Permeability
Note: the drywall perm ratings are published in the general literature; the stucco and plaster perm ratings are from Straube.
What is permeability?
Permeability, as rated in "perms," is a measure of the water vapor transmission through a material. It should not be confused with breathability, which implies a significant amount of air exchange through a material. According to Chris Magwood, straw-bale author and editor of The Last Straw, "Vapor transmission is good; it indicates that moisture trapped in the wall or naturally moving from one side of the wall to the other will be able to escape into the atmosphere. Air flow is not good; it indicates points of leakage, and these will compromise the thermal insulation and result in loading the wall with much greater volumes of moisture than desirable."
Why is permeability important?
Permeability is important because trapped water vapor within a wall cavity can lead to condensation, resulting in eventual moisture damage, rot, and mold formation.
How permeable are earth and lime plasters compared to more conventional finishes?
Earth and lime plasters are significantly more permeable than painted drywall surfaces or concrete stucco. This means that earth and lime plasters will readily allow any water vapor within a wall cavity to escape, but more conventional wall finishes actually act as vapor barriers and will trap water vapor within a wall cavity.
How do these facts impact the construction details for a natural wall assembly?
When building a natural wall assembly (including both straw-bale and straw/clay wall assemblies), one should specify air-tight construction methods in combination with permeable wall finishes. This means that any locations prone to gross air leaks should be eliminated through proper detailing (around electrical boxes, windows and doors, and at foundation and roof interfaces), and that wall finishes should have the highest "perm" ratings possible (using earth and/or lime plasters).