If the air inside the coop is very humid (either b/c of poor ventilation or poor coop management, or because you are in a period of prolonged very wet cold weather and the *outdoor* air is persistantly humid *too*) or the chickens are in a persistant breeze they can't get away from, then you can get frostbite at not much below freezing.
Under normal conditions i.e. basically-dry draft-free air in the coop, a few roos with very large or elaborate combs (think Andalusians, Buttercups, that sort of thing) can have trouble not much below freezing, but well-chosen breeds are typically fine well down towards 0 F, many of them considerably colder than that.
You just sort of have to keep an eye on them, see how it goes.
Good luck, have fun,
Pat