Air and vapour barriers
Choosing the right house wrap / weather barrier, which is best?
...retarders. The higher the perm rating or Ng, the quicker moisture can evaporate.
*Ng stands for Nanograms, or billionths of a gram of water. That 60 Ng number refers to the amount of water that will pass through a square meter of a material in a second [ 60 ng /( Pa.s.m2 )].
1 U.S. perm is actually 57 Ng, that 3 bi...