Frugal and Energy Efficient Living

or recesses?

Do you have pelmets over the windows? Heavy curtains? Without, natural convection will be constantly moving your heated air across chilled glass.

Make frames that are snug internal fits for frames or window recesses and instead of plastic wrap use the same heavy vinyl that's used for weatherproof patio screens. (It may well be cheaper to buy the screens and cut the zippers off than purchase the material off the roll.)

And just thinking about that idea. Move the frames to the outside and permanently affix them to the walls. This time don't cut the zippers off, but instead sew matching zippers onto 90% shade cloth. A batten across the bottom will stop the wind geting underneath. Windproof in Winter, and screened for light and insects in Summer.

Also what floors do you have? If boards, then underfloor insulation is a must.