What is the permability of half inch (0.5″) XPS?
This is related to my other question: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/41561/what-minimum-permbility-dyring-exterior-zone-#comment-91231 but different so I’ll create a new question.
I see from this link that less than 1″ XPS is above 1 perm: http://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials/view.
I”m in Halifax, zone 6. I have a basement/pony wall sheathed with OSB that is not flush with the main part of the house which is older and sheathed with planking. I’d like to add 0.5″ XPS to bring the pony wall to the same plan as the first story. I might be able to use a smart vapor barrier on the inside which would make this question moot but I’m curious regardless.
Is there way to calculate the permeability of a given thickness of XPS or EPS? I vaguely remember this being asked before but can’t find anything about it.
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Wesley,
According to this document, the water vapor permeance (not permeability) of 1/2-inch XPS (in this case, Owens Corning Foamular) is 1.5 perm.
Thanks. I couldn't find the datasheet for 1/2-inch.
It varies a bit with manufacturer, and some half-inch XPS is intentionally manufactured with thin facers to lower it's vapor permeance. Half-inch XPS without facers would sometimes be a hair north of 2 perms, but not much higher than that. Many thin XPS products used as siding underlayment will have the facers for adjusting vapor permeance downward (which can be a good thing, behind brick cladding.)