Here in Maine I have NEVER insulated or heated my coop. These birds will grow enough feathers to insulate them. Keeping the coop to air tight will create a over load of amonia fumes (my belief), creating health issues. For years I had a chicken house with aluminum roofing (no insulation), it was 5 feet high (to keep the heat down into the coop) and wide boards. In the fall I would put plastic over 3/4 of the outside walls to stop drafts and NEVER lost a single hen.
Now I have a shingled front and where its "in a shed" so it has tar paper around the back side of it to stop a draft.
If your power goes out, you must realize that this is when your flock is at its highest risk of freezing to death. You are killing them with kindness.
I can see if you heat your water or leave a light bulb on to lengthen days, but my power goes out several times a winter, often weekly and I want my girls to know how to suggle up.
Oh, hand I have about 2 1/2 ft of clear plastic for the human door so that when I go in, not all heat is lost when I open the door. This is also another reason the "deep litter" method is best for Northern climates.
Good luck on your choices!