R-value is used to measure materials ability to resist heat or in other words, resistance of a material unit to heat transfer. R-value figure is usualy provided by the manufacturer of a particular insulation product and is shown in the product description. R-value figure can be calculated from the equation:

R value = thickness (m)/Thermal Conductivity (W/mK)

Unlike thermal conductivity, 
thermal resistance (R-values) solely depend on the material’s thickness. Thicker insulation has higher thermal resistance and lower thermal conductivity.