The unit of measurement of permeance of a substance to water vapor. The lower this number, the smaller the amount of water vapor that can pass through the membrane. See "Vapor Barrier."

A vapor transmission rate of 1 grain of water vapor per square foot, per hour, per inch of mercury pressure difference (1 pound = 5,760 grains). The lower the perm, the better the resistance to water vapor transmission.

A unit of water vapor transmission defined as 1 grain of water vapor per square foot per hour per inch of mercury pressure difference (1 inch mercury = 0.49 psi). Metric unit of measure is ng/m2 s Pa. 1 perm = 55 ng/m2 s Pa

The time rate of water vapor migration through a material of one grain per hour, per square foot, per inch of mercury pressure difference.

A measure of water vapor movement through a material (grains per square foot per hour per inch of mercury difference in vapor pressure).

Measurement of one grain of water vapor per square foot, per hour, per inch of mercury pressure differential (0.491), used to measure the amount of water vapor passing through a material to help in the selection of waterproofing materials and vapor barriers.

a unit of water vapor transmission defined as 1 grain of water vapor per square foot per hour per inch of mercury pressure difference (1 inch of mercury = 0.491 psi). The formula for perm is: P = GRAINS OF WATER VAPOR/SQUARE FOOT * HOUR * INCH MERCURY