The diffusion resistance factor μ (mu) shows the vapor permeability of a material. It indicates how many times it will be more difficult to pass the water vapor through the material then through the air. You can see in the table below that the insulating materials from renewable insulation materials have a small value, therefor are mu-vapor-permeable, and the insulation materials from petrochemical raw materials have a large mu-value and be vapor-tight. Therefore use insulation material from renewable raw materials.