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Epdm roof with double vapor barrier

quantumgirl | Posted in General Questions on

I’m in climate zone 2, hot and humid.
I have an EPDM flat roof on part of my house. The EPDM is glued directly to the plywood deck. There’s no exterior insulation under the epdm. 
I now need to insulate my 2×6 rafter bays. My spray foam guy recommends using closed cell foam. He says in the winter time, interior moisture can travel through open foam and cause mold issues on the plywood by condensation, since no exterior insulation was added between plywood and epdm to keep the plywood warm enough.

While I do get his point, wouldn’t closed cell foam create a double vapor barrier since I have EPDM on the outside of the plywood? If the EPDM ever leaks, there would be no drying potential to either side, correct?

Should I got with open or closed cell foam?

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  1. quantumgirl | | #1

    No one have any insight?
    So my walls in the same room are 2x6 with 1.5” polyiso on the outside. I was definitely gonna go with open cell there, as it’s cheaper, and I don’t think I need to be concerned with condensation since I have enough exterior rigid foam to keep the sheeting warm, correct? I’m still trying to wrap my head around the physical aspects of all of this.

  2. brian_wiley | | #2

    Hi quantumgirl, while I can't speak specifically to your details, I thought this article by Martin Holladay may help out:

    And, it may be helpful to list out the full assembly details of the wall just so that we’re not overlooking a critical detail. .

    1. quantumgirl | | #3

      Thank you for the article. Seems like Martin is sayings that a double vapor barrier isn’t necessarily dangerous, as long as the roof doesn’t leak. The EPDM is a diy job and while I feel like I did a good job, I probably wouldn’t trust it to last past 20 years. 🤔
      Personally, I would feel safer using open cell foam, so roof leaks can be recognized easier and I have that inward drying option. I wish I would have added exterior insulation underneath the epdm, but it’s too late now as I’ve already build a deck on top. Poor planing on my part for sure.
      In your opinion, what would be a more likelier problem: 1) a roof leak and no drying potential to either side when using closed cell foam. 2) condensation issues from interior moisture in the winter time with open cell foam?

      I think I did a better job with the wall assembly, after doing a lot more research:
      2x6 stud wall construction, plywood sheating with taped seams, Tyvek, two 3/4” layers of poly Iso with offset and taped seams, rainscreen with 3/4 treated ply furring strips, hardie siding.
      And I plan on filling the entire wall cavity with open cell spray foam and then Sheetrock. Does that sound like a good wall assembly? It’s actually gonna give me way more R value than the required R13 for my climate zone. Are there any cons to having too much insulation?

      1. brian_wiley | | #4

        I believe your roofer is correct: that un-insulated plywood would be a condensing surface for any interior moisture, and that open-cell spray foam won't do much of anything to stop it from reaching that surface.

        As for the discoverability of leaks with/without closed cell spray foam, Martin Holladay responded to a post (#2 in the comments) regarding that. There's more to his answer, but basically he said this: "Eventually, all roofs leak. If your roof eventually develops a leak, you will know it. The first sign is usually a stain on your ceiling. These facts are true whether or not your roof sheathing has been sprayed with closed-cell foam."

        It seems like you're damned if you do/don't relative to leaks, (although I understand your anxiety relative to the deck up there…) so it would make sense to me to focus on preventing the vapor reaching the condensing surface of the plywood.

        Also, in a 5.5" deep cavity, it seems like you'll have trouble achieving r-38 with open-cell. Closed-cell will just get you there, so there's that benefit as well.

        On your wall system (which sounds great from my understanding of it), I don't think there are necessarily any cons to too much insulation unless the ratios of exterior to interior get imbalanced. However, that doesn't apply to zones 1–3, so you're set there. You could make the argument that you're probably in a diminishing return on investment situation though. BEOPT software would probably tell you where that line is.

        One other thing to mention is that given your wall system, you don't really need to use spray foam. You're probably aware, but it has much higher embodied carbon than diy-options like mineral wool or fiberglass. And if you're already paying for an installer, you could always switch over to dense-pack cellulose, which is super low on that scale. Just something to consider. In any case, as someone with a very leaky r-13 assembly, I'm jealous of your wall.

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