I live in OK in a 33 year old home. My return ducts are located in the attic and have a 1 inch fiberglass insulation blanket around it. The blankets exterior is white plastic. The return duct box has no insulation around it at all, only the tube part is insulated. Is this adequate or should I add another blanket to the tube part as well as the return duct box. If so, what type do you recommend and how thick? Installation techiques are appreciated!