I am in the midwest where we range from anywhere from -5 to 30 F in the winters and I have an unheated coop w/a dirt floor now, just as we had when I was growing up. I have a thick layer of lay and pine shavings on the floor and I think they girls are fine. I empty the water when I shut them up at night and refill it with fresh every morning (not as much ice breaking required that way) although I have thought of getting a water heater. I have been sorely tempted to put the red heat lamp I used to brood them in with them this winter but I'm resisting unless it gets below 0.
