You did not say specifically where you are located.
My reference to temperatures were absolute NOT including wind chill. When I read your post I realized mine may not have been clear.
Good luck!
I live in Canada, average winter temperature is -15C/5F some periods as low as -30C/-22F.
Chickens do not need any additional heat in the colder months. If you add heat and have an electrical failure your chickens can be severely impacted negatively as they are not conditioned to the colder...