GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Air and/or Vapor Barrier for Tongue-and-Groove Finished Wall

timgodshall | Posted in General Questions on

I am interested in using horizontal tongue and groove boards instead of drywall for the inward-facing plane of my exterior walls on a house in climate zone 4 (Harrisonburg, VA.) I have two different wall assemblies I am dealing with:

1)  2×4 framed walls with taped zip sheathing on the exterior to create my air barrier at that plane,  1″ of polyiso foam covering the zip sheathing, fiberglass batts between studs.

2) CMU block walls with 1″ of polyiso foam adhered to the inside plane with taped seams and canned foam around edges to create air/vapor barrier. 2×4 stud walls with fiberglass insulation between the foam board and the room.

In either of these scenarios, is any sort of air (or vapor) barrier recommended between the studs and the tongue and groove wood?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-2310254 | | #1


    You could put a smart vapor retarder such as MemBrain on the interior, but you need to be careful to not tear the material when installing the boards. (See for more detail.)

  2. Expert Member
    BILL WICHERS | | #2

    For #1, yeah, you need an air barrier of some sort to keep air from entering the wall and potentially causing moisture problems. Smart vapor retarders can work (MemBrain, Intello), but note that MemBrain seems to be unobtanium at the moment. Drywall works too, and is pretty much the standard for this sort of thing. You could also use plywood or anything else that can act as an air barrier while still being vapor permeable to allow for some inward drying.

    For #2, you don't need an additional air barrier, since the taped polyiso should already provide that function for you.


    1. timgodshall | | #3

      Thanks for your response, Bill. In wall assembly #1, since I am air sealing my wall on the exterior with taped zip sheathing, wouldn't that provide the same level of air barrier as #2?

      1. Expert Member
        PETER G ENGLE PE | | #4

        You need an interior vapor retarder in Zone 4. The taped Zip gives you an air barrier on the exterior, but you still want to limit interior moisture from reaching the cool sheathing. 1" of exterior polyiso is not enough to keep your sheathing above the dewpoint for reasonable interior conditions. So, an interior smart membrane, or just drywall painted with a vapor retarder paint. You want the interior as airtight as possible as well, so that you don't get convection cells set up inside your walls to deliver moisture to the sheathing.

  3. Jon_R | | #5

    You have more than enough exterior foam such that you can and should use a Class III vapor retarder on the interior side. The wood and/or insulation facing will provide this (so no need for MemBrain or VR paint). Being closer to 1 than 10 perms, there is lots of margin.

    IMO, well taped Tyvek CommercialWrap on the interior would be a good enough interior side air barrier. Better than the often used drywall that isn't detailed to be an air barrier.

    See here section 4.2.2 which says that the exterior air barrier alone is OK in Z4. But conceivably this wasn't written with very leaky T&G interior in mind.

    Always test to verify air sealing.

  4. timgodshall | | #6

    Thanks for your responses, Peter and Jon. My understanding based on GBA articles is that 1" of foam is plenty to avoid condensation on my sheathing in climate zone 4 with 2x4 walls.

    When I read the GBA article linked by Steve Knapp in reply #1 above, it seems like a second air barrier is not really necessary if the first air barrier has been well-detailed. I plan to fill stud cavities full of fiberglass insulation to avoid convection loops. Wouldn't the kraft facing of the fiberglass, if properly installed, constitute the necessary vapor barrier?

    1. Expert Member
      BILL WICHERS | | #7

      The kraft paper is an old-school "smart" vapor retarder, that's it's purpose. It's all you really need here vapor retarder wise (note that you do NOT want a vapor BARRIER here). I would still detail the drywall as an air barrier for some extra protection though. It's always a good idea to keep indoor air from getting into exterior walls as much as possible.


    2. Expert Member
      PETER G ENGLE PE | | #8

      The R5 on the exterior with 2x4 cavities exceeds the prescriptive requirements for reducing risk of condensation. In my experience, it is not quite enough to eliminate that risk. I've tested lots of houses with R3-R5 insulation in zone 4 and I've found that most suffer from significant dampness during the winter. Once you get closer to R-10, the sheathing is generally very dry, even in winter.

      Use of an interior vapor retarder is a belt-and-suspenders approach, and it's cheap. Yes, the kraft facing on FG batts is an adequate vapor retarder for your application. Just be diligent about installation, and staple the paper tabs to the face of the studs, not the sides.

      1. Jon_R | | #9

        > I've tested lots of houses with R3-R5 insulation in zone 4 ... most suffer

        How many perms were on the interior and exterior? Was there an interior side air barrier? Vented cladding?

        Prescriptive levels of exterior foam (0" (IRC) and R3 (ABTG) for Z4) usually avoid excessive moisture accumulation (but do allow some condensation). If it's excessive then it's important to document when/where this happens.

        1. Expert Member
          PETER G ENGLE PE | | #18

          Jon, Sorry for the slow reply. I've been on the road a lot lately.

          Most of the houses I've tested with exterior insulation had EIFS or stucco claddings, drained but unvented. Overall exterior perms for most of these systems is about 1. Most had interior-side kraft paper vapor retarders, but no attention to air barriers other than typical drywall details.

