Bubble wrapping my single pane windows has made a massive impact on retaining the heat in my house. If you haven't done so, I absolutely recommend it. Clean windows, spray with soapy water, face bubbles toward outside and smooth out. I wish I would have done it 2 years ago.
@Embers embarrassingly, when I cleaned the windows, I had to question whether I had ever cleaned them since I moved in 2 years ago. They were so dirty, it's a miracle i could even see out of them. Lol