Basically. You are forgetting about the importance of a coop design that provides copious ventilation as well as draft protection at roost level. Those are critical parameters that permit the birds to keep themselves warm.So basically, if I understand correctly from you guys - there is no need to worry about any heater in coop - it is all about natural immunity of chickens against cold, sufficient light, and fresh pullets every year (which I don't think I am ready to replace), even in -20, -30C. I get it. So basically those are the reasons not to get an adequate heater - no need?
No, you don't need to provide supplemental heat in those temps.