the window, and make sure that there is atleast 1/2 inch to 1 inch space between the foils and the glass. For radiant barrier to work it needs to have an air space ot it becomes a conductor of heat rather than a reflector.

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