Sorry for butting in, how are your girls doing? Mine are not laying and not eating a ton which I assume is normal with the short days. They huddle together quite a bit with the cold in the coop at night. I have straw in there but I wonder if I need to do more. This is their first winter and they are 10months old.
The key for chickens is DRAFT FREE coop with TOP VENTILATION. They poop a lot as they sleep which leaves a lot of moisture in the air. This can lead to frostbite unless the warm moist air can rise and exit the coop at the roof line. If you get those two requirements and your chickens are healthy with plenty of food and water, no need to worry they will be fine.