Metal siding does not have insulating properties. Therefore, if you want heat to stay within the area that has metal siding, then insulation is necessary. However, if the metal siding is just in a garage where you store your car, the effort it would take to install insulation may not be worth it. Metal siding and insulation do not go hand in hand and you do have a choice on the matter. If you do decide to insulate the metal siding, Styrofoam insulation would be a valid type of insulation to consider. Alternatively, you could use fiberglass insulation with a moisture barrier to prevent the build-up of condensation as metal is prone to heating and cooling from day to night.