On a sunny day it can feel downright balmy in there.
It's all a big experiment, of course. We don't know how much the greenhouse will protect the plants from the cold, especially if it gets really cold. The glass is single-paned and there's no heat except for the winter sun. Still, it should protect from wind and ice (like sleet and freezing rain), so that's a good thing. There's a thermometer in there that we check nearly every day to get a feel for how cool it is. So far it's been a few degrees higher than the lowest of the morning temperatures outside. That could make a difference.