We ending out getting down to -29. One coop I put a small heater, which kept the temperature at zero. They may have been fine without it, but I have some that are molting in there so I was worried. The other coop is a very large hoop coop and there is no way to add heat. Those birds were fine too, albeit with a little frostbite. We've already had -25 this winter, so mine are fairly acclimated. We used to get -40 at least once a winter, but that was before I had chickens, and I wasn't sure what the lowest tolerable temperature is for them. (I still don't know, actually, but -29 is okay).