Ever wondered why we don’t use conventional stucco on our buildings? Because modern Portland-based stucco is not vapor-permeable and it traps moisture. Natural plasters, such as clay and lime, are much more permeable.
Typical permeances of various plaster are:
6 mil poly vapor retarder .006” = <1 US perm
Cement-Sand stucco 1.5” = 1 US perm
Cement-Lime-Sand stucco 1.5” = 4-9 US perms
Lime-Sand plaster 2.0” = 9 US perms
Clay plaster 1.5” = 20 US perms!
For more, check out this article about the Stucco-Pocalypse!