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

Cathedral ceiling insulation: cut and cobble?

jmlorax | Posted in General Questions on

I’m working on finishing my attic in Alexandria VA and want it to be comfortable and as efficient as I can manage. I have attic trusses as part of a 10 year old addition, good soffits /ridge vent, 3/4″ sheathing, 9” deep bays. I’ve spoken to 2 spray foam guys who’ve assured me that 5 inches of open cell is just what I need and thermal bridging doesn’t matter, and a fiberglass guy who could get me R30, and but said the fiber would fall out of the knee walls in a decade or so.

I’ve read the articles on cathedral ceilings, OK to skimp and cut and cobble, and I’ve bought 66 sheets of 2.75″ reclaimed fiberglass faced polyiso. I’m planning on using 2 1 x 2’s as spacers (I have them), for a 3″ vent, two layers of the polyiso between the rafters (5.5″), and another sheet inside/across the rafters for the slopes, and one sheet outside the studs on the knee walls (maybe? See question 3). I’m going to buy canned spray foam, caulk and some tape to see what holds, I have managed to decently friction fit the polyiso (nothing is installed, still trying stuff out) and like that idea better than lots of canned foam. 1. Are there problems with this that I maybe missed other than a ton of work? 2. Should I have an air barrier other than drywall?

The ceiling below the attic room subfloor is filled with I think open cell sf? While the remainder of the truss space is blown in fiberglass, that I’d really rather not pull out, but I know that I should bring my ductwork inside the envelope. 3. Would there be any problems with/benefit to putting just one sheet of the polyiso between the rafters, maintaining the vent, outside the kneewalls and leaving the fiberglass, for a kinda conditioned space? I’d also like to do it in the old attic, but across the rafters (stick built/only gable vents that I would block) for the air handler if I’m not completely sick of polyiso.

11 trusses 2 ft on ctr, 43 ft wide, attic room will be 14×22.

Love this place. Thank you!

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
    Brian Pontolilo | | #1

    Hi Jeannie,

    Drywall can be your air barrier, but there are a lot of details to take care of to make it work really well. Here's a helpful read on that topic:

    In my experience, it's much easier to not friction fit rigid foam in stud, joist, and rafter bays, but instead to cut it a bit short on all four sides and use canned spray foam to seal it in place. It's easier work and air tight.

    You mentioned that you plan to run rigid foam on the outside of the kneewalls. I assume that you'll also have cavity insulation there. If you have spray foam in the floor beneath your kneewalls, that will help, but keep in mind that those kneewall stud bays should be sealed on all six sides.

    I'm not sure I understand where your ductwork is, but I think it is in the attic space outside of the kneewall. If that is correct, and with all the work you are doing, you should consider making that area into conditioned space. These two drawings show the different approaches.

  2. jmlorax | | #2

    Hi Brian,

    Thanks for your response! I hadn't seen that article before, thanks. I'm attaching a sketch to clarify the ductwork situation. All of my ductwork is currently in a very hot attic, and is solely used for A/C as I have a combo of hot water underfloor and baseboard on the first floor.

    I was quoted $9k for less than code of open cell spray foam for the whole roof, with me pulling out all the blown fiberglass in the addition and the old mungy batts under t&g in the old attic. Not something I'm willing to do.

    But I would like to cool off the ductwork as much as I can, so was wondering if leaving my existing attic floor insulation in place and adding maybe a sheet of the reasonably priced reclaimed polyiso in the rafters of the addition would be better or worse than putting that second layer between the studs of the kneewalls? Creating a semi controlled temp space, and directing my vent straight up the bays instead of the air swishing around the attic space. Should I forgo the outside of the kneewall sheet and put both in the rafter bay? It would only be R 28-30 ish. I'd love to put 3 in there (I only have access to the 2.75", for 3 it would stick out with a 1.5" vent space), but I'm not sure I can swing that much material right now, and I'm not sure how much time I'm willing to spend in there. I hope this is clear enough, thanks for your time.

    Full disclosure, 3 of my 22 bays are taken up with skylights in the triangles, not the attic room, so no vent there.

  3. GBA Editor
    Brian Pontolilo | | #3

    You could certainly insulate the roof plane. That is what one of the two drawings I posted illustrates and what I would recommend with your duct work in the attic space. But notice that in that illustration the thermal barrier is connected from the walls below, to the top plate, and then up the roof plane, and everything is air sealed. Installing the rigid foam on the bottom of the rafters without air sealing in all of those areas isn't going to do you much good. Unfortunately, there aren't great budget solutions here and half measures tend not to work so well.

  4. jmlorax | | #4

    Hmm. Thanks for the input. It's busy enough up there that I'll have a tough time getting that done well. Sounds like my best cost/effort scenario might be going with at least two layers maybe three of the polyiso on the knee walls and maybe try to blow in more on my ducts (extending the baffles where I have them).

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |