I helped a friend assemble a 30x40 metal shop. The kit came with insulation blanket that was installed before you put on the siding. That proceedure sounds too late for you. But you might try to get some of that stuff. It came in about 6' wide rolls and had a plastic coating on it like vinyl. Fiberglass was only accessable from the sides. That could be taped up. Just a thought.
Edwin