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Community and Q&A

Question on cut-and-cobble rigid foam in a vaulted ceiling

Brian Gray | Posted in Green Building Techniques on

Hello all,

About a year ago Martin Holladay wrote an article on GBA regarding the practice of cutting rigid foam into 14″ wide sections and installing them between framing as a poor man’s alternative to spray foam. Being a poor man (frugal is more accurate), I’ve decided to implement this in our new master bedroom (which was our old garage). We intend to vault the ceiling and removing as many of the old ceiling joists as our village will allow.

I was able to find 4″ thick 4’x8′ reclaimed Polyiso from Insulation Depot. I’ve already purchased the insulation. If it matters these sheets are fiberglass backed. They are being stored in our new garage for now.

First, here is how I’m thinking of assembling this:
1) Importantly, I plan to keep this as a *vented* roof. Once I get a good weather day I will install a ridge vent along the roof ridge. I debated an unvented roof but I’m too nervous to try it.
2) Using my radial arm saw I’ve already cut ~30 14″ wide strips that I will cut and cobble into the rafter bays.
3) My rafters are 2×6. I will leave 1 1/4″ for venting and install the 4″ thick polyiso flush with the interior edge of the rafters.
4) Here is where my assembly will be different from those described in Mr. Holladay’s article. Once I’ve cut & cobbled all of the rafters, I’d like to double up with another layer of 4″ polyiso. My thought is to install 2x4s (plus a furring strip to get me to 4″) horizontally every 4′. I would then slide full 4×8 sheets between these 2x4s.
5) Finally, I would add furring strips vertically over all of this assembly. These furring strips would be attached to the 2x4s I installed horizontally and would be used to install the drywall. For added R-Value I could add 3/4″ XPS or polyiso between these furring strips. I have not decided whether that is really necessary though. 8″ of polyiso is already R56.

So I do have some questions about this:
1) My plan is to meticulously air-seal the second layer of 4″ polyiso (the layer comprised of full 4×8 sheets). I figure this will be the easiest place to air seal since I have a lot fewer joints. My question is this: If this layer is well sealed, do I need to worry about the small gaps in the first layer (the layer between the rafters)? I’ve already installed ~1/3rd of the cut & cobble section and my joints range from nil to ~1/4″. If I need to worry about these small gaps it would obviously add a lot of labor. My preference would be to leave these small gaps alone.
2) Is there anything inherently stupid about this plan?

Thank you,

GBA Prime

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  1. Charlie Sullivan | | #1

    I don't see anything wrong with your plan. Two suggestions:
    1) I don't think you need true air-sealing for that first top layer. But a 1/4" crack could have cold air flowing along it, cooling the rafter more than it would get cooled otherwise. So it might be worthwhile to stuff the crack with something squishy when possible, like maybe sill plate gasket foam or something along those lines. Even if it was something air permeable, it would greatly reduce any airflow there--there's no pressure trying to force air through if the rest is sealed well. I don't think that's essential but I do think it would help some if it's easy to do.

    2) You might rip 2x6s down to 4", rather than adding 1/2" to 2x4s. You could use cut-offs, perhaps ripped in half, for your furring step afterwards. Perhaps you already considered that and decided adding is easier than subtracting.

  2. GBA Editor
    Martin Holladay | | #2

    Charlie's advice is good. Another approach would be to skip the 2x4s + 1/2" shims and just install the interior polyiso with long cap nails. Then install furring strips with long screws, screwing through the polyiso to your rafters.

    I don't have any data on the energy penalty associated with cut-and-cobble foam without air sealing, but the idea bugs me. If I were doing what you are planning, I would (at a minimum) meticulously air seal the first (topmost) layer of polyiso, as well as the last (bottommost) layer of polyiso (the continuous layer installed on the interior side). I don't know if this level of fussiness is worth it, but that's what I would do.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Brian, the gaps will let air go places you don't expect. Not a moisture problem just an R reducer. Worth sealing time.

  4. Brian Gray | | #4

    Thank you for the responses. Martin, I like your suggestion of sandwiching the foam with long cap nails rather than encasing them in framing. Not only will this reduce mantime to build layer 2, it will be more energy efficient.

    After 2 full days of work, the cut & cobble is pretty much complete. From my experience thus far, cut and cobble is a bit of a pain in the derriere but using thick 4" rigid foam along with a table saw and chop saw has made it bearable. I cannot imagine pancaking multiple layers of foam in each rafter. Wow, that would be a lot of work. What made this project bearable was that my 4" foam meant that each rafter bay only needed to be custom fit once.

  5. tr24 | | #5

    Hi, I have vented shed/mono 'low' slope cathedral ceiling with 10" roof joists (91/2").
    Similar to above, could I put 31/2 inch EPS (seal it well) then add 51/2" or 31/2" inch of rock wool and then 4x8 sheets of polyiso or eps?
    Questions then are, is half inch air space between first layer of EPS and sheathing enough and if not, is 21/2 ok if I use 31/2" eps and 31/2 in rock wool? Also, is 1" enough of the 4x8 sheets on last (bottom most).
    Also, I have enough 31/2 EPS to double up / sandwich, but wondering if the eps/rockwool will save the second layer of cobble time.....
    Thanks, Todd

  6. tr24 | | #6

    To add to above in case relevant, off grid cabin, 4 season use, Ontario (zone 6 I believe), wood stove heat for occasional
    winter use and no AC in summer (perhaps window unit in july for a few hours a day off generator to remove humidity for comfort/sleep

  7. tr24 | | #7

    sorry for another, but read martins building cathedral ceiling....I will be putting tongue and grove as the interior finish and wondering if the proposed solution/question still requires drywall or can tongue and grove be installed right over inner layer of rigid?.

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