With a stabilized 3/4" air space on one side of this insulating board its R-value at 1" increases to R10. 1/2" Celotex Thermax was rated at R 3.8, increasing to R 6.4 with the air space we specified above. 4" Celotex Thermax was rated at R 28.8, increasing to R 31.6 with the air space we specified above.