I personally wouldn’t suggest lining the inside of a coop with styrofoam unless the chickens don’t have access to it. We tried to line the a spot on the ceiling with the blue styrofoam and the chickens pecked at it. I worried that they would eat it so we took it down.
This is a great question. I would also love to hear others thoughts. I currently use an electric cord to my coop in the winter and plug in a metal heated base. I do think it is pretty safe, uses low wattage, and shuts off when it gets above freezing. I would like to convert to non electric, but...