The frame building and the roof finishing even for the barn needs a complete functional insulation. There are several ways to do that and to achieve the best results with the roof insulation as much as the walls that can be used for any other roof or wall to. So, there are: post frame building roof – furring; post frame building roof – draped; post frame building ceiling – retrofit; post frame building wall – retrofit; post frame building wall – furring; post frame building wall – draped
For the benefits and the steps of its construction on the images bellow. An easy step by step guide that will clarify many questions.

Source.

...