I've slept in a car (a VW bus with the seats down to make a flat space) without the heater running, in below-freezing conditions (maybe 10-15 degF) with another

I've slept in a car (a VW bus with the seats down to make a flat space) without the heater running, in below-freezing conditions (maybe 10-15 degF) with another person. It was actually pretty amazing how well it worked. We had sleeping bags, but woke up to find the interior of the car quite warm. It actually seemed downright cozy and luxurious compared to sleeping in mountain lodges where it's typically miserably cold in the sleeping areas outside your sleeping bag. My takeaway: this experience seems to suggest that a few people in a small space that's wind-tight and maybe has a bit of insulation (clearly not great in a car with all that glass) can keep things reasonably warm with only body heat...