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Community and Q&A

Interior Smart Vapor Retarder

wiscoguy | Posted in General Questions on

I’m not sure I’m thinking of this correctly so please help me out. I’m putting 2.5” of exterior foam to get my 33% outer insulation then rockwool.

After that I’m thinking of doing intelligent on the walls.

what about the roof though? The roof will just be blown in insulation amd be vented so I’m not sure if I should be using 15 mil plastic on the roof to keep moisture from going up ? Or continue the intelli everywhere?

Is just drywall with 2 layers of latex paint enough or do is it preferred to do the smart vapor barrier?

appreciate any thoughts on the subject.

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    1. wiscoguy | | #4

      Thanks for the reply.

      That’s a big part of my question is wether intello is necessary given that drywall can provide the same protection. I suppose that even with intello an errant screw or wire or wife hanging pictures could lead to air penetrations. Do you feel this could still be a problem? I will be doing my best on all penetrations but as things progress I’m not sure how vapor will work through out the building deaths ility of the structure will occur especially in the roof. I do have a small section of roof where there is a cathedral thatvwilll have proper vents so I will need to pay special attention to that area.

      Thanks appreciate any more thoughts.

  1. Expert Member
    BILL WICHERS | | #2

    You don't generally need any vapor retarder or barrier in the ceiling under an attic floor in a vented attic. The reason is that the small amount of moisture that might make it up to the attic by diffusion through the drywall is so low that the venting for the attic shouldn't have any problem taking care of it.

    In the walls, it's not usually really required (unless by code) if you have sufficient levels of exterior rigid foam for your climate zone, but it does provide some extra insurance. In my own home, I have enough exterior rigid foam, but I also have a smart vapor retarder under the drywall on the interior. The smart vapor retarder doesn't hurt, but it does help to minimize the chances of your having any moisture issues in the walls over time.


    1. wiscoguy | | #3

      Thanks for your reply.

      My wall is a simple 2x6 standard frame wall with rockwool insulation and then 2.5” of exterior foam for a r 12.5 on the exterior.
      I won’t have a service cavity. We will have a whole house dehumidifier and humidifier kept at 35% roughly. And an erv which I don’t think will help.

      I’m a little concerned about missed screw in drywall wife hangin pictures small holes in the drywall that would allow water vapor into the cavity. Am I over thinking this and shouldn’t need to worry about this or could it lead to a problem down the road?

      I’d prefer to just do drywall because the intello is kinda pricey and time consuming. If drywall is acceptable an won’t cause problems it could save some time and money.

      Appreciate any thoughts.

      1. Expert Member
        BILL WICHERS | | #5

        A hole in the drywall that has a screw in it isn't an air leak. Just make sure that if you take the screw OUT, you patch the hole. Aside from that, it's not a big deal to hang some pictures on the wall. I'd worry a lot more about hitting a pipe or wire than I would about puncturing the drywall itself.

        I used CertainTeed's MemBrain myself, which is a lot cheaper than Intello. The problem is that MemBrain seems to have dissapeared from the market as of late, which CertainTeed has apparently been telling people is due to supply chain issues.

        Regarding the "can I just do drywall?" question, what climate zone are you in? If you exterior foam is sufficient for your climate zone (there are articles here on GBA about that), and your wall is air tight (which isn't hard to do, just install your drywal using the airtight drywall method, also described in articles here on GBA), then you don't really need a vapor retarder unless it's required by your local codes. I personally would still try to include a vapor retarder in the assembly for extra insurance though.


        1. wiscoguy | | #6

          Thanks I forgot to mention I’m in zone 6 just north of Chicago. For my area it says that 11.25 on the exterior of the building on a 2x6 wall is what’s required by code and meets the 33% idea of exterior insulation as well.

          I have been searching for membrane from certaintees it doesnt exist currently but intello does just pricey as all get up. If I could find the membrane it would be a no brainer but the 6k to do my house is significant for intello. I will check into the airtight drywall method.

          Thanks again

        2. user-7061227 | | #7

          Do you think that in Wiscoguy's case (and mine) if we do not intall a smart vapor barrier behind all the drywall that there are certain ares where it could be of extra benefit - like in the bathrooms, laundry and kitchen/areas that produce high moisture. I am thinking of installing it at least on walls and ceiling of bathroom (behind drywall). Thanks

          1. wiscoguy | | #8

            I too would be interested to hear these things

          2. Expert Member
            BILL WICHERS | | #9

            I don't think it hurts to put it in in places that may be higher risk, and I agree places like bathrooms (and I suppose laundries and kitchens too, to a lesser extent), would count as "higher risk" in this case. I'm a big fan of exterior rigid foam in "thick" (much more than code requires) thicknesses, but I still think an interior vapor retarder is some extra insurance and nice to have.

            I would only bother with the vapor retarder in places that make sense though. That means NOT on interior walls, and NOT behind things like showers if they're installed with impermeable membrane backer products (Kerdi, Ditra, etc.). If there is already a vapor barrier product like one of thoe poly tile backer membranes installed, a vapor retarder isn't really going to do anything in that area of the wall.

            Note that I ALWAYS detail exterior wall drywall as a primary air barrier too. It's not always necassary on interior walls, depending on how things are framed and if there are any penetrations.


  2. LaciB | | #10

    Hi Bill,
    With the amount of humidity that is created by teens showering, I think smart vapor retarder in bathrooms behind non ditra/waterproof membranes is a good idea.
    Also, I will be installing 4” rigid on the exterior and roxul on interior. Plan to install Hp windows over wrb at the sheathing plane (in between idea). I am choosing between Inline windows that have an extended exterior buck and Alpen without. How would you suggest we detail the Alpen windows?

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