Assuming you have access to the attic space, a blown-in insulation seems appropriate in this case. A technician could quickly apply insulation in the space, adequate for insulating the living space, while leaving ventilation space. This could be done without any disruption to the roof or ceilings. Inaccessible spaces might be able to have insulation blown in through holes drilled in the ceiling. This project is best left to a professional installer and his experience with specialized tools and techniques.