It is best to cover a fully cooked ham with foil when baking it. Fully cooked hams can become dry when reheating, and covering them tightly with foil prevents excessive dryness.
To prevent a fully cooked ham from drying out during the reheating process, place it in a roasting pan, put water in the bottom of the roasting pan and cover the ham with aluminum foil. One may also cook the ham in a slow cooker or in a cooking bag; both of these methods prevent dryness while reheating. The key is covering the ham to prevent loss of moisture.