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To vapor barrier or not to vapor barrier?

jibba1 | Posted in General Questions on

Hi all,

Pardon my ignorance or lack of proper nomenclature, but have a question regarding vapor barrier/retarder (I know they are not one and the same) and insulation.

We live in NH. We have an office/den that was an addition built over a very shallow crawl space. Two of the walls are exterior. We ripped up the floor, joists, etc, and, in doing so, had to remove some interior tongue & groove paneling that runs horizontally about 3.5′ up from the floor. Above that, the walls are covered in drywall, which we’re leaving. We cut the existing faced fiberglass about 3′ up between the studs to get at the electrical and run new wiring, etc. We’re ready to put new insulation where we removed it and are planning to use unfaced Roxul, but my question is whether or not to put up any vapor barrier/retarder on top of the insulation along the (2) exterior walls. Our plan is to put shiplap over it. Externally, there is siding, and I can’t tell if they used a housewrap or not. There is certainly not any exterior insulation.

The room is heated somewhat in the winter, but not air conditioned in the summer, if that helps. Grappling with whether or not to put anything on the interior of the new insulation before we put on the shiplap.

Appreciate any insights!

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  1. Expert Member
    Dana Dorsett | | #1

    Location matters is this US climate zone 5 (southern & coastal NH) or zone 6 (most of NH)?

    It definitely needs an interior side air barrier, the vapor retardency of which depends a bit on the exterior sheathing & siding, and which climate zone. Ship lap is not an air barrier, but is somewhat vapor retardent (varies by species and finish). The best approach is a full gypsum board detailed as an air barrier, but whether it needs other treatment (smart vapor retarder membranes, standard or vapor retarder primers, etc) depends on your climate zone.

    If it has plywood or OSB sheathing, prior to take the time to lay a bead of polyurethane caulk (not the cheap stuff) at the intersection of the the sheathing to the framing (studs + top & bottom plates) in every stud bay, as well as a bead at the intersection of the bottom plate & subfloor.

  2. jibba1 | | #2

    Dana - Thanks for your message. We're on the border of zone 5/6.

    The exterior has horizontal planks under the siding. We are unlikely to put gypsum under the shiplap for a variety of reasons, though we're aware that that is a recommended option.

  3. GBA Editor
    Martin Holladay | | #3

    You should take Dana's advice and install a durable air barrier between the shiplap finish boards and the studs. If you don't want to install taped drywall, at lease install Intello membrane, detailed as an air barrier.

    You'll have to be creative when it comes time to air-sealing the horizontal joint between the Intello and the drywall above. You'll probably need to install horizontal 1x3 nailers between each stud to make this horizontal seam airtight. These 1x3 nailers can be screwed to the drywall before you install the shiplap boards.

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    What is the sheathing and siding stackup? (It matters!)

    I wouldn't trust a membrane vapor retarder detailed as an air barrier be good for 5+ decades under shiplap wainscot, but taped wallboard painted with half-perm primer should. Intello is pretty tough stuff compared to 2-mil nylon (and 3x the price), but still...

  5. jibba1 | | #5

    Hi Martin, thanks for your note. What would the recommended depth be for the drywall?

  6. GBA Editor
    Martin Holladay | | #6

    1/2-inch drywall is inexpensive and works just fine.

    -- Martin Holladay

  7. jibba1 | | #7

    Last question - does it need to be mudded & taped, or will just taping suffice? (It won't be the appearance layer, so asking for sealing purposes, not aesthetics.)

  8. GBA Editor
    Martin Holladay | | #8

    You can't tape drywall without some mud. You just need to apply the tape with mud as usual. You can skip the second and third coats of mud, though.

    -- Martin Holladay

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