My Bielefelders have had no problem with the cold. They are laying just as good now as they did when it was warmer. None have died or had weight loss problems. They have a metal coop to go inside, it has no insulation. Temperature wise it is a couple degrees warmer than outside. The water still freezes inside the coop when it is cold.
My younger ones from a week old to a couple months grow a little more every day. They are in a brooder. Only the smaller ones can get underneath the box with the heat lamp. Same deal, water freezes inside the brooder when it is real cold.