W. R. MEADOWS developed the Perm Calculator to help you compare the amount of moisture that is transmitted, via vapor diffusion, for materials of varying permeances as compared to the requirements laid out in ASTM E1745 of 0.1 perms. The calculator is designed to show the estimated amount of moisture that is transmitted through a material of a given permeance over a user defined area and time period.
For the sake of comparison, the pressure differential had been set as a constant at 0.106 psi. [Calculated using a slab exposed to a 70° F (21° C) and 30% relative humidity (RH) environment at the top and 55° F (12.7° C) and 100% RH at the bottom.]
Please contact W. R. MEADOWS technical services at 800-342-5976 or email us for assistance with this calculator or for questions arising from its use.
a) Enter the perm rating of the vapor barrier/retarder to compare.
b) Enter the floor area in square feet for analysis (for example: 1000).
c) Enter the time frame in hours for comparison (for example: 24 for one full day).
The resultant calculation will be the amount for water vapor that is transmitted through a given material of your choice in a variety of units for your comparison. The result will also be compared to the 0.1 perm requirement as defined in ASTM E1745. If you would like to compare the result to the materials with the industry’s lowest perm ratings for critical applications, select “Show all W. R. MEADOWS products.”
Footnote: The calculator is provided for illustrative purposes only and should not be used as the basis for design or to determine specification values or limits. W. R. MEADOWS disclaims all liability from the use of the calculator; use of the calculator or values obtained shall in no way imply or provide a warranty of any kind. Please contact W. R. MEADOWS technical services at 800-342-5976 or email us for assistance with this calculator or for questions arising from its use.