After having laying chickens less than a year, I cannot believe that there is any benefit to insulating a coop. We live in Canada and that was one of the decision points we made in building our coop. We went without. See our coop here. https://www.backyardchickens.com/a/lair-of-the-layers The venting requirements will nullify any benefits of the insulation. On a really cold day, fully open half the widows in your house. Your insulation will do nothing to keep your house warm. (If you are building a coop, do not under-estimate the venting requirements.) On top of that, there is a risk the chickens will start eating the insulation if they can get at it. As well, if your coop is made of wood, insulation will reduce the ability of the wood to absorb water vapour. In my view, this is also a reason not to paint the inside of your coop. If you are thinking about having some form of heat and restricting air flow, the humidity will be quite high, and if the heater fails, there is a real risk your chickens will freeze to death. One point I wish I had considered was the location of the vents. I wish I had one or more vents lower in the coop, given that the chickens will roost as high as they can. Having vents lower down reduces the risk of drafts while the chickens are roosting.
There could be special circumstances where insulation and heat would be beneficial. But I would agree, most cold hardy healthy birds are fine without. Controlling the humidity is paramount.