First I would say that provided your coop is wind proof and draft free the wind chill does not count towards the temperature your chickens experience, they are not in the wind.
I have found my birds sometimes seem uncomfortable when it is below zero F. If it is below 0 and windy, they will choose to stay in their coop for most of or all of the day. I check on them often and bring them food and water in their coop (not the norm for me, I generally keep food and water in the run), and watch and make sure they come down from the roost to eat/drink and are active within the coop. Being active helps keep their body temperature up.
Every chicken is different, and while my flock has done ok without heat/insulation, it does not mean they will always be fine and it does not mean that your flock will be fine. I did lose a young (maybe 8 week) chicken once that I attribute to cold and the stress that put on her. Some birds are more sensitive than others and weak for various reasons, maybe they are old, maybe they have had an illness in the past or respiratory problem that surfaces when stressed, maybe they are a tiny frizzle bantam, you name it. The most important thing is to check on them often and learn the behavior of your birds. What is normal for them? What is not normal for them? A lethargic bird is not a bird that is healthy and strong. Let the flock dictate when they may need heat. If they need heat, your goal should not be warm, like the inside of your human house, your goal is warmer than the outside air temp. Maybe if it's well below zero you want you coop to stay at at least zero. There are passive ways to help keep the coop warmer... a dark color roof, water jugs in the coop, more mass will retain more of the sun's warmth and take longer to cool off at night. Also collect eggs often, they will freeze solid and split open pretty quick at those temps. When the ground is frozen, provide a dust bath.
I have found my birds sometimes seem uncomfortable when it is below zero F. If it is below 0 and windy, they will choose to stay in their coop for most of or all of the day. I check on them often and bring them food and water in their coop (not the norm for me, I generally keep food and water in the run), and watch and make sure they come down from the roost to eat/drink and are active within the coop. Being active helps keep their body temperature up.
Every chicken is different, and while my flock has done ok without heat/insulation, it does not mean they will always be fine and it does not mean that your flock will be fine. I did lose a young (maybe 8 week) chicken once that I attribute to cold and the stress that put on her. Some birds are more sensitive than others and weak for various reasons, maybe they are old, maybe they have had an illness in the past or respiratory problem that surfaces when stressed, maybe they are a tiny frizzle bantam, you name it. The most important thing is to check on them often and learn the behavior of your birds. What is normal for them? What is not normal for them? A lethargic bird is not a bird that is healthy and strong. Let the flock dictate when they may need heat. If they need heat, your goal should not be warm, like the inside of your human house, your goal is warmer than the outside air temp. Maybe if it's well below zero you want you coop to stay at at least zero. There are passive ways to help keep the coop warmer... a dark color roof, water jugs in the coop, more mass will retain more of the sun's warmth and take longer to cool off at night. Also collect eggs often, they will freeze solid and split open pretty quick at those temps. When the ground is frozen, provide a dust bath.