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A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · X · Y · Z   entire Web this site permeance A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4°F (23°C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury. Related category    • HUMIDITY AND MOISTURE CONTROL Also on this site: Encyclopedia of Science Transport Concepts & Designs (partner site) BACK TO TOP