Gosh, I'm probably going to ruffle some feathers, here, but I'll add my .02. We did insulate our coop, but think it is even more important to stick with breeds that can cope well with a cold climate.
Since you don't yet have chickens, you have plenty of time to give some thought to breeds. You might consider roundish bodied, well-feathered breeds with small, thick combs. They should cope well with Michigan winters. Those with single combs can suffer frostbite even in an insulated coop, so we avoid those breeds (we live in the northeast). Most of ours have rose or pea combs. Silkies might be more uncomfortable in the cold because their feathers don't insulate as well.
Winter hardy animals can cope with a wide range of temperatures, as long as the temperature gradually changes and their body chemistry has time to adjust. This is what normally happens as the seasons change. For this reason, we believe that heating the coop is unnecessary, and we don't do it.
I have mixed feelings about crowding them into a small space in order to allow body heat to keep them warm. Crowding also predisposes to cannibalism. We have occasionally had problems with feather picking, but only in winter, when they all stay in the coop to avoid the snow and wind. It seems to help to reduce crowding, even though fewer bodies in a given space means less heat.
In short, I don't think keeping them warm is hugely important, as long as you have winter hardy birds not susceptible to frostbite, they have excellent protection from wind, rain, and snow, and plenty of space to allow them to avoid conflict.