What is perm?

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Perm is a traditional unit of water vapor permeability, that is, the ability of a material to permit the passage of water vapor. If we want to keep things dry, we wrap them in something having low permeability. A material has a permeability of one perm if it allows transmission of one grain of water vapor per square foot of area per inch of mercury (in Hg) of pressure difference per hour.

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