perm

   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 (inHg) of pressure difference per hour. The value depends somewhat on temperature, however. At 0 °C, one perm equals about 5.721 x 10^-11 kilograms per square meter per pascal per second (kg/(m²·Pa·s)) or about 0.2060 mg/(m²·Pa·h); at room temperature the equivalent is about 5.745 x 10^-11 kg/(m²·Pa·s). [The SI unit, kg/(m²·Pa·s), simplifies to seconds per meter (s/m).] The lower the perm value, the better the vapor barrier.