You don't know what a draft is, in a coop. A draft is a cross breeze, that will ruffle the bird's feathers. This coop is built tight. No cracks in the corners, no gaps up by the roof. When the coop is in winter mode, with all the windows, and the entry door shut, there is no pathway in the coop for the wind to blow through. Even with the wide open front wall. I've had 35mph+ winter winds blow directly at the open front. You go inside the coop, and shut the door, it's as calm as it is in your living room watching TV. In the book, that is described as the air cushion effect. I get snow down here too, and no matter what it does, it never amounts to anything in the coop. Nothing ever worth worrying about.
If you were to have say, the eaves open, all the way around the coop, plus a ridge vent, I would bet that you would have more felt air movement in that coop, then mine. I'd bet $$$ on it. You would be providing that pathway through the coop. Could get drafty.
You keep chickens in a poorly ventilated box, with the misguided idea that by trapping "BodyHeat" you are doing them a favor. You are setting up a high humidity environment, combined with the cold, you will have frostbite, and probably respiratory issues. I've had winter temps to just below zero, not including any windchill, and I've ( Really, my chickens) NEVER had any problems what so ever, NONE.
Again, read the book