The R-value refers to insulation’s resistance to heat flow. It is sometimes called “thermal resistance” and the most important thing to remember is that the higher the R-value, the greater the insulating power. Any materials with the same R-value, regardless of type, thickness, or weight, are equal in insulating power.
The R-value of different materials is based on testing methods established by the American Society of Testing and Materials (ASTM). The R-value is determined by material type, thickness, and installed weight per square foot, not by thickness alone.