Insulating ranch with vaulted ceilings and no basement

[Racine Insulation]

Looking for advice here on my sister's house in southern Illinois. It is a ranch with vaulted ceilings throughout, except above the central hallway where there is a small attic. There is no basement and the heat ducts are just under the flooring in the slab. There are vents at about 9 feet high but they are not ducted. They allow passive air flow from room to room through this small attic space. I do not know if there is any insulation in the vaulted ceilings or external walls. Since the attic is connected to the rooms via a vent I have concluded that the attic is part of the conditioned air space and within the thermal envelope.