Insulation is measured in R-value, the capacity of an insulating material to resist heat flow. The higher the R-value, the greater the insulating power. When you’re purchasing insulation, buy or specify R-value, not inches, as R-values of materials vary.
Achieve a higher total R-value by "combining" two different R-values. For example, R-38 added to an R-11 results in R-49.
Fiber Glass Insulation Fact Sheet (US)