Have you hung curtains?
Plastic is good but if you hang heavy curtains over the windows as well it will be far warmer.
I know curtains are not considered fashionable any more but those itty-bitty valances don't stop drafts!
I hate valances! Well, I have a lace "swag" over the kitchen sink for summer. Don't like venetian blinds either. We have curtains but not heavy ones. Yet. Being an old house, front and back are double windows (76 inches across), sides are single. That's a LOT of curtains. lol
I love the place. It's a classic but Mr Cheap didn't take care of it.