There are several ways to insulate a post frame building. When building a post frame building you should consider different types of insulation and methods. Some options may improve energy efficiency, reduce energy costs, and may actually qualify you for tax credits. That being said, an example of one good way to insulate a post frame building is to first, add building wrap. When adding building wrap make sure it is well sealed, and that you tape all the seams and if you spot any tears. You can then install an interior set of wall girts and apply a two-inch or thicker high R insulation sheeting inside of the girts. For the ceiling, once it has been drywalled, you can have insulation blown in on top. You should aim for at least 15 to 20 inches of insulation.