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Caulk plywood sheathing seams from inside?

nj_homeowner | Posted in General Questions on

Hi!

We are gut-renovating a house in NJ, USA.  The new 2×6 walls have plywood sheathing, and the old 2×4 walls have ye olde T&G sheathing. 

Both the new and old walls have tons of gaps in the sheathing, many as wide as 1/4 inch, along the whole length of the wall.

Should I caulk those gaps from the inside as much as possible?

Will this present any problems for drying of the wood?

Is a product such as Big Stretch a good choice?

Outside the house has regular Tyvek installed, with Hardie Plank soon to be added on top.  The wall cavities will be insulated with rock wool or fiberglass.

Thank you!

Brian

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Brian,

    If you search the site, you will find a few threads on this topic. The challenge with caulk is maintaining the seal over time. Tape is probably a better option, but you have to consider the cost. If you have flexibility on the budget, Aeroseal might be a better solution.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Brian.

    I don't think caulk or tape are the best options here for long-term performance or durability. If it's too late to do your air sealing from the exterior, with, for example, a fully-adhered WRB, I do think your project is a good candidate for a flash-and-batt or cut-and-cobble (or partial cut-and-cobble approach).

    1. Tim Janson | | #7

      Brian, why do you say that tapes would not be a good long term solution? I am thinking of all the tapes that are standard in long term applications - flashing tape, Zip tape, etc.

  3. Expert Member
    Zephyr7 | | #3

    Duct mastic might work here. Apply fiberglass mesh drywall tape over the gap, then a heavy coat of duct mastic. Sort of an improvised extra beefy tape.

    I’ve used canned spray foam here, but I’m not sure of the long term durability. There has been some conversation about that here in the past, and the usual thinking is that the more flexible “door and window” version might offer better longevity for sealing things like this compared to the more rigid regular stuff.

    Bill

    1. Expert Member
      Dana Dorsett | | #4

      +1 on the duct mastic approach, or if the plywood is clean enough, housewrap tape reinforced with duct mastic.

      Can foam is not going to hold up over time for sealing a seam on something as flexible and hygroscopic as plywood sheathing. Specially formulated low expansion can foams that don't need water for curing can seal old plywood to something as rigid as a 2x4 stud, but not for sealing a sheathing seam that spans between framing.

  4. nj_homeowner | | #5

    Thank you everyone for these thoughts.

    It sounds like everybody thinks sealing the sheathing somehow is definitely OK and definitely a good idea, it's just a question of best method.

    I was trying to avoid using spray foam everywhere, but it appears that flash-and-batt with closed-cell foam would be the easiest, most cost-efficient, and most durable. Does everyone agree?

    What if I do nothing to seal the cracks? Will the Tyvek covering everything provide any air leakage protection?

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi Brian.

      Tyvek is an air barrier in the sense that air cannot flow through the material, but mechanically fastened water-resistive barriers are difficult to detail well to actually keep air out of the walls. If all of the seams are taped, the top and bottom courses are taped or caulked to the building and the flashing at all of the penetrations (windows, doors, etc.) are done right and air sealed, the Tyvek will help, but it won't likely get you to the kind of low air exchanges that you see discussed on GBA.

      If you decide to go with a flash-and-batt system, check this out: Flash-and-Batt Insulation. You'll find a lot of useful info on how to do it right.

      Also, consider verifying your air sealing work with a blower door test and doing some blower-door directed air sealing before installing drywall and finishes in the house. Here's some info on blower door directed air sealing: Blower-Door-Directed Air Sealing

    2. Expert Member
      Zephyr7 | | #8

      I doubt flash and batt is the most cost effective. I would do the more conventional air sealing work with canned foam, urethane caulk, and some duct mastic as explained above. It actually goes pretty quickly if you have the wall open. After that, put in your batts normally.

      I’m a big proponent of spray foam, but only where it makes sense and regular walls aren’t usually one of those places.

      Bill

      1. nj_homeowner | | #9

        Thank you. If I do it that way, isn't there a risk that water will condense on the warm side of the wall in winter?

        Also, it seems like it would take forever to try and seal everything this way, hence the notion that maybe we should just do foam?

        1. GBA Editor
          Brian Pontolilo | | #10

          Hi Brian.

          If you seal the gaps with tape or mastic or such and fill the walls with batts or mineral wool, and assuming that you have no exterior insulation and you are not installing the siding over a rainscreen, then you can install an interior vapor retarder if you'd like (the codes in your area may call for a class I or class II vapor retarder in this situation).

          I would recommend class II, like kraft faced insulation or a smart vapor retarder like Intello or Membrane (avoid polyethylene). But the best thing you can do is air seal, not only the sheathing, from the exterior, but from the interior, using the drywall or your vapor retarder, too. You're unlikely to have consequential wetting issues if you keep moist air out of the walls.

        2. Expert Member
          Zephyr7 | | #11

          Moisture only condenses on cold surfaces. You will never get moisture condensing on a warm surface. You get condensation when warm, moist air comes in contact with a cool surface.

          If you have a lot of air sealing work to do, the great stuff “pr0” gun can be a big help. You can do an entire decent sized wall in under an hour if it’s open. There are some articles about what and where to seal, but basically you’re sealing the perimeter and any penetrations in top/sill plates (around wires, pipes, things like that). Use great stuff for penetrations and bigger gaps, and caulk thinks that are too tight a fit for the foam to get in and seal. Mastic is good for things that might move or flex with temperature changes, like those gaps between plywood panels.

          Bill

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