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Community and Q&A

Sealing Sheathing Seams Between Framing Members

jamesboris | Posted in Green Building Techniques on

Hello, building in Zone 2A (not my first rodeo, but trying to up my air-sealing game). I plan to detail both my exterior sheathing (½” ply) and interior drywall as air barriers. So, belt and suspenders. In that case, what is the purpose of taping or sealing the seams between framing members?

E.g. between double studs, double top plates, around openings, etc… I can see doing it where the bottom plate meets the subfloor, and maaaaybe at corners, but it generally seems like belt, suspenders, and gluing your pants to your body. The framing members are already sandwiched between two air barriers.

I’m not trying to be cheap here, I just don’t see the point — but am willing to be enlightened! Really appreciate all I’ve learned from this community!

EDIT: Here is a JLC article that recommends this: Air-Sealing Framing | JLC Online. “Built-up jack studs to king studs, headers, and corners are all common culprits. When assessing existing construction with a thermal camera, we often find corners where the framing has separated and allowed a stream of cold air to infiltrate the space. That’s why we emphasize using a proper sealant to air-seal built-up framing. Pay close attention to inside and outside corners, ganged jack studs and king studs, multiple-ply headers, and double top plates in new construction.” But again, where is that air even coming from, if the drywall and the sheathing are both properly detailed (as air barriers)?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    James,

    I agree.

  2. Expert Member
    ARMANDO COBO | | #2

    "You Must Learn from the Mistakes of Others. You Will Never Live Long Enough to Make Them All Yourself" ~ Sam Levenson
    Build two houses, one with full sealing details and the second with a half job... then do a door blower test on both houses and see the results. I'll guarantee you, you'll never ask that question again. 😎

    1. jamesboris | | #3

      Hi Armando, thanks for the reply, have learned a lot from your posts here. But my question is: What do you define as "full sealing details"? I think that detailing both sheathing/plywood + drywall as air barriers is a pretty high standard. I'm asking why one should tape/seal framing members in addition. If the drywall and the plywood work, then the framing members shouldn't come into play.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        James,

        I agree with Armando in that whatever strategy you decide to use should be done diligently. However I also think one well detailed primary air-barrier is sufficient, a secondary one is good insurance, but beyond that where do you draw the line? Why not add a membrane behind the airtight drywall for a forth? Unless sealing between the framing and sheathing is a necessary part of one of the air-barriers, what's the point?

        1. jamesboris | | #6

          That makes sense to me Malcolm -- as did your first reply haha. I'm curious how you'd reply to Bill below.

          1. Expert Member
            MALCOLM TAYLOR | | #11

            James,

            As a general proposition, my approach would be the same as for any other task: know what you are aiming for and perform it diligently. I don't see much good coming from randomly deciding to seal things which don't represent an air path to the exterior.

            I also don't see the current trend among high performance builders to add unrelated tasks to the framers primary job as leading anywhere fruitful. If the the air-sealing strategy is going to involve sealing stud-bays, it needs to be part of a separate task done by others. Framers spend their days, and dream at night, about attaching wood in various ways. Not making sure your roof underlay is lapped properly, or your windows diligently flashed. When you bring those jobs under their care you are always going to get at best their distracted attention.

  3. Expert Member
    BILL WICHERS | | #4

    I would call that "protection against sneaky air pathways". You would think that if you have some kind of sheathing over a doubled stud, for example, that you'd have an air seal. What you actually end up with is a leak between the studs that sneaks out the end, where the radiused corners of the stud don't really seal against the sheathing. Yep, it's small, but lots of small leaks add up. Like Armando said, if you do a halfway job, you WILL see it in a blower door test. I saw it myself in my vaulted roof, where we sistered a bunch of rafters. I thought "hey, I put in ccSPF, and more than code! I have a membrane roof! I have drywall on the inside!". What did I see in the frost patterns? Big thaw lines -- you can SEE the sistered rafters like an x-ray in the frost. I think it's some convection currents between the the sistered rafters because I didn't put any sealant in the gap. A lesson for next time -- don't be like me! :-)

    If you do belt and suspenders, one of your air barriers is likely to catch the stuff that the other barrier(s) missed. I like to detail my vapor retarder airtight, for example, but I detail the drywall airtight too. I also put up exterior sheathing and detail that airtight. All that "detail airtight" means in any of these cases is a bead of sealant around the perimeter, so it's not a big deal to do. Interior walls are a bit fussier with the ocassional electrical box to seal, but it's not too bad.

    Just remember that if you don't seal ALL the gaps, you'll still have "sneak paths" that find their way out using the elusive third dimension. Joe Lstiburek called these "complex 3 dimensional airflow networks", which is just a fancy way to say "the air found a sneaky way to leak out".

