We're in Wisconsin with silkies and frizzles and for eight years have heated our coop enough so it wouldn't freeze, so to about 40F. They come out when it's 15-20F though and no windchill. We do that somewhat for the chickens, but more so for us, so no frozen water, eggs, or wattles this way.
For the other pens, they each have a hutch, and we use a radiant panel heater (Cozy Coop) in each as the one pen has young ones in it.
I always recommend if anyone is hung up on heating or not heating and a bit worried, to just get one of those Cozy Coop type heaters. Those are safe, shut off if they tip over, they won't heat much of an area, but if chickens are suffering, they can sit/stand next to it.