R, the eighteenth letter of the alphabet, in the construction industry, this letter is used to denote how well a material can resist heat transfer. The higher the R-value, the more efficient the material can resist transferring its heat energy.

Wool insulation at 100mm thick (4in) R= 0.1m / 0.07 = 1.42 W/m2K

Limestone at 100mm R= 0.1m / 1.26 =  .079 W/m2K
 
Clearly not a good choice as a material for insulation, but there is more variables that need to be considered. When an exterior wall is said to be R45 or code requirements a minimum R, it is for a single material. Sandwich a bunch of different material all at various R values, thickness, and what we should be aiming for, is the U value of the total assembly.