OP is asking about temps when chickens are out in the elements not inside coop.
I'm also curious what others say.
Deep litter in the coop. I have found that if the outside temp is too cold, my chickens will choose to stay in the coop for the majority of the day. Their instincts keep them alive. When they do venture outside, they stay in the sun. Mine aren't fond of snow so we built a roof outside the coop that offers a bit of protection from the elements.