I'm in southern Australia, the lowest temperature we get here is -5 degrees C, I know that's nothing compared to what many get, but here's my two cents worth:
Our coop is insulated and draft free. It has insulation in walls and roof and a green roof for summer heat.
Because they're cooped up, and don't have access to dust baths, I make them a box and put the sifted fines of woodash and our mountain soil in it. (the soil is clay based and will help dry out any mites etc, but you could also use DE.) We get a lot of rain here, and between the wet and the cold they love it, and use it often; I hope it will help them with mites etc over colder months when immune systems are a bit challenged.
I make them a warming and nutritive mash, and include seaweed, nutritional yeast, stone dust and shell grit, crushed corn, sprouts ( i grow in the warmest room in the hosue), sunflower seeds (oils for feather production esp in autumn), flaxseed oil, tahini, flaked oats, seeds I have spare, coconut, some dried fish, kefir, kombucha.. ( really whatever's spare in my pantry..), and always a clove of garlic and 1/4 tsp of chili seeds and a pinch of turmeric, per bird. And usually a grated carrot and beetroot. The garlic/ chili/ carrot/ turmeric is good for worms but the chili will also help raise metabolism, and garlic good for immune function. In summer I chop lots of fresh herbs into it.
Wicking, perpetual grazing bed: I have made my girls a vegie box (mines 1.2 x 2.4 metres), for 4 chooks. The soil level is about 10 cm ( 4") below the top of the box, which is covered with 1" square galv weldmesh. They can walk on the weldmesh and eat greens that grow through, this means plants hould have enough root systems not to get pulled up, and the trimming effect gives extended harvest. Grow the cold hardiest greens you can (rye, lupins, beans, brassicas, comphrey or whatever is local and green in cold weather), and you could grow under lights or heat, if needed. Our box is outside, with walls of concrete blocks to provide some thermal mass on the days when the sun is out. Or at least get an earlier start on the growing season.
You could think about incorporating compost heap proximal or under perch or something, I've seen systems in the UK where they put the compost/ manure under glass and use the heat given off to grow seedlings... ??