By building inexpensive frames and using Astrofoil Build it solar says they were able to save a 10 degree drop in indoor temperatures at night. That could add up to some comfort level and some pretty good savings.
Insulating Window or Door Shutters Using Astrofoil Reflective Insulation
http://www.builditsolar.com/Projects/Conservation/Astrofoil/AstrofoilShutters.htm
