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OSB, Rockwool, and Vapor Retarder

Renovations102 | Posted in Building Code Questions on

Hello everyone, new member here with some questions I’m sure are easy for you folks. I’ll try to be clear & to the point.

I air-sealed my stud bays in the garage and will use tape on the smooth side of the OSB horizontal joints before insulating, will Zip tape work well long-term for this? (Interior, smooth side of the exterior sheathing). Garage will be conditioned on days of excessive heat or cold + excessive humidity w/ a mini-split.

Interior walls in the garage will also be OSB (except wall leading to home). I was going to use kraft-faced R-19 glass but now am thinking about R-23 Rockwool. However, I’m in Martin’s neck of the woods, so I assume that if I do the rockwool, I now need to deal with a vapor barrier? If so, what is recommended?

Hand-in-hand w/ that is whether or not one can just use a vapor barrier primer on the smooth side of the interior OSB I’m going to hang to achieve this? I have “read” that painting or priming an entire wall of OSB is not recommended…

If a barrier is needed, is the rockwool worth the extra effort to do both steps instead of just using the kraft-faced R-19 in one step…?

Thank you all. If you need clarification, please ask.

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  1. brian_wiley | | #1

    Hi, Reno, welcome to the forums.

    Martin Holladay has a couple of articles that would be of interest in regards to your tape question. They’re great in that they explore a lot of options, some of which may be more effective and cheaper in your case.

    When you say “ Interior walls in the garage will also be OSB (except wall leading to home)“, do you mean that you’ll be hanging OSB as a replacement for the drywall? That is, OSB will be on the interior walls of the garage?

    1. Renovations102 | | #3

      Hi Brian.

      Yes, 3 of the 4 interior walls in the garage will be OSB. Drywall on the ceiling & on the wall joining the house.

    1. Renovations102 | | #5

      Thank you. I read it, but it does not specifically answer my question...

  2. Deleted | | #4


  3. Renovations102 | | #6

    Additional reading has not clarified the issue. Some are saying in zone 5, you need a vapor barrier like membrain over the rockwool, on another site I saw this statement "Because of its greater density and water resistant properties, mineral wool acts as a vapor barrier and, unlike fiberglass, does not need an additional vapor barrier to be effective."

    To be clear, the garage will be heated & cooled via mini-split as needed, OSB exterior walls, no idea what is between that and the vinyl siding. The interior side walls will be OSB as well, the ceiling will be drywall, with R-38 dense-packed cellulose above it (air-sealed & insulated bonus room above garage), and the wall abutting the house is drywalled as well.

    So I still dont' know if a vapor barrier is needed in this situation, or what the benefit would be...

  4. GBA Editor
    Kiley Jacques | | #7

    I realize your question is hyper-specific but I do think you should read this article, Five Walls that Work. It will shed light on some of your control-layer-related inquires.

    1. Renovations102 | | #8

      Thank you, looks like I need a membership or something to read it though...

  5. brian_wiley | | #9

    I’d say go with a smart vapor retarder such as MemBrain if you’re use Rockwool (and, I would go with Rockwool as well for the higher R-value).

    That said, I think your wall would still likely work if you omitted the vapor retarder as you’re air sealing the sheathing and you have vinyl siding, which inherently allows the sheathing to dry as though it had a rainscreen. The other component that helps
    is that this is a garage, so it likely won’t see the moisture loads that a usually occupied space would see.

    These are all assumptions of course, and any one or all could change in the future, which is why I’d probably include the smart membrane if it were me.

    1. Renovations102 | | #10

      Thanks again, Brian!

  6. Jon_R | | #11

    Re response #5

    Table 2A. Say that your exterior layer (just OSB?) wet perms = 4. This leads to a Class I interior side vapor retarder in Z6 and a Class II in Z5. Say that interior layer OSB dry perms is 1.5. In this example, you are OK in Z5 but not in Z6.

    Air sealing the interior side is a good idea.

    1. Renovations102 | | #12

      Hi Jon,

      I saw that, I just don't know how figure the wet & dry perms, never done it before.

      And for info, this is rockwool techs response to vapor barrier:

      "ROCKWOOL is a vapor open product… air and vapor can slowly diffuse through it so will not act as an air or vapor seal."

      And I'm air-sealed with tape, some contolled bead foam from pro gun, and NP1 caulking, depending on what was being sealed...

      1. Renovations102 | | #13

        Thanks for all the help. I went with Rockwool & Membrain...

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