WILL 2 LAYERS OF EXTERRA STILL BE PERMEABLE?
No. Two layers of Exterra will not meet the minimum requirement of being “permeable”, which is 60 ng/Pa-s-m2, or 1 perm.
A single layer of Exterra maintains it’s breathability up to a maximum thickness of 2” (71 ng/Pa-s-m2 or 1.2 perms at 2”).
Exterra panels are laminated on both sides. Exterra’s laminate layers are perforated and those perforations allow moisture to pass through so the wall can dry to the outside.
However, when two layers of Exterra panels are used that means there are a total of four layers of perforated laminate and, in that quantity, the perforations cease to allow enough moisture to pass through to achieve a permeance greater than 60 ng/Pa-s-m2, or 1 perm.