The higher the R-Value the better thermal resistance the product will provide.
How is it derived? Simple
R Value = thickness of insulating material(m) ÷ thermal conductivity of material. (W/mK)
Thermal Resistance is the most important material characteristic that should be defined when specifying insulation. From the formula for calculating, it is apparent that there are two factors affecting the thermal resistance: the thickness of the insulation and the thermal conductivity of the material. Simply specifying thickness of material is not enough.
To achieve a specific thermal resistance, increased product thickness is more economically efficient than using a product with a lower thermal conductivity.
R-value is measured through a series of industry standard tests and ethical manufacturers have their values certified by SABS. With the passing of SANS 10400-XA all insulation products must state the R-value of the product on the packaging.