XPS or EPS for venting cathedral ceiling cavity.
I’m planning to create a vent space in a cathedral ceiling slope. Cavity will be 13″. I am planning on making a “U” shaped channel out of rigid foam, putting that against bottom of roof deck then dense packing the cavity. It seems to me that EPS is a safer bet here because of its greater permeability over XPS. But XPS is easier to get and stronger. I am a little concerned about dp cellulose crushing 1″ of XPS. My understanding is that XPS is rated at 1.1 perm @ 1 inch thick, while EPS is 5.5. Is the XPS still a pretty safe assembly? How much difference is the EPS’s greater permeability going to make. I am also weighing the possibility of netting the dp cellulose with a smart membrane limiting airborne moisture into the cavity and allowing better drying to the interior.
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