The water vapour diffusion resistance number, also known as the μ value, indicates how great the resistance to vapour diffusion by the component is and thus describes the tightness against moisture in vapour form. The value is evaluated in relation to a stagnant air layer (in meters) under the same conditions. The lower the μ value, the lower the diffusion resistance, i.e. the lower its tightness.
Unit: none.
Source: bauwion
