Thermal Performance
Thermal performance of insulation
THERMAL RESISTANCE (R  VALUE)

The ability of a product to resist the transfer of heat.

To calculate the Rvalue you need to know the Thermal Conductivity (Kvalue) and the thickness of the material being used:
(Rvalue = Thickness in meters)
K Value
Calculation of Rvalues for typical roof/ ceiling construction products

Total Rvalue of a construction element (wall or roof) can be obtained by adding the Rvalues of the different components.

The higher the Rvalue the better the products thermal resistance will be.
THERMAL CONDUCTIVITY (K / λ  VALUE)
All materials conduct heat, some better than others depending on their thermal conductivity.
Good insulation materials must have a LOW conductivity.
 Eg. A material with a low conductivity allows heat from one side of a wall to travel very slowly towards the other side of the wall.
Thermal conductivity is calculated in W/mK (Watts per meter Kelvin degrees).
If you test two products with similar densities at the same temperature and one has a Kvalue of 0.040 W/mk and the other has a Kvalue of 0.045 W/mK, the material with the lower Kvalue insulates better which means that the heat will travel slower from a higher temperature to a lower temperature until and equilibrium temperature is reached.