Adding perms
I’m working on an earthen structure and the relevant code requires that “The vapor permeance of the combination of finish materials shall be 5 perms or greater to allow the transpiration of water vapor from the wall.” I take it that means that any finishing material I use (e.g., plaster and insulation) must each have a perm rating of 5 or greater. Is this correct, or is there some calculation that must be performed when materials of different permeability are used together.
I’m currently planning on using unfaced mineral wool for insulation. Are there other high R, high perm insulations I should consider? EPS seems like another option, but I’d be grateful for additional ideas and resources.
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Jacob,
Is this a rammed-earth structure, or an underground house?
Look at the section bellow "How to calculate the total resistance to water vapor transmission in a building envelope assembly" bellow:
https://www.buildingenclosureonline.com/blogs/14-the-be-blog/post/85873-the-difference-between-permeance-and-permeability
To have an above 5 perm assembly, depending on the number of layers and their individual permeability, you'll need much more than 5 perm for each layer.
Brilliant. Thanks for the reference, Akos.