Which is better Knauf Dritherm 32 or 37 Insulation?

Knauf Dritherm 32 has a lower thermal conductivity so makes a more effective insulation per millimetre of material thickness.

What is thermal conductivity?

Also called the lambda (λ) value or k value, thermal conductivity is the number of Watts conducted per metre thickness of the material per degree of temperature difference between one side and the other - W/mK.

For insulation the lower the thermal conductivity the better, because the material conducts less heat energy.

Dritherm 32 has a lower thermal conductivity making it a better insulator. Thermal conductivity is a property of the material and does not take into account thickness. Two different thicknesses of the same material still have the same λ-value.

What is thermal resistance?

To compare the relative performance of different thicknesses of materials means working out their thermal resistance (units: m²K/W). Thermal resistance is calculated by dividing the thickness of the material by its thermal conductivity, giving an R-value specific to that thickness.

So the R-value gives the overall insulation performance, taking into account the thermal conductivity and the thickness of the materials.