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Getting the details right – unvented cathedral ceiling

brentwilson | Posted in General Questions on

I have asked previous questions on this site as part of the process of trying to finalize the roof insulation plan for my parent’s house.

It looks like this will end up being an unvented flash and batt assembly. Climate Zone 6B. Roof Pitch 5:12. Build for 100-pound snow load. Minimum R-50 (hopefully R-60+). Roof rafters 12″ (about 11 7/8″ actual) BCI I-Joist type rafters.

Here is the basic build-up I am thinking to go with, from the exterior inward:
1. Standing seam metal roofing
2. Roofing underlayment and Ice & Water shield (at least where required)
3. 5/8″ Zip System sheathing
4. 11 7/8″ BCI rafters with the following insulation in the rafter bays:
          a. 5.5″ closed-cell spray foam (approximately R-35.75+)
          b. 6.375″ dense-packed cellulose (approximately R-22.3)
5. 1/2″ to 1″ of rigid foam board insulation.
6. Taped and mudded drywall (only if needed)
7. Pine T&G ceiling

Does this sound okay? I would like some specific brand/type recommendations for layers #2 and #5.

What kind of roofing underlayment do you recommend? Does it extend over the top of the ice & water shield?

What about the ice & water shield? Am I okay just doing the ice & water shield on the lower portion of the roof and in the valleys? Or is there a huge advantage to covering the whole roof with it?

What kind of rigid foam board do you recommend? Should this be foil-faced and taped? Or not faced? Other?

Thanks for your advice regarding this!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Brent.

    The first thing that pops out at me is the use of ZIP roof sheathing plus ice and water protection and an additional roofing underlayment. Why not save some money and use OSB or plywood sheathing if you are not relying on the ZIP underlayment?

    The use of ice and water membrane at the eaves and under the roofing of closed valleys is what the International Residential Code calls for. you'll have metal lining your valley, which also meets these codes, I believe. Using a fully adhered membrane over more of the roof is an option that some people take. Because you will have a vapor-closed material beneath the sheathing, if you also cover the roof in a vapor-closed material, it is important that the sheathing is dry when you do so. Otherwise, builders typically choose synthetic underlayments for metal roofing. More on that here: Choosing Metal Roofing.

    Your assembly need to be able to dry inward. If you install continuous interior rigid foam beneath the I-joist rafters, it needs to be vapor-permeable. EPS would be a good choice. Another option would be to just run strips of rigid foam along the bottom of the I-joists to mitigate thermal bridging and make more room for cellulose in the cavities.

    The drywall installation is a good idea for air sealing.

    1. brentwilson | | #2

      So if I use Zip sheathing, I don’t need an underlayment? Or, inversely, if I use underlayment, I don’t need Zip? Either way I still need ice and water shield down near the eves, right? And with the metal valley pieces for the roofing, it sounds like the ice and water shield may be optional in the valleys? If I use Zip with the metal roofing above it, what seals the screw holes from the roofing? Would the spray foam below the sheathing help to seal those penetrations? Or is just the fact that there is a screw in the hole enough to seal it?

      I like the idea of foam strips on the bases of the I-joist rafters. Would we just cut those strips down from foam board sheets? I think this would be cheaper to do the strips and use thicker cellulose rather than covering the whole thing in rigid foam - especially if I would need drywall between the EPS sheets and the T&G anyway. I am wondering about thickness and screw length though. If I attach strips of 1” foam to the undersides of the rafters, then drywall, then pine T&G, am I going to be okay without also adding horizontal strapping? And if I do the strips of foam, should it still be EPS?

      Thanks for the great information you provided!

  2. Expert Member
    Akos | | #3

    Metal roof or not you always want I&W at the valleys. This is a prime area for leaks, extra protection never hurts. Metal roofs typically require an underlayment to allow for expansion. I don't think the coating on ZIP qualifies for this, but check with the manufacturer of your metal panel.

    Also I&W at the base of ZIP would be considered a reverse lap, which can be dealt with but adds extra steps plus more failure points. Simpler to stick to plywood + underlayment and I&W where needed.

    You should figure out what your local codes require for ignition barrier and also what your insulation installer wants.

    If you go with foam strips, might be simpler to go with taped OSB with longer screws through the foam strips into the rafters, you can then nail your T&G onto the OSB.

    If you are paying for the labour, it would be cheaper to go with deeper I joists than going with foam strips though. Also keep in mind that every time you add insulation, you need to also add spray foam. The incremental cost of that usually negates any savings from reduced energy use.

    Make sure you have somebody that can dense pack, around here it is more expensive than getting batts installed.

    1. brentwilson | | #4

      With 5.5" of spray foam, there would already be plenty of R-value in the spray foam to offset the little bit of additional insulation added with the deeper bays without having to add more depth to the spray foam.

  3. brentwilson | | #5

    Okay, based on your helpful comments here, this is a revised build-up from the exterior inward:
    1. Standing seam metal roofing
    2. Synthetic roofing underlayment (Ice & Water membrane at eves and valleys)
    3. 5/8" OSB or plywood sheathing
    4. 11 7/8″ BCI rafters with the following insulation in the rafter bays:
    - a. 5.5″ closed-cell spray foam (approximately R-35.75+)
    - b. 7.375″ dense-packed cellulose or batt insulation (approximately R-25.8)
    5. 1″ thick rigid foam strips attached to the bottom of the i-joist rafters (creating a deeper rafter bay and helping to minimize thermal bridging)
    6. Taped and mudded drywall
    7. Pine T&G ceiling

    1. Expert Member
      Akos | | #6

      That should work.

      Ceiling thicknes might be too much for nailing up the t&g, you won't get much embed. Either reduce the foam or go with osb instead of drywall. One option could be to rip Zip-r for you foam strips, that would give you something to nail into.

      With the spf, this will be a pretty expensive roof, so best to only put enough for condensation control for your climate.

      Another lower foam use option would be to go with SPF for the rafters that can't be esaly vented in the valleys and a vented assembly elsewhere. This way you would only have to spray foam maybe 15% of your roof area. Even if you have to bump up you rafters to 14" TJI to get the R value, it would still be way cheaper than all spf roof.

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