The blocking value or Sd value describes the tightness of the component in terms of water vapour diffusion compared to an equivalent air layer thickness. It is a feature especially for vapour retarders and vapour barriers, which are intended to prevent moisture from penetrating into the insulation through water vapour. The higher the value, the denser the component. Vapour barriers should maintain an Sd value of 2 m to 1500 m. If the value is above 1500 m, it is called a vapour barrier. The Sd value is calculated from the water vapour diffusion resistance factor μ and the thickness of the component "s".
The unit is meters.
Sd = μ x s
Blocking value = Water vapour diffusion resistance index x layer thickness
Source: bauwion
