Thermal performance of insulation
THERMAL RESISTANCE (R - VALUE)
-
The ability of a product to resist the transfer of heat.
-
To calculate the R-value you need to know the Thermal Conductivity (K-value) and the thickness of the material being used:
(R-value = Thickness in meters)
K Value
Calculation of R-values for typical roof/ ceiling construction products
-
Total R-value of a construction element (wall or roof) can be obtained by adding the R-values of the different components.
-
The higher the R-value 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 K-value of 0.040 W/mk and the other has a K-value of 0.045 W/mK, the material with the lower K-value insulates better which means that the heat will travel slower from a higher temperature to a lower temperature until and equilibrium temperature is reached.
The ability of a product to conduct heat:
- Steel is a good conductor of heat (heat flows faster through steel)
- Wood is a poor conductor (heat flows slower through wood thus making wood a better insulator)
The lower the K value the slower the heat flow through the product