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.
Joe
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Replies
The permeance of EPS varies with it's density. Only lower density 1lbs/ft^3 "Type-1" EPS has a permeance in the 5s. Going with 1.5lb "Type-II" EPS gives you better structural strength, and only lowers the permeance to about 2.5-3 @ 1" thickness. That's still 2x or better drying rate than you'd get with 1" XPS.
EPS/XPS of any density can be supported mid-span between rafters with wood or EPS stringer glued/tacked to the roof deck, parallel to the rafters, and you should be able to dense-pack cellulose even against 1" Type-I EPS if it's not spanning more than a foot between supports. Code requires a minimum of 1" of channel space in a vented cathedralized ceiling, but bumping that to 1.5-2" may be measurably better in colder-wetter regions, or if the slope of the roof is less than 4:12.
But with a smart vapor retarder membrane on the interior the lower drying rate of ~1-perm XPS isn't going to be a moisture problem, since it'll still be drying into the channel toward the exterior even in the middle of winter.
Joseph,
This article discusses your question: Site-Built Ventilation Baffles for Roofs.