Cheap insulator
Insulating glass windows is crucial for staying warm in the winter. You can also insulate doors, especially larger ones like garage doors. The garage is usually a very cold place and can lower the temperature of the whole house.
Prevent drafts from entering the living room and bedrooms by insulating the garage windows and doors. You can do this with store-bought insulation or things in your house, such as heavy curtains or other thick blankets and rugs.