One year in university, I had an uninsulated bedroom. (It was intended to be a sunroom so had lots of windows. It was kind of like winter camping with walls and a roof to cut the wind and snow. That was a really cold winter. A glass of water on my bedside table would freeze overnight but I was super toasty in my bed with my mattress heater. It was a gift from my grandparents and over 20 years ago, so I have no recollection of the make or model but, man, it worked really well, as long as I didn't drink too much before bed.