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Polyiso has a high R-value per inch compared to other insulation products. Polyiso is unique in that the R-value increases with the thickness of the foam, so three inches of polyiso has a higher R-value per inch than two inches. Johns Manville Polyiso foam sheathing insulation boards provide a continuous layer of insulation to reduce thermal bridging and improve energy efficiency. Polyiso consists of a uniform closed-cell polyisocyanurate foam core and provides one of the highest R-values per inch of any rigid insulation.