I have 3 speckled sussex and 1 golden comet. They have a small sleeping house (which I think now is badly designed, but this is my first time with chickens) and a run that it sits in. Their run is mostly encased in plastic but I think some draft still gets into the sleeping house and because I put the pirch too high the draft may be on the chickens. It couldn't be much, but still a draft is a draft. They have been sleeping out there in this cold winter and are obviously still alive but it does appear they may have some frostbite on their combs and wattles. This makes me so sad because it was my job to protect them and I didn't build their coop and run for such a cold winter. We haven't had a winter like this in ages, last I remember it being this cold was 15 years ago! So needless to say I did not design this with such cold weather in mind. The sleeping house is small and there's no way I'd put a heat lamp in it, running an extension cord for it as well, uhg.
Anyway, on one night it was forecasted to be -15F with a windchill of -28F, I decided that was too cold even for very cold hardy birds like sussexes, when there's only 4 of them to keep each other warm. I set up a large dog kennel I wasn't using in my kitchen, put some carefresh in there and brought them all in. I covered it with a flannel sheet so they wouldn't fling any poo in my kitchen. The temp ended up being a -18F. That's ambient air, I don't know what the windchill was and even though they are mostly out of the wind when in their house, I did think that that was just too cold for only 4 hens to try to stay warm in a small drafty house.
I'm not sure where the line ought to be drawn, though. Tonight is supposed to be another very cold night. Not forecast to be quite as cold as the night I brought them in, but I wonder what the line is. I'd hate to go out there and see one of them dead. I know everyone says they're hardy and were bred to be in the cold, but the fact is animals do freeze to death (or, when it's hot, die from heat stroke) even ones that were bred to stand it. I figure I'd rather be safe than sorry, you know? Poor chickens already have frostbite it would break my heart if they froze to death out there.