a measure of the flow of water vapor through a material

The amount of water vapor that passes through an area in a certain period of time.

A rating that rates the permeability of a film or membrane. The perm rating measures the density of the holes or perforations in the material. The higher the rating, the easier it is for air to pass through it. Coverings rated at greater than ten perms are generally considered to be permeable and breathable.