Water vapour diffusion resistance factor (Sd) is a diffusional equivalent of the air layer thickness that creates a barrier for the water vapour diffusion.

Sd is given in meters. The bigger Sd is the bigger diffusional resistance is and vice versa. Sd 40 m means that particular material creates a barrier for the diffusion of the water vapour comparable to the 40m thick air layer. Sd is a non relative value, not dependable from temperature, pressure, or humidity. Sd is given in meters. The bigger Sd is the bigger diffusional resistance is and vice versa. Sd 40 m means that particular material creates a barrier for the diffusion of the water vapour comparable to the 40m thick air layer. Sd is a non relative value, not dependable from temperature, pressure, or humidity.

ATENTTION:

• Metr [m] is the Sd unit.
• Sd is an absolute value.

Division of membranes and vapour-barrier foils according to the Sd factor: