I know I am not in 100% the same climate but.....I have kept birds for many many years. When in the country my birds had a horse stall....zero insulation in a metal barn. Zero incidents of frostbite in the 5+ years we were there. We had a 9 day stretch without power due to an ice storm. Lows...
One thing not mentioned here yet.....mice. Mice love the nice cozy spaces created by adding insulation.
A link to Dobies Mulligan coop.....
https://www.backyardchickens.com/articles/the-mulligan.74743/
How many birds?
I keep a thermometer with a humidity gauge inside the coop and another outside. My goal is to keep them close in reading. If my inside humidity is 10% higher than outside I open windows, remove all poop, and add fresh dry pine shavings. My coops have fully open sofits. So far I...
Fresh air is much more important than warm air.
Chickens create a LOT of moisture from their breath and the droppings. Ventilation up high allows that moist air to escape (along with heat). The birds have built in down coats to stay warm.