We have been sitting ~20 below zero (F) for the last few nights, and I am seriously contemplating popping another hole in the wall for more ventilation. The reason is that while 26 chooks have been able to keep the coop around freezing, they are putting so much moisture into the air that I needed to scrape nearly an inch of frost off of the
inside of the windows. That means the moisture is not getting out and is endangering them.
Normally I open the door and let them have a little more ventilation during the day, but the last snowstorm left a mess of ice and such in their run that I haven't been able to clear out yet. Without that extra ventilation, the windows went from lightly frosted to what I mentioned. It was so thick that when you went inside during the day, you would have sworn it was dark outside.
That much moisture can lead to serious frostbite and lung problems. It goes against what you might think by suggesting you allow more ventilation, but all of that weatherstripping and caulking is stopping your air flow and could lead to moisture problems as bad as mine. Just because we caulk and weatherstrip our houses doesn't mean that it is the best thing for the coop. You actually want the air exchange without any major drafts. Little drafts through a crack under the door or along the roof line are not the dangerous ones that people warn about. Remember, moving air will take moisture with it, and the air that comes in from outside will be drier. Drier air, even at a lower temperature, will allow the chickens to stay warmer because they will not be dampened by humidity. Experiment for yourself...A wet hand in cool air feels colder than a dry hand in colder air. Moisture wicks heat away faster than does air.
To prevent water from freezing, use an electric model or put a heat lamp over the top of the water can and it should stay liquid. The temp inside the coop was 15-20 degrees today when I got home from work and the water was fine. They handle themselves overnight without any extra heat aside from the lamp over the water, and I know it gets colder in there at night. As a bonus, all of their poop freezes, and frozen poo doesn't stink!
Good luck!