Windows can be a source of free heat on sunny winter days. Open curtains and blinds on south-facing windows in the morning so sunlight can warm your rooms naturally. Even on cold days, direct sun can noticeably raise the temperature of a room, which means your heating system has a little less work to do.

Once the sun goes down, close those same curtains to help hold heat inside. Heavier or insulated curtains can add a useful layer between the cold glass and your living space. If you have older single-pane windows, this habit can make an especially noticeable difference in comfort.

For drafty windows you rarely open, clear plastic window insulation kits are an inexpensive option. They create a thin layer of trapped air that reduces heat loss, and they can usually be removed in spring without damaging paint. It is a simple step renters can often take too, though it is wise to check your lease first.