Low-E coatings on glass control radiant heat gain through insulated windows.  Windows manufactured with Low-E coatings typically cost about 10% - 15% more than regular windows, but they reduce energy loss by as much as 30% - 50%.  Although Low-E coatings are usually applied during manufacturing, some after market films are available for do-it-yourselfers who wish to retrofit existing windows.  These films are inexpensive compared to total window replacements, last 10 – 15 years without peeling, save energy, reduce fabric fading, and increase comfort.