If it is possible, I would start by insulating the area underneath the floor in the unheated parking space. This will help keep the cold air from gravitating upwards. You could then concentrate on the upper part of the floor. Keep in mind that you will be changing the elevation of that floor from the rest of the house. Check with carpet manufacturers. At one time they sold insulated padding for this very purpose. It was very popular in basements. Before giving an absolute answer on the upper part of the floor, you must decide on what type of flooring that you are going to choose. Check also on the floating wood floors that are sold in all of your major hardware warehouses. They also are laid with an insulated pad. GOOD LUCK!!!!