Interesting.
When I use to backpack in the winter we used a four man tent. There was the tent and then there was a fly over the tent it's self.
Believe it or not we lit a collapsible candle lantern and along with the temps of our bodies kept the temps in the tent OK as long as the wind wasn't blowing to hard. We never had the candle on if we were not awake.
When it was in the single digit range we packed snow or leaves up along the base of the tent and a small ways between the fly and tent.