    When you sister things, run a bead of sealant between them. Tape helps if you don't use sealant, but try to wrap the tape over the end (top and bottom plates in the case of a sistered stud), to make sure that the radiused corners don't provide a sneaky gap at the connection. Sealant around the perimeter of whatever panel product you use on a wall (plywood, drywall, etc.) helps a lot too. Seal BOTH sides of the wall to be doubly sure, which also helps to minimize moisture ingress.

    Bill

    1. jamesboris | | #7

      Thanks for the thoughts Bill! Some followups:

      "What you actually end up with is a leak between the studs that sneaks out the end, where the radiused corners of the stud don't really seal against the sheathing. Yep, it's small, but lots of small leaks add up."

      How is air in the stud bay at all? The sheathing is taped to itself at corners, and taped to the wide face of the framing at openings.

      "Like Armando said, if you do a halfway job, you WILL see it in a blower door test."

      Yes, but Armando is attacking a straw man. I didn't say "hey how about a halfway job?" Rather, I said "I'm creating *two* well-detailed air barriers to the level recommended by seeming experts here and elsewhere (e.g. Zephyr7 ;)). Think I should also do this additional thing of taping or caulking double studs, etc.?"

      "When you sister things, run a bead of sealant between them."

      I've always used the term "sistering" for joists/studs/rafters in remodels or to extend length. Are you saying to run a bead of sealant between a king stud and the neighboring jack stud? Between a top plate and a "crown" plate (i.e. between the top plate and the "double top plate" as it's confusingly called in TX~). If so (see above).

      1. Expert Member
        BILL WICHERS | | #9

        Air always finds a way. Since I'm an EE, I like to think of it similar to the decibel scale: you can reduce things and reduce things and reduce things, but you can never actually get to zero -- you can only make things smaller and smaller. The same goes for air leaks: you can reduce the leaks and make them smaller, but you'll never get to 0.0ACH -- something will ALWAYS leak a little.

        Tape can detach, sealants can skip over small voids or cracks. All of that adds up. It all depends on how far you want to go. Redundancy in multiple layers is a good way to help make sure you aren't missing anything, and ensures that leaks tend to be very small.

        Anytime you have two framing members joined, it doesn't hurt to put a bead of sealant between them. It's quick, and it doesn't have to be precision. Top and bottom plates are probably most important, as are any other areas that tend to be towards the outside perimeter of a wall, roof, or floor section. Things in the middle tend to be less of an issue, since leaks tend to be worst at the edges of things. You absolutely need to seal between sistered studs if you have a joint between sheathing panels at that location. Try to think about how your structure goes together, and make sure anywhere there is a joint of any kind you have a seal there. If you just seal the face of the sheathing, don't forget to wrap around the edge of the panel too, otherwise you miss a 3/4" or so small gap. Sealant helps to seal this by oozing out into other parts of the gap, but you can't count on that for a full seal.

        I'm going to have to say that I completely agree with Armando here, this isn't a "straw man". If you want to eek out as much performance as possible, you need to be diligent with your air sealing. If you do the basics, you're a lot better than nothing, but if you want to push the limits, the small details really do matter.

        Bill

        1. jamesboris | | #12

          Guys... *I am not asking if it's worth it to air seal.* I have built 3 tiny houses with near-Passive House levels of blower door results *and I have never sealed a framing member.* I'm willing to be told that that was wrong of me. I'm willing to learn new details. I assume Malcolm (who agrees with me in Reply #1 above) has about 5 times my experience.

          "You absolutely need to seal between sistered studs if you have a joint between sheathing panels at that location."

          *Assuming the drywall and plywood are airtight,* why? How is it backup? Because if, e.g., the tape detaches from your plywood, all the sealing between studs in the world doesn't matter -- air can get into your walls.

          1. Expert Member
            BILL WICHERS | | #17

            I should probably say there are multiple ways to do some of these things. What I (and probably some of the others) are basically saying is that there is the basic stuff, which probably gets you 90% of the benefits of air sealing, but the last 10% is harder to do and requires more detail.

            Using that sistered stud example, if you tape the plywood sheathing, and you don't seal the sistered studs, you're in that "90%" zone. If you seal the studs, you do a bit better. Exactly how much "better" you do depends on how well you do everything else. If you wrap the tape around the edges of the plywood at the top and bottom, then sealing the studs makes less of a difference, because your tape job is better. I don't particularly trust tape at the far edges though, because it has a tendancy to peel back a little bit. Sealant is more reliable.

