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Community and Q&A

Unvented Roof Assembly with Open-Cell Spray Foam

Doelman | Posted in General Questions on

We’re looking at constructing an unvented 2:12 pitch skillion roof with a cathedral ceiling in zone 3A using this detail
except we would be using open cell foam insulation.

I’ve read that there can be moisture problems at the decking with open cell, but there seems to be a lot of conflicting information regarding what causes the issue and what the solution is.  There seems to be two schools of thought, you need the ceiling to be air and vapor tight or you want it to be leaky so humidity doesn’t build up in the roof assembly.  I’m not sure what to think here, could leaky recessed lights actually be a viable solution here?  A couple small inconspicuous vents at the top and bottom, inside the house, to essentially ventilate the roof with conditioned air?

According to Lstiburek, installing a supply and return air duct in the roof assembly will solve any condensation issues with this type of roof.

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  1. Expert Member
    ARMANDO COBO | | #1

    You got problems, and it’s that pesky little thing called Building Codes… Read R806.05

    1. brendanalbano | | #7

      Whether or not it's a good idea is a different question, but I believe R806.5 allows an open-cell spray foam unvented roof in climate zone 3. Per 806.5.4, the "air-impermeable insulation" only needs to be a class II vapor retarder in climate zones 5 and higher. It's the "shall be a class II vapor retarder" part that requires the use of closed-cell foam rather than open-cell foam.

      By my reading, open-cell foam in climate zone 3 would satisfy 806. and thus be allowed.

      Am I reading that right? I haven't worked in climate zones warmer than 4, so I may be off base. (And the Oregon Residential Building Code essentially follows the zone 5 IRC rules for unvented assemblies, even though plenty of Oregon is zone 4C)

      That said, I would be hesitant to install an all-open-cell cathedral ceiling in any climate, but again, I'm not a warm climate expert.

  2. Expert Member


    Can you post a link to Joe's advice on mechanically ventilating these roofs?

    1. Doelman | | #3

      You can find his comment here

      "There was no mechanism for moisture removal in this attic - a simple approach is the use of a supply air duct coupled with a return path. This was not done. In many cases it has proven to be unnecessary when supply ducts are leaky and ceiling planes are leaky - typical construction - the air change provided between the attic space and the house with leaky ducts and a leaky ceiling serves to remove the moisture in the attic. Where supply ducts are tight and where the ceiling plane is tight this mechanism does not work and an active supply duct and deliberate return path is necessary. This should not be news to experienced researchers. It is pretty well understood by many installers using spray foam. But it is apparently not well understood by everyone and clearly a code change would be helpful. No air change measurements via tracer gas were done to provide information about the lack of communication between the attic and the house. Modeling was used by individuals with little or no experience with this type of construction. The modeling was reviewed by people with even less. The comments on moisture sources and energy impacts was also misleading."

      1. Expert Member
        MALCOLM TAYLOR | | #5


        You are mixing up a few different things:

        - A cathedral roof, and an attic with spray foam insulation on the underside of the sheathing.

        - Air leakage into assemblies, and providing balanced ventilation to condition an attic.

        There is no situation where air leakage, whether inadvertent or intentional is good for vented or un-vented cathedral ceiling. You want the underside of the assembly to be as airtight as possible.

        Open cell spray foam in attics is problematic, but the risk can be mitigated by conditioning the attic and limiting humidity.

        Open cell spray foam in un-vented cathedral ceilings is problematic period.

        1. Doelman | | #10

          what's the difference between open cell foam and batts? they're both permeable and detail #3 here shows batts being used

          It also appears Holladay says interior air sealing on this type of roof assembly isn't required.

          CarsonB | Dec 18, 2020 01:16pm | #55
          " the closed-cell spray foam is the air barrier". So out of these assemblies, only #1 and #2 would truly require an interior air barrier, with #2 the interior air barrier likely being the interior side foam. For #3, 4, and 5, the exterior foam boards or spray foam would be sufficient to prevent condensation issues, so interior air sealing is not required, only an extra precaution.

          GBA Editor
          Martin Holladay | Dec 18, 2020 02:03pm | #56
          Yes, you're right.

          1. Expert Member
            MALCOLM TAYLOR | | #12


            You are mixing and matching advice again. "The closed cell spray foam is the air-barrier." That's what is called out in all the assemblies with spray foam. You can't simply substitute open-cell foam and have them function the same way.

  3. user-2310254 | | #4


    Damp sheathing seems to be more of a problem in colder climate zones. See for more info.

    Have you considered installing a vapor diffusion port? See for more info.

    1. Expert Member
      MALCOLM TAYLOR | | #6

      User ...254,

      "Damp sheathing seems to be more of a problem in colder climate zones. "

      I'm not sure that's what the article says. Their take away is: “Open-cell foam is risky in all climate zones”.

      1. user-2310254 | | #8

        Hi Malcolm,

        That's correct. The authors also state: "If you choose to install open-cell spray foam against the underside of roof sheathing in a humid climate, your HVAC system be designed to condition the attic air and lower humidity levels in the attic."

