And with energy prices at their highest levels in years, it's important to get the most for our heating dollar. Let's start with the idea that Lori is considering. She's on the right track. Air can be used as an insulator.
Storm windows are important because they create an airspace between the two windows.
That pocket of air will keep the cold air out and the warm air in. You can use sheet plastic on the inside of your window to create airspace. It works whether you have storm windows or not. You can attach the sheet plastic to screen frames, cardboard or wooden frames, or even simply tape it to the inside of the window opening. Another option is to hang blankets in front of the windows.