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

Under cathedral ceiling – EPS or polyiso?

kstario | Posted in Energy Efficiency and Durability on

I have a vented cathedral ceiling that is as follows. I am in climate zone 5 so I assume this is an R-38 assembly:

1. Asphalt Shingles
2. Roof decking
3. 12″ I-beam joists full length with styrofoam vent channels stapled to the underside of the decking
4. 10.5″ fiberglass batts in between I-beams
5. Visqueen sheeting stapled up underside of I-beams over entire underside of roof.

I want to finish some of the space under one side of the roof. I am planning on putting up rigid foam board to the underside of the joists to add a bit of R-value and prevent thermal bridging. I will then install furring strips, and half inch drywall . My questions are this:

1. Should I leave the visqueen in place, and put the rigid foam over it?

2. Should I use PolyIso or EPS? EPS is cheaper, so I would prefer to use it.
I am planning on using outward facing foil board. I will get about a 1/2″ air gap between the board and the sheetrock to increase the R a bit.

3. If EPS is OK, is 1 inch EPS OK? I would use the Lowes RMax stuff.

4. What am I missing, if anything?

Thanks for the advice.

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. GBA Editor
    Martin Holladay | | #1

    If you are planning to install interior rigid foam, you won't get much drying to the interior (especially if you use foil-faced polyiso). That's fine, because your roof assembly has ventilation channels to handle any incidental moisture.

    So go ahead and leave the polyethylene in place. It will do no harm.

    Either EPS or polyiso will work fine. The thicker the rigid foam, the better the roof assembly will perform. One inch of EPS has an R-value of about R-3.7 or R-4. If you double the thickness, you'll get an R-value for the EPS of about R-7.4 or R-8.

    One inch of polyiso in this location has an R-value of about R-6.5. Two inches would give you about R-13.

    The thicker the rigid foam, the more the foam will reduce thermal bridging through your rafters.

  2. kstario | | #2


    Thanks for the response. Any opinion on the polyiso vs EPS? or does it make no difference?

  3. GBA Editor
    Martin Holladay | | #3

    I just edited my first answer. Refresh your page, and your latest questions will be answered.

  4. kstario | | #4


    Great, thanks. One last question - would taping the seams on the rigid foam be worth the added cost and effort?


  5. GBA Editor
    Martin Holladay | | #5

    Q. "Would taping the seams on the rigid foam be worth the added cost and effort?"

    A. Yes.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |