I use heated water bowls, but no heat in the coops. We got as low as -26 one year and I had no losses, even with small bantams and leghorns. Frostbite was minimal, just a little on the tips of a few of the largest combed birds, but it healed without any intervention. I see too many coops burn down each winter with heat lamps so I'd really think hard before using one (heck, I don't even use one for baby chicks, I have an ecoglow)..