Aluminum Foil will reflect heat from your windows to keep the heat inside your house.
During wintertime you can put some foam insulation inside the window frame and then put some aluminum foil over those drafty windows that you're not using.
Then the heat will hold better in your house over the wintertime and your house will be warmer and your heating bill will go down.
I had to do this with my old drafty windows in an old house I was renting that had terrible drafts.
The house was over 100 years old with poorly maintained windows.
So I bought some foam insulation and cut it to fit into the window and then stapled some aluminum foil onto the foam insulation.
The heat is then reflected off that aluminum foil which helps hold the heat in.
Aluminum foil is metallic and shiny which makes it a great heat reflector.