          When testing these systems in late winter or spring (we generally didn't do testing in the depths of winter) in NJ and surrounding states (mostly Z4), We routinely found background MC in wood sheathing in areas away from any likely sources of water leakage to float between 15%-25% when there was about 1" of exterior foam insulation. With 2"-4" of exterior foam insulation, we rarely found background levels of moisture above 12%. We also regularly found MC above 20% in wood sheathing with vented claddings and no exterior insulation.

          While 25% MC in cold sheathing is unlikely to allow mold growth, it is far too close to saturation for me to be entirely comfortable. IMVHO, the added R-value and moisture protection of another inch of exterior insulation makes sense in this climate.

          BTW, thanks for the link in #5 above. I was not aware of the new 2021 VR requirements. I guess I've got to spring for the new codes.

  5. Expert Member
    Akos | | #10

    One thing to watch is insulating the CMU with foil faced rigid insulation. The problem is that if any liquid water leaks make it through the CMU it will get stuck behind the layer of foam and cause mold. Usually the best way to insulate masonry walls is with spray foam, the next best thing is coating the CMU inside with a liquid applied WRB before permeable (not foil faced) rigid is installed. See details in Fig 11 here:

    1. Expert Member
      BILL WICHERS | | #11

      You can mitigate the risk of mold behind the polyiso by painting the block with a mold killing paint or primer prior to putting up the polyiso. That's what I like to do, for a little extra insurance. I coat the rim joist and the tails of joists with coppercoat prior to insulating that area too. An ounce of prevention and all that... The same goes for vapor retarders when using exterior rigid foam, unless you're using "thick" exterior rigid foam (well beyond code minimums).


      1. Expert Member
        Akos | | #12

        I've just replaced some faced rigid over masonry with mold issues. The problem is that once liquid water makes it into the space, it has a very hard time drying and gets musty in no time. Maybe permeable rigid insulation could have saved the wall, hard to say and I'm not taking the chance.

        With everything is detailed properly, it can work but not a robust assembly, takes very little for it to develop problems.

        If you want a wall with no issues, SPF is the way to go for a masonry wall.

        1. Jon_R | | #13

          Add this to experts advising not to use < 1 perm rigid foam on the interior side of damp concrete. 1 perm is about right.

          IMO, for a generally dry concrete wall, mold killing/moisture blocking paint plus approx 1 perm rigid foam is OK. Don't add risk with lower perm faced foam.

          > SPF is the way to go
          Being fully adhered, agreed, spray foam is even lower mold risk - but if thick enough to be < 1 perm, I'd expect some increase in "water on the floor" risk.

          1. timgodshall | | #15

            Thanks for the response. Are you recommending open or closed cell foam?

          2. Expert Member
            BILL WICHERS | | #16

            You would absolutely want closed cell spray foam here. Open cell spray foam may appear to help with moisture for a while, but it's going to act like a sponge over time and get soggy -- they you'll have problems.


  6. timgodshall | | #14

    Thanks for all the responses. I'm gathering that the best approach at this point would be the following:

    -- for the 2x4 stud wall with 1" of foil-faced polyiso foam on the outside, a well-detailed air barrier on the inside would prevent the potential for high moisture in the wall (even though 1" of foam is code accepted.)

    -- for the block wall, using a >1 perm foam on the inside of the block is necessary to allow any moisture that comes in through the block to dry to the inside. Spray foam is preferable because it is fully-adhered to the block, although if the block is generally dry, a mold-killing paint + >1 perm foam board is fine. If I go the spray foam route, and I keep the studs 1.5" off the wall, that will allow for 5" depth of open cell foam which I understand has a perm rating of about 10 at that depth. Is that too vapor permeable? Would it be better to do just a 1-2" thick layer of closed cell foam and use fiberglass between the studs?

    One followup question - I am covering the outside of the block wall with vertical board and batten siding to match the framed portion of the house. I will do a rainscreen for the siding with horizontal 2x4 strapping with vertical furring behind that. I need to bring the wall plane out an additional 1.5" to match the existing house, so I was thinking I would adhere 1.5" of xps foam to the outside of the block wall. If taped, this foam would keep the block from getting wet from the outside, but would also limit it drying to the outside as it would be less than 1 perm. Would it be better to eliminate the exterior foam and just use thicker furring to achieve the desired wall thickness?

  7. Expert Member
    PETER G ENGLE PE | | #17

    Foam on the outside of the block wall is a better choice than inside because it will keep the masonry warmer in winter and that reduces the potential for condensation. Doing both is Ok too. EPS foam would be fine. Recycled foam saves $$ and greenhouse gases. You can tape the surface of the foam to act as WRB, but this can be less effective with some EPS foams. XPS has a better surface for taping. You could also use a fluid-applied WRB/Air barrier directly on the block. This would make the liquid water leakage issue discussed above far less of a risk. You probably don't need the vertical strapping, especially if there are still grooved mortar joints on the block (as opposed to a smooth stucco finish). The grooves would provide plenty of drainage ability, and wood clapboard siding has plenty of vapor permeability to dry. You could consider 1.5" of EPS foam on the inside and outside of the block, and FG or other "fluffy" insulation inside the stud cavities. That system has some drying ability to the interior and exterior, with little condensation potential. A Class II or III vapor retarder would work on the inside.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |