Most rigid insulation does not have the proper fire rating to leave it exposed. Look for something that does or plan on covering it. You'd need about 4 inches of extruded polystyrene or about 2-1/2 inches of isocyanurate foam to get R-19.
Thermax makes a faced iso board that is rated for exposure. I've seen it used inside some metal buildings. I have no idea about cost or availability. I would look for something that says it has a UL "Class A" flame spread rating.