        In zone 3A, open cell applied against the roof sheathing is common. Not an expert, but my personal experience is that it's very difficult to find contractors who are interested in spraying closed cell. In a couple of my homes, I used open cell to create conditioned attics. But I made sure those spaces received adequate supply air, and I continuously monitored the humidity conditions. One home was fairly tight, and the monitoring was showing elevated humidity levels despite the supply air. To keep things safe, I installed a whole-house ventilating dehumidifier.

        All that said, I now have a better understanding of best practices. If I was doing those projects today, I'd find a way to install closed cell. (Or, more likely, try to avoid using foam at all.)

        1. Expert Member
          MALCOLM TAYLOR | | #9

          User ...254,

          I agree. This thread is a continuation of one on what type of assembly to use, and all the discussion was around safely venting a 2:12 roof. I think figuring that out is a lot easier than chancing using open cell foam in a cathedral ceiling.

        2. Doelman | | #11

          you're right, here in 3A all the spray foam guys use open cell, they all swear by it, and no one has ever heard of any roof deck water issues stemming from the foam. Some of these companies have been around for decades so I would hope they're being honest about their experiences, but its hard to know.

          1. Expert Member
            MALCOLM TAYLOR | | #13


            Lots of assemblies that are potentially damaging work just fine. If you are content with some risk then sure, go ahead and accept the advice of local installers and use open cell spray foam.

            I think it's useful to look aback at what lead to you having two fairly unpalatable choices for how to deal with this roof. The easiest way to avoid these situations is at the design stage, by not including assemblies that require risky or expensive solutions. On your project it was as simple as raising the roof pitch to 3:12 or specifying deeper rafters.

  4. Expert Member
    ARMANDO COBO | | #14

    About that pesky Building Code...
    1- R806.5 To build an unvented (should be Conditioned Attics) attic in CZ1-8, you have three choices:
    a) 5.1.1 All rigid foam on top of the roof decking.
    b) 5.1.2 Rigid foam on top of the roof decking and PERMEABLE INSULATION under the roof decking, and shall be in accordance with the R-values in Table R806.5 for condensation control.
    c) 5.1.3 Rigid foam or ccSPF directly under the roof decking and PERMEABLE INSULATION directly below the ccSPF, and shall be in accordance with the R-values in Table R806.5 for condensation control.
    2- 5.2.7 Vapor diffusion ports are ALLOWED in CZ1-3 for roofs with 3:12 pitch or higher, which means Mr. Doelman cannot use it with his 2:12 roof.
    Vapor diffusion ports are installed at the ridge of gable and hipped roofs, and requires the sheathing to be cut back on both sides of the ridge. I do not believe they are used in shed or skillion roofs, or at least, I’ve never heard of one.
    Some folks say you could LESSEN the risk of rotted sheathing in CONDITIONED ATTICS by installing a full amount of code required impermeable insulation (R38 in CZ3) PLUS providing ventilation at a minimum 50 cfm/1k ft² of attic area. In addition, installing a dehumidification system helps a long way.
    The insulation industry has lied for many years about installing 6” R21 ocSPF against the roof decking to be price competitive and with the pretext of using the prescriptive code, which is a wrong interpretation. What they don’t tell clients is that ocSPF is an air-permeable insulation (30 perm/in), even though it’s a decent air-sealer. The worst part is that ocSPF is NOT moisture impermeable, which means, ocSPF allows for moisture vapor to migrate to the underside of the sheathing.
    You may get away creating an ocSPF unvented attic, but are you willing to risk your own house or reputation and a possible lawsuit on your client's home?

  5. alliedinsulation | | #15

    If an ocSPF has an air permeance of <0.02 L/s/m, then would it be okay to use at an unvented cathedral ceiling assembly? I don't see a requirement for moisture impermeability in R806.5 for climate zones 3-15 in CA (where we do most of our work).

  6. begreener | | #16

    Why not spray closed cell foam first to a sufficient depth that the surface would stay above the dew point & then open cell to get the sufficient R-value?

    That's what I did for a "hot roof" application in my house in NH

  7. alliedinsulation | | #17

    The cost of closed cell is prohibitive and I wanted to see if the open cell was sufficient for our climate zone. Now that they are making open cell foam that is air impermeable (air permeance of <0.02 L/s/m), I was hoping we could forego using closed cell without any problems with inspectors (and without developing condensation in the future).

    1. Expert Member
      MALCOLM TAYLOR | | #19


      It's the vapour permeability of the open-cell that is the problem. The moisture isn't piggybacking on air-leaks, it is moving through the foam by adsorbtion.

  8. begreener | | #18

    Even if you just did a "flashing" of closed cell foam to completely seal the underside of the roof deck, I think you would be a bit "safer" in your assumption ...

    If the open cell with low air permeance is the belt, the flashing of closed cell could be considered your suspenders!

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