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Vapor permeable rigid insulation boards???

quantumgirl | Posted in General Questions on

Is there such a thing? I’m assuming all rigid foam boards are not vapor permeable?

I have a home addition with a flat EPDM roof. I missed adding insulation underneath the EPDM since it’s technically not required in climate zone 2. Now I wish I had.
so im thinking of open cell spray foam in between the 2×6 roof rafters. Then adding some other piece of sheet type continuous insulation that can be screwed or nailed to the bottom of the rafters before my Sheetrock goes up.
id like to add R-value and eliminate thermal bridging which is probably quite significant since the rafters are 12”oc. Since I already have a vapor barrier on the outside (epdm) I assume I would need something permeable on the inside so I don’t create a double vapor barrier?

anybody have any ideas or tips on what could be used?

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  1. user-2310254 | | #1

    Different types of foams have different levels of permeability. What type of foam did you install and how much? Why are you concerned about your installation?

    1. quantumgirl | | #2

      I don’t have any insulation installed yet. I have epdm on the roof which is a vapor barrier. I want to put open cell spray foam in the rafter bays and add some other type of rigid insulation on the underside of the ceiling. That insulation needs to be vapor permeable so I don’t trap moisture inside the roof with a double vapor barrier. Or am I overthinking this?

  2. Expert Member
    Dana Dorsett | | #3

    .>"I’m assuming all rigid foam boards are not vapor permeable?"

    That assumption would be incorrect. All rigid foam board that does NOT have foil or plastic facers have at least some vapor permeance, and in your stackup that's probably a GOOD thing.

    There is such a thing as (semi) vapor permeable polyiso, and the vapor permeance of Type-1 or Type-VIII EPS isn't super low either, eg:

    The problem is that the open cell foam is too vapor permeable to fully protect the roof deck, and even if one installs interior side foam rigid foam that's in the Class-II vapor retardency range it could be deemed a code violation, even though in most locations it would be pretty safe.

    How thick can your rigid foam layer be? With open cell foam between the rafters you technically need the foam layer to be under 1 US perm to meet codes designed to protect the roof deck, but the amount that it really takes varies by climate zone and the solar reflective index of the roofing (black EPDM has a very low SRI, so the sun help keep the roof deck dry.)

    Unless you can give up 4" of headroom you're probably better off with fiber-faced roofing polyiso (0.5 perms due to the characteristics of the facers), or using at least 2" Type-II EPS ( 1.25-1.5 perms @ 2"), or thinner unfaced EPS and a membrane type "smart" vapor retarder such as Certainteed MemBrain, or simply paint the ceiling gypsum with "vapor barrier latex" primer, which brings it down to 0.5 perms.

    There is no way to "...eliminate thermal bridging..."- the thermal conductivity of the rafters will always move more heat than the in-fill of open cell foam.

    1. quantumgirl | | #4

      So if I understand you correctly, you’re saying I want a vapor barrier on the inside? I thought vapor barriers should always go on the outside in my climate zone, never on the inside. Now I’m even more confused. Im in Houston, hot and humid climate zone 2. We have the heat running maybe 3 weeks per year. Rest of the time the AC is on.

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