Because I had leftover insulation from building my house, I insulated my coop. My coop is small, built from the same materials as my house (2x6, plywood, metal siding) and didn’t cost me too much. The roof isn’t insulated, but is plywood covered in corrugated metal. It is unheated and in winter is a few degrees warmer than outside. I have plenty of ventilation so the interior humidity is never higher than exterior.
I’m in the process of designing my second coop and don’t know if I will insulate or not. I do have leftover insulation, metal siding, and some lumber, but this second one will definitely cost me a lot more than the first. Especially since I plan on making it five times larger than the existing one.
My summers average temp is in the 90s with at least two weeks in the 100s, and winters are supposed to average in the 20-30s, but this winter has been in the 50s with hardly any snow and the last two winters were in the -20s with record-breaking snowfall (over 100 inches) so I have to be prepared for it all.
Ultimately for my second coop, insulation will come down to budget. I’m not worried about mice getting in the walls because we built the coop like we built our house. Even the floor is insulated and completely enclosed. Because of weather extremes, and lack of protection from wind and sun, I am leaning towards insulating my new coop - but time will tell. Where I live there is no protection from the elements. We are on top of a hill surrounded by alfalfa and clover fields. Trees do not grow here unfortunately.