What is a perm rating?

Home Energy May 21, 2014 Print Friendly and PDF

It is a standard measure of the water vapor permeability of a material. The higher the number, the more readily water vapor (in the gaseous state) can diffuse through the material. A perm rating of less than 0.1 is considered a Class I impermeable vapor retarder (which is also considered a vapor barrier); perm rating between 0.1 and 1 is considered a Class II semi-permeable vapor retarder; a perm rating between 1 and 10 is a Class III permeable vapor retarder; and a perm rating greater than 10 is highly permeable and not considered to be a vapor retarder.. The U. S. Department of Energy's has more information about vapor barriers and vapor retarders .

Connect with us

  • Twitter
  • Facebook
  • Pinterest


This is where you can find research-based information from America's land-grant universities enabled by eXtension.org



Home Energy Efficiency

Meet all of "our people"      


This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.