The first house I bought many years ago had single pane casement windows built into cinder block walls. The windows would frost in the winter so I built interior wood frames to attach sheets of thick plastic that helped insulate the windows. The house was warmer but I could barely see out the windows!
Our current home was built in 2012 using modern Energy Star rated double pane windows. I've been surprised that these windows still allow cold air in the house. So I went shopping at our local hardware store thinking I would need to build opaque plastic frames again.
Luckily, the store recommended a new product using clear shrink film made by Frost King which is very easy to install and so clear that I have not needed to use a hair dryer to remove wrinkles. The product comes in various sizes and quantities - with an average cost of $1.50 per window. I'm already amazed at the qualitative results feeling our home much warmer than before!