I live in Upstate NY and we get subzero temps. As long as it's not snowing, or the temps are in the low 20's my hens like to go out. That said, we have snow and low temps a lot. I have a tractor and have 5 hens ( asked for 4, 5 were sent ) so I am at the absolute min. sq. ft. and they did get nasty with eachother by the end of the winter. If I could afford to do it all over again, I'd trash the tractor idea and just build it a bit larger than they need for the many days they may spend inside. The tractor isn't any cheaper to build in my opion and there is less cutting of wood, etc. I also put in the insulation board and that helped out a lot. The key as I'm sure you've read already is keeping out the drafts.
Have fun with your new birdies.
Have fun with your new birdies.