It was -5 here this morning and 8 above right now. I have bantam crosses buff orpington and welsummer hens in my main coop, More buff O and Welsummers in a smaller coop and 4 roosters (standard sized) in a small third coop. Since bantams are not noted for being particularly fond of cold temps (mine are mainly OEGB) I moved all my extra cockerels into the main coop to help provide body heat, made sure there were no drafts (plenty of structural ventilation going on) locked everybody inside and checking frequently during the day. So far everyone is fine. No frostbite. A bit of attitude but mainly because they want OUT AND THEY WANT OUT NOW!
I'd keep an eye on the boys for frostbite, grease them up with Vasoline if they show signs and like everyone is doing, pray for spring or at least a break in the cold wave.
As along as they have shelter, no drafts and can hunker down together on a roost they will do fine.