When you're walking through the store and see a big bag of insulation printed with "R-20", you're seeing the R-Value of the insulation based on a cavity depth, or recommended installation depth.

Proper installation is critical - over compressing is bad, settling in the wall or attic will affect the R-value, as will the presence of moisture and the building materials used around the insulation.

The R-Value per inch of a material is a better reflection of how well the material will perform, and the U-Value is a good thing to know too. Many products won't have U-value, with the exception of windows.

The number that we really should know is the EFFECTIVE R-Value of the assembly - walls, basement slab, ceilings. The EFFECTIVE R-Value is the combined R-Value of all of the materials in the assembly.