My best insulated window
Eight or ten years ago I put insulation in our little used dining room /office window. This has been the best window insulation I've tried. And believe it or not, the curtains are lace! Other methods have led to condensation and mold.
The window jambs are painted wood, the sashes are painted metal (not sure if aluminum). There is brush where the upper and lower sashes meet, but the brush is worn, allowing more air passage. There are nine inches between the window pane and the front edge of the sill, which gives me a lot of room to put insulation.
I'm examining the layers today because I'm washing the curtains and room-facing layers of fabric. Here is what's in use, starting at the glass and working toward the room.
BUBBLE WRAP directly on the window. Not adhered with water as often recommended, but lightly cello-taped.
BEDSHEET cut to fit the space and hung with tension rod.
MYLAR reflective emergency blanket, carefully sewn to the sheet. I think I put a band of tape down to put stitched through so that the mylar would not rip.
BUBBLEWRAP ROLLS collected from mailing and packaging. Stacked and stuffed passively into the space, one at a time as acquired.
2 SPUN POLYESTER BLANKETS cut to size
BEDSHEET --this sheet and the two poly blankets are stitched together at the top and hung on a tension rod. The weight and grip of this sandwich is what hold the rolls of bubble up.
LACE CURTAIN suspended from old-fashioned traverse rod mounted on the outside of the window frame. These are permanently closed and just finish off the space visually.
All the materials hang so that they just touch the sill, except for the curtains which skim over it and fall below.
I do have to wash these curtains once a year though, because in a very small house with no ventilation from the kitchen cooking, the curtains do get dirty. But today is the first time I will have washed the first layers of fabric below the curtains. They are dusty and it would have been good to have done it sooner.
Anyway, this has worked really well for a window that never needs to be used.