GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Wall to vented cathedral ceiling detail

JakeG | Posted in General Questions on

I’m going to be building a garage with an apartment above in Zone 5a.
im going to be doing 2×4 wall with Zip R6 and Rockwool in the stud bays. The ceiling will be 2×10 with baffles and Rockwool. I will also be putting 2 inches of EPS on the underside with taped seams and strapping with 1×4 to get my R value up and air seal. Wondering what’s the best way to connect the air barrier from the zip to the EPS. Would taping both the zip and EPS to the top plate suffice or should an Air Barrier of some sort be taped to the Zip run over the top plate and then taped to the foam be the best call. Will having a EPS foam on the warm side cause any problems?

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
    Michael Maines | | #1

    I like to drape a strip of high-quality WRB over the top of the wall before setting the roof trusses or rafters, and tape that to the sheathing and the ceiling air barrier. You can also use tapes or sealants from the top plate to the back of the sheathing, you'll just have more discontinuities at joints.

  2. Expert Member


    "Will having a EPS foam on the warm side cause any problems?"

    Not with a vented roof.

    1. JakeG | | #3

      Would there be any advantage/disadvantage to using foil faced foam?

      1. matthew25 | | #4

        Foil facers are usually 0 perms. What type of WRB will be on the outside of your roof sheathing? If it is also low perms, you could be making an impermeable sandwich that cannot dry to either the inside or the outside.

        I am a little confused on what the ceiling of the garage insulation plan is, versus the ceiling of the apartment insulation. Where is the EPS going, roof of apartment, ceiling of apartment, or ceiling of garage?

      2. Expert Member
        MALCOLM TAYLOR | | #5


        Not any appreciable difference either way. The roof can dry to the vent cavity above. 2" of either faced or un-faced foam with painted drywall provide an adequate vapour-retarder. If you are using the foam as your primary air-barrier, faced might be easier to detail.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |