As long as they are not in a direct draft and you have decent ventilation, chickens don't even notice 20*F weather. They are wearing a down coat after all. The problems come in when they are in a direct draft (wind chill is a real thing) or the ventilation is not good so humidity and ammonia from their droppings build up. You get good ventilation without having them in a direct draft by having your ventilation openings higher than the roosting chickens.
It was 8*F when I took this picture. That is ice on the ground.
It was 8*F when I took this picture. That is ice on the ground.
