GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

XPS or EPS for venting cathedral ceiling cavity.

Decumanus | Posted in Energy Efficiency and Durability on

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.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    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.

  2. GBA Editor
    Martin Holladay | | #2

    This article discusses your question: Site-Built Ventilation Baffles for Roofs.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |