Curtains can be crazy good insulators. I lived with my grandma after grad school, and she liked to keep the house at 80. I am warm most of the time anyway, so that wasn't gonna be good. I also had to take the door off my room (only location for cat's literbox was where the door swung), so I hung up a privacy curtain. Ran my portable AC in my bedroom and my room and the rest of the house were SIGNIFICANTLY different temperatures, with very very little gradient between. You'd pull the curtain aside and step into a fridge. It was beautiful.

Also I just hung my curtain off of a couple command hooks, easier than mounting a whole curtain rod.