Chickens are really cold hardy and are most comfortable with the temperatures below 60 degrees. Most seem to feel frisky and happy when temperatures are about 30-50 degrees. Chickens suffer more in hot weather than cold and are perfectly designed to hold heat in against their bodies. Feathers are great insulation. We get pretty cold here and my chickens do fine in a pole shed for a co-op with a west facing door always open.
You should still encourage them to go inside for protection from precipitation and drafts, and predators, just make sure the coop isn't closed up and is well ventilated. Keep the air dry and fresh and the chickens will take care of keeping themselvesself warm.