For getting down to -40, the best two things you can do IMO are a) make the coop as LARGE as possible, with a slab floor, for thermal mass; and b) build it with 6" stud walls (not the usual 4" studs) and insulate the bejeebers out of them and the ceiling both.
I have a 15x40 coop of the above description; our winter lows get down to -35 C, with -20 to -25 C being typical. With minimal ventilation (and it doesn't need much for just a dozen or two chickens; it'd need more if you had it more packed) the building doesn't get below about -8 to -10 C (that's like 20 F). I run a small greenhousified leanto run as a solar heater during the winter, more for my own amusement than because it's needed; this gives a boost of about 5-10 F on sunny days, and presumably contributes *some* to keeping the coop warm at night (thru heat stored in the internal structure of the coop) but the building never did get below 20 F even *before* I started using the solarized run, either.
Good luck, have fun,
Pat