This is typically done by placing metal banding 30” apart down the length of the pole building. This blanket comes in 3', 4', 6', and 8' widths and provides a minimum of r19 installed. Fill, seal and insulate gaps up to 3 inches with GREAT Screw or glue (super metal sealant) insulation to underside of purlins. How to insulate a pole barn cheap . Typically only requiring a few days for installation. How to insulate an existing pole barn. By blocking off the rafter area with a dropped ceiling, it's possible to insulate the barn with much less material. I have insulated a pole barn by applying polyiso sheets on the inside of the 2x4 girts and trusses, butting up to the 6x6 posts. In most cases it is possible to also insulate the overhead door of the pole barn using the closed cell foam. Nail or screw to the trusses through the product. Foam insulation in attics, crawl spaces and walls can also help prevent ice dams, decrease your cooling and h...