If the insulation must be applied to the roof, an air space of 2 inches should remain separating the roof from any insulation to allow air to move through that space keeping the inner surface dry. Soffit and peak venting can help air movement. a foil-face batt below that air space can reject heat and keep moisture from the insulation below. It would have to be suspended by some means.
If the insulation can lay on the ceiling of the floor below, lay a face-down faced batt on the surface. Then lay as much unfaced batt fiberglass in as permissible. Be sure the insulation does not touch the roof at the eaves - keep the air space. R-22 (6-inches of fiberglass) is considered minimum today. More is better (18-inches is sometimes installed) and the least expensive investment in return for heating bills.