I have BO, PBR, Australorps, and RIR. our winter this year was freezing and below -0 .. with windchill some days -20. My birds coop is the entire barn. They free range so no doors. They are all healthy and well. They stopped laying eggs for the first time ever this winter for 1.5 months due to freezing temps. Warm oatmeal breakfast and warm water is a treat during this time. Other then that they are obvious very cold hardy birds. So I can't say what is too cold.. some breeds are hardier then others.