            Your assumption of "airtight" drywall and plywood is assuming a perfect installation. In reality, there are always some missed spots, areas the tape didn't stay stuck, etc. That's where the "10%" comes in. If you do some of the extra details, which can be done in different ways, you start getting into the 90+% range. Note that I'm making those numbers up, but I'm probably close -- the basic idea is that to get into the really high performance area, you need to start thinking about smaller and smaller leaks.

            What I typically do is seal sistered framing in the "edge areas", but not usually in the middle of a wall or ceiling unless I know there is another weak spot in the same general area. I seal the perimeter sheathing, the drywall, and the vapor retarder (I almost always use a smart vapor retarder as extra insurance). That gives essentially three air barrier layers, and a few "spot seals" in areas like sistered perimeter pieces that are more likely to leak. I'm shooting for better than "good", but I'm not quite going all out, either. I'm looking for something of a benefits vs effort/cost balance.

            If you're doing a good job sealing in a particular way, the other backups might not be as necassary. Keep in mind though that the larger the house, the more small leaks there will be, so the bigger overall impact they are likely to have. Some types of seals tend to be more reliable over time than others too (which is why I generally like polyurethane sealant over tape on wood pieces).

            Note that my "backup" for tape is usually to sandwich it in between other things. Tape something under the drywall, for example, and the drywall helps to clamp the tape in place forever.

            Bill

  4. Expert Member
    ARMANDO COBO | | #8

    Your curiosity is understandable, but the results are the difference between getting ≥1ACH50 and ≤3ACH50. So where do you want to be?
    There are too many opportunities for air to infiltrate in a building envelope, from bad or missed sealing spots, to tape and sealer failures, material expansion and contraction, etc. Many of these mishaps can develop into problems within a short time or years after.
    Old geezers like me, have learned over the years, how far we want to go to get expected results, in my case its 1ACH50. I include many sealing details on my plans, plus I go to the jobsite to train Builders and Subs on proper air sealing. I’ve always believed that if I have details on my plans, I increase my chances of getting it done right. If my client wants PHIUS tightness, we can do an Aerobarrier job, but as you may know, that comes with a premium.
    Today we can get very inexpensive thermal imaging phone adapters and cameras that I believe any GC worth its mustard should have in his/her pocket to double check air-sealing jobs, and the more complicated the wall and roof design, the more the reasons to follow thru.

    1. jamesboris | | #13

      Armando, I will PayPal you $1000 right now if you can link a study showing that taping framing members *in a house that already has properly taped plywood seams, gaskets, and airtight drywall* will improve ACH from 5 to 3. You can't -- because there's no way that the house I'm describing is a 5. In fact, I've been on builds that get under 3 without taping/sealing a single framing member -- we relied on airtight sheathing and airtight drywall. I've seen under 2 on simple buildings with nothing but airtight plywood. Drywall was a bonus, took it down to 1.5. Anyway.

      Again, I'm not asking "is it worth it to air seal?" My goal is usually 1-or-2ACH50. I'm simply asking for someone to explain how sealing framing members to one another or to the sheathing improves an assembly *that is already sealed at the plywood and drywall.*

  5. plumb_bob | | #10

    For my $.02, I think doing an excellent job of sealing your primary air barrier is good, and can even be great if done as a holistic system from the foundation up. Sealing a secondary plane is the next level, and with 2 sealed planes the house should be super air tight if done diligently. When I see acoustic sealant applied between king and jack studs, around headers etc. I think it is missing the point of using sheet goods.
    The real kicker is detailing penetrations, and for a high quality product the approach taken should be part of the overall system and well thought out prior to foundation being poured.
    I have seen some houses recently that have passive house type air tightness (.6 ACH) that used the sealed WRB as the only air barrier, interior poly was left unsealed.

    1. jamesboris | | #14

      "When I see acoustic sealant applied between king and jack studs, around headers etc. I think it is missing the point of using sheet goods."

      Yes, this is what I'm thinking. I'm willing to be proved wrong, I'm having a hard time communicating this because it seems like no matter what I write, it's being interpreted as "I'm trying to chintz on air sealing, whaddaya think?" I'm not... I'm saying that, scientifically, I'm having a hard time seeing how a bead of sealant between two studs makes a house tighter *if that house already has airtight plywood/drywall.*

      1. Expert Member
        MALCOLM TAYLOR | | #15

        James,

        It's an interesting discussion with wider dimensions than just air-sealing. One of the most important things builders learn as they gain experience is where to put their attention, and what makes an appreciable difference.

