In the winter, there are about 16 hours a day when windows aren't performing their intended function. Instead, they are an enormous heat drain, which provides a good reason for finding ways to insulate them. These windows gain more heat during the day than they lose at night. Night insulation is very effective. Solar heat is gained through east windows during morning hours and through west windows in late afternoon.
North windows receive no direct sunlight and are always losing heat.
All window coverings closed all hours the air conditioner is operating. If air conditioner is not in use, close those windows and window treatments receiving direct sunlight. Use shaded windows for ventilation. Many insulating treatments incorporate substantial quantities of plastic foam and films and synthetic fibers. If exposed to extreme heat or flame, these may constitute a smoke and fire hazard.