There is a looong thread somewhere on here about cold weather coops, with a lot of input from people in Alaska.
Our weather here isn't as cold as yours, we seldom get down below -35 C (that's, what, minus twenty-something F?) but we do have a number of days when the high is below -18 C (0 F).
Your best bet is to have the chickens in an insulated/ventilated coop within a large outbuilding, like a barn. The large structure, even though unheated, will considerably buffer the outdoor nightly lows.
Failing that, I would suggest building an AMPLE SIZED coop (see below) and insulate it as heavily as finances permit. 6" stud walls, filled with insulation, are great, they offer nearly twice the insulation value of normal 4" stud walls. And insulate the ceiling real well too. See my ventilation page (link in .sig below) for discussion of designing and using ventilation for the winter. Insufficient ventilation causes humidity, and
humidity (much moreso than cold per se) is what will give you frostbitten chickens.
There are three reasons IMO for building an AMPLE SIZED coop, like as big as you reasonably can. First, larger buildings are much more thermally stable. They contain more thermal mass, more air volume, and proportionately less surface area. This means they will tend to hold their heat better, on a real cold night, than a small coop would. The difference between, like, an 8x12 coop and a 4x4 tractor is considerable. If the coop can be within a larger building, or even just attached by one wall to it, then you get even more benefit.
Second, when it is -40 out, your chickens are not going to be spending lotsa time outside. And winter confinement stress easily leads to picking and cannibalism, which habits are not always curable even by reducing flock size once theyv'e started. You really want to give them as much living room as possible. Winterizing the outdoor run so that it is largely protected from the elements will help too. Personally I allow about 15 sq ft per chicken indoors (plus run), and would not go below 10 sq ft per chicken indoors. The less you allow, the poorer your odds, although of course some of it depends on luck.
And finally, no matter *what* the climate, it is cheaper to build a larger coop now than to decide two years from now that it's not big enough. Adding on is not always possible, and awkward at best.
At this point, you or someone else reading this is probably thinking "Oh, no, you have to have a small coop for cold winters, to concentrate their body heat". This is NOT TRUE AT ALL. There is no such thing as too big a coop. If you find yourself wanting to concentrate their body heat, it is easy to knock together a temporary roost box or hover or partitioned 'coop within a coop' type arrangement, so they have a smaller warmer area to retire to but still have the use of the whole indoor space. REally truly.
You might also do a 'search' on popcan solar heaters and other passive solar schemes. For instance I plastic-wrap my very small lean-to front run, and then by opening the window and popdoor inside it, I can get a considerable solar-heat boost. My arrangement does require manual opening and closing, but, hey
Good luck, have fun,
Pat
