I got the thick 4mil, does it work better with a space between the window and film? My neighbor puts the film right on the window, does that help? I think a still-air space would work better. How much does it really help, 5%?
You are correct that an air-gap would provide much better insulation than the film directly in contact with the window, and, I would expect it to improve your heating expenditures by no more than about 15% or so.
yes the air gap will help because air is an excellent insulator, however, make the gap very small, because if you make it too large then you can get lots of convection(where the air can circulate inside the gap) which will significantly lessen the insulation qualities. This same idea is used in regular fiberglass insulation, the air gaps are created by very small holes in the insulation, and these small holes also help to keep the convection down.
