Thanks. I plan to shut the pop door. My concern is I only have 5 hens in a very large space (140 sf) with tall ceilings with lots of ventilation very high up. So any body heat will just go straight up and out. Thinking of hanging something inside to 'lower the ceiling' from a heat point of view.No, I don't provide heat.
At about 10°F, I keep the people door shut (it's usually open all day). When it gets closer to 0°, they don't want to go outside, so I give them a bowl of snow to go with the bowl of feed in the coop and shut the pop door too.
The only frostbite I've had to deal with was on my Buff Orpington's comb and wattles, and it was fairly minor.
The openings in the coop are covered with air filter material, so there aren't breezes blowing through. I have 10 hens and about 62 sf in the coop. There's a thick bed of pine shavings (mostly shoved to the back) and the make nests in it.
I'll check on them every couple of hours when it gets that cold.