The insulation type with the highest R-value per inch is polyisocyanurate structural insulated panels, which has R-values ranging from 5.6 to 7.7 per inch, according to HouseLogic. Closed-cell polyurethane spray foam also has high R-values ranging from 6.0 to 6.5 per inch.
Polyisocyanurate SIPs typically must be installed by a contractor and are best used for new walls, floors, ceilings and roofs, according to HouseLogic. They are frequently faced with foil that acts as a moisture barrier. This type of insulation is expensive, costing $22 for a 4-by-8-foot sheet with a 1-inch thickness. Other types of insulation include batts and blankets, loose-fill insulation and spray foam.