NCFI. Made from up to 20% renewable agricultural products, InsulStar closed-cell spray polyurethane foam insulation provides R-values higher than R-6 per inch, helping to lower energy bills by as much as 60%, the maker says. The material requires only 2x4 studs for complete insulation, as opposed to the 2x6 studs required by many batt materials. This makes InsulStar ideal for reinsulating homes with 2x4 exterior walls. The material is designed to resist settling, shrinking, compressing, and sagging for the life of the structure. 866.678.5283.