I kept a bit of heat in the coop through the coldest spots, it was maybe -10 in the coop at the coldest, -20 outside. But the broodies and chicks did fine there. We've been pretty lucky with the temperatures this winter from a chicken perspective, it's been a lot warmer than usual. But as long as you can keep water liquid for them, I think they'll be fine. Make sure they're eating though, we just lost a broody mama who had neglected that part.