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Community and Q&A

Taping seams of tongue and groove sheathing

Khkiley | Posted in Green Building Techniques on

Hello, I am remodeling a powder room in a 1928 brick colonial.

While the wall is open I would like to air seal and insulate the cavities from inside the house.

To the best of my knowledge, the sheathing appears to be 3/4″ Tongue and groove. I would like to tape the joints and the areas along the studs with 3M All Weather Flashing Tape. What is the recommended overlap of the tape ? It comes in a 4″ width, will 2 inches be adequate ?

Also, would this tape be expected to survive the seasonal expansion and contraction of the T&G ? It had the best adhesion in Martin’s tape test. Would it be a good/bad idea to leave a little slack at the seam to allow for expansion or is this not needed ?

Would a primer be recommended ?

Any advice or recommendations are appreciated.

Thank in advance,


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  1. GBA Editor
    Martin Holladay | | #1

    The work you contemplate is fussy and may not be durable. If you really want to perform air-sealing work on the interior side of board sheathing, your best bet is to use spray foam. I advise you to purchase a two-component spray foam kit.

  2. Expert Member
    Dana Dorsett | | #2

    Before you go the spray foam route, or do anything else, is there any tar-paper on the exterior of that sheathing, and is there an air-gap between the sheathing & brick veneer?

    What is your location (or US climate zone)?

    How deep are the studs?

  3. Khkiley | | #3

    Hi Dana,

    From outside: Brick, air gap, tar paper, sheathing, 2 x 4 studs.

    Central NJ, 15 miles west of Manhattan.

    I have given sprayfoam a lot of consideration and its probably not a route I want to take unless absolutely necessary. I plan on using Roxul between the studs and possibly furring out the studs a half inch with PolyIso as a thermal break before sheetrocking with half inch.

    I don't like that spray foam is almost impossible to remove once installed, and if my house is still standing another 87 years from now the owner might want to install something more modern for the times and a cavity full of spray foam would make that a nightmare.

    I'll admit I might be a little nutty in that regard.

    And another thought, the air sealing my be overkill. It may not be perfect but I think the brick veneer is doing a good job. With the studs exposed, the bathroom is cold but we haven't detected drafts or breezes even with the wind whipping up outside.



  4. Expert Member
    Dana Dorsett | | #4

    Are they full dimension 2 x 4s (common in 1920s construction), or milled 2x4s that are 3.5" deep? Batts are only designed for milled framing, and are a lousy fit for full-dimension rough 2x4s, in which case a blown fiber insulation would be more suitable.

    The tar paper is a "smart" vapor retarder on the "wrong" side of the assembly , but sufficiently vapor open that the sheathing can dry into the cavity between the brick & plank sheathing in winter. Your location is at the zone 5/zone 4 boundary. Consulting the IRC, you'll note that in zone 5 with "Vented cladding over wood structural panels." it's legitimate to use only a class-III vapor retarder such as standard latex paint on the interior (see: ), and that would also give the planking much higher drying rates toward the interior.

    Using closed cell foam in the cavity would reduce the drying rate toward the interior (even an inch of foam is on the boundary between class-II & class-III vapor retardency at about 1-perm, falling with additional thickness) and since it's not necessary to protect the sheathing from interior moisture drives, using a more vapor permeable way to get it air tight would make it more moisture resilient.

    Rather than 1000 yards of tape, try a membrane approach. Housewrap lapped over the studs. It gets tucked & stapled at the stud / sheathing intersection, is pretty air tight, and would still allow you to use batts (if they fit) or blown fiber in those cavities. Caulk the top & bottom planks to the studwall plates, and tape or caulk the top/bottom edges of the housewrap to the planks- it'll help, even if it's not perfect. You can install the housewrap fairly loosely to keep it from ripping at the staples- the insulation will be springy enough to press it flat against all sides of the cavity.

    Using cellulose rather than rock wool or fiberglass would give the assembly even more moisture resilience, since it buffers and "shares" the moisture burden with the structural wood, keeping both the plank sheathing & studs drier. Cotton batts would have a similar buffering capacity, but are more expensive than blown cellulose. But it's milled 2x4s 16" o.c. which can work well enough, the rock wool batt approach works too.

    Cellulose blown at 3-3.5lbs density would be my personal first-choice, followed by new-school fiberglass blowing wools at 1.8-2. lb density, with rock wool batting being third. As a DIY the rock wool solution is obviously an easier choice, since most people don't have an insulation blower kicking around in the garage.

    With any of these solutions, a "smart " vapor retarder such as Intello Plus (imported and expensive, or at least it WAS before the Euro crashed), or Certainteed MemBrain (much cheaper but hard to find in some markets) between the gypsum & insulation adds quite a bit of resilience to the assembly as well. It's cheap insurance if using rock wool or fiberglass, less of an issue if using cellulose or cotton.

    Foil faced polyiso over the whole interior side would protect the sheathing from interior moisture drives, but would block all drying toward the interior, and even condense moisture inside the cavity during the summer if you air condition. If you can give up 1.5-2" of interior space, a more resilient and higher performance solution would be to use polyiso only on the stud edges, at a stud depth + polyiso thickness that leaves you 5.5" of cavity depth, at which point could then use R23 rock wool in the cavities instead of R15, and the performance would beat IRC 2012 code min for climate zone 5 with some margin, due to the R9-R12 thermal break.

    Don't forget to caulk between the subfloor & bottom plate, and the seams between any doubled-up top plates before adding interior layers.

  5. LucyF | | #5


    I used Prosoco R Guard joint and seam sealer ( to seal the seams on the inside of the house we built. I used a Siga caulk gun I bought from Small Planet Workshop ( which is the only place I could find the Prosoco products. I love this stuff. It is fast, very easy to use, vapor permeable, and long-lasting. It is expensive, but if you are sealing a small powder room, you probably won't need more than 2 tubes (sausages) which would cost about $40.

  6. Khkiley | | #6

    Thank you very much Martin, Dana and Lucy for taking the time to answer my questions and help me out.


    Thank you for the recommendation and links, I am going to do some research on "R Guard" It looks interesting.


    Wow, thank you for the very in depth response.

    I like the membrane approach a lot, you have given me a lot of great advice.

    You have hit upon my biggest concerns, moisture resilience and the best compromise for my DIY approach.

    The studs are finished at 3.5" deep.

    My original idea was to use polyiso at the stud edges, but only half an inch. I might be able to squeeze in 1", I am constrained by the rough in for the toilet. I thought that might perform (weakly) as thermal break for the stud and give me a little bit more room for insulation.

    From your recommendation it doesn't seem that half inch is worth the effort, or a Roxul product would not be readily available to fill a 4" cavity.

    I am going to give the cotton batts and cellulose a look.

    Thanks again,


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