empirical unit of water-vapor permeance; one perm = one grain (avoirdupois) of water vapor per hour flowing through one square foot of a material or construction induced by a vapor-pressure difference of one inch of mercury across the two surfaces. Note: Mass flow rate may be restated in other units consistent with the defined units. One perm is the generally accepted maximum allowable permeance value for the residential building envelope in a 5000 heating degree-day climate.