But that's outside, not inside. Even an old building with lesser insulation and double-pane windows will hold a higher temperature when inhabited. Humans radiate more than a few watts. It's not comfortable, it's definitely a health concern, but it's still not 0°C.

An anecdote from earlier this winter when the electricity prices peaked in Europe: my girlfriend and I live on 79 sqm in a modern apartment building with triple-pane windows, and because of the landlord being a stingy d*ck we went through several weeks without any heating. During the worst 7-10 days we had a temperature of -4°C outside, but never below 15°C inside. When the temperature rose to 5°C we had about 18°C inside.