        There is a man down the road from me framing up a small house by himself. I wandered along last weekend to chat, and he proudly showed me that all his framing is at 12" oc. The walls are veritable birds-nests of studs and blocking, but beyond giving him comfort, they provide no real advantage, and many downsides. If he was clear in his aims - more seismic resistance? less deflection? - he could have found effective ways to achieve them. The time and expense could be so much more usefully spent elsewhere in the build.

      2. huey_ce | | #21

        James,

        I started reading these replies in agreement with you and I am slowly being won over to see a legitimate reason for sealing framing members even with air-tight sheathing and drywall.

        Have you ever heard of the swiss cheese risk model? You look at what you are assessing as a stack of slices of swiss cheese. Each hole in a slice represents a risk. The idea is to have none of the holes line up so you could never fall all they way through the stack of cheese. If a series of holes line up then you have a problem.

        Picture the drywall as a slice, the stud bays as a slice, and the sheathing as a slice. Not sealing the framing members is deciding to leave a hole in the middle slice of cheese. It seems like you are reaching your desired performance levels with this hole in your cheese.

        Some of the other commenters have concerns that unplanned holes may be left in the outer slices of cheese, unadhered tape along the sheathing, or maybe an electrical box that wasn't sealed along the drywall. These commenters are concerned that these unplanned holes might line up with your planned hole in the center slice, thus leading to problems.

        Risk management is pretty subjective and everyone has their own goals. Some of these replies do seem oddly aggressive at times, but I'm going to chalk it up to a misunderstanding of tone over an internet forum. Think about the swiss cheese model and make your own decisions. I know I wouldn't sweat sealing the framing for a renovation on my current home, there are bigger fish to fry, but I would consider it on a high-performance new build depending on what my performance goals were.

  6. Expert Member
    ARMANDO COBO | | #16

    Most folks come here for advice about better ways of doing things. I feel you came here to justify that what you are doing is good and there is no need to do more than you have to.
    As we’ve said, set your goals, and if getting 1-2ACH50 is good enough for what you are doing then great, you got the answer you were looking for. For many of us here is more than that.

    1. jamesboris | | #19

      Much of what I do is based on what I've learned here (including stuff directly from your posts). I'm not trying to justify what I do. I'm asking for an explanation of why you or others would seal seams. Bill is getting into that more now (above), which I appreciate. I didn't even say that 1-2ACH was good enough. I said that I don't see how taping seams would have any effect at all, scientifically, and I'm trying to understand that.

  7. plumb_bob | | #18

    There are many decisions that go into building design, and if energy efficiency is the goal I believe how you detail your air barrier is down stream of big picture items such as building size, shape, etc.
    Getting to sub 1ACH on a simple rancher is a different beast than sub 1ACH on a 3 storey monster with 24 corners and 6 roof lines.
    I would trust the experts on here that say exceptional results take exceptional detailing. However, if 1ACH is an acceptable goal for you (it certainly would be for me) then your plan above should be fine.

  8. Expert Member
    MALCOLM TAYLOR | | #20

    It's perhaps worth remembering that the tightest house ever measured (o.05ACH50) used one diligently executed strategy - and that was a well sealed poly air-barrier.
    https://www.greenbuildingadvisor.com/article/meet-the-tightest-house-in-the-world

  9. jamesboris | | #22

    Bill, thanks for the further explanation. In your example, would you seal the framing at the interior face or the exterior face? Do you ever seal studs to the back (interior) face of the sheathing as well?

    1. Expert Member
      BILL WICHERS | | #24

      My usual method is to run a bead of polyurethane sealant around the perimeter framing, then put up the panel (plywood sheathing or drywall depending on what I'm doing). With exterior sheathing, I tape the seams too. Belt and suspenders and all that. On the interior side, I seal the vapor retarder too.

      My thinking is "caulk is cheap", and it only adds a few minutes to run a bead. It's a lot more difficult to come back to seal something you missed later, so my thinking has always been to do a little extra when the wall is open and it's easy, to avoid having to come back and fix anything later.

      As a sort of analogy, I do the same thing when sweating copper pipe fittings. I put some effort into getting the pipe exterior and fitting interior nice and shiny with emergy cloth and/or scotchbrite pads, then I flux both sides, then I assemble and sweat the fitting. The little bit of extra prep work might not always be entirely necassary, but it's a lot easier than redoing a fitting that didn't solder right and has a very small leak. I almost never have any issues if I make the surfaces shiny and bright first, so I always do that even if I might do a bit more than is absolutely necassary.

      Bill

      1. jamesboris | | #25

        That makes a lot of sense, thanks. I always do that with copper flares too, which I've been doing more of than solder lately. Just one followup, RE: "My usual method is to run a bead of polyurethane sