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what? Keep chickens at 90 degrees? My RIR's and Sumatra's don't have any insulation or heat. The RIR's have a draft free coop, and the Sumatra's...being Sumatra's refuse to be cooped and live in a spruce tree. In Upstate Ny it is normal to have zero degree nights, with 10 degree days, for months on end,and I've NEVER lost a chicken to the cold. We had one night this year that was 30 below zero F. Everyone was fine. I think of it this way. A wild bird does NOT have the option of a t stat. They grow feathers to protect themselves and huddle together for warmth. Now if you have a lightbulb set to keep the temp at 90, and that light blows on a very cold night, then your chickens won't be prepared for it, and may die.
This year I may add a light, but mainly to prompt egg production, not for warmth.
So to answer the op's question. Insulation is a good idea. Make sure there is good ventilation as well. Heat source is optional. Assuming we are talking about a cold tollerant breed, which imho is all you should consider in your climate.