Permeability versus Permeance: Know the difference

April 29, 2014

by Richard Maurer
Revised 9/11/2014

Much has been written recently about moisture and vapor transmission of waterproofing membranes.  Some of the interest stems from changes in requirements for steam showers after the TCNA adopted a moisture vapor transmission of 0.50 perms for vapor barriers. 

We believe there is confusion regarding testing and reporting.  The following may help clarify some of the issues.  Two terms are used to describe the effectiveness of moisture and vapor barriers:

• Permeance is the moisture transmission rate of the membrane.  It is a performance test and is stated in perms.  Lower numbers indicate better performance.
• Permeability is another term used to describe moisture transmission of a material.  It is a theoretical number calculated by multiplying permeance by thickness of the polymer.  It is reported in perm inches

Permeance should be used to compare the performance of waterproofing membranes.  Ensure that the waterproofing membrane you select has the permeance needed for your application.   

Following are permeance values for Noble waterproofing membranes:

Chloraloy                       0.08 perms      ASTM E96, Procedure E
NobleSeal TS/CIS        0.15 perms      @ 100 °F & 90% Humidity
Wall Seal                       0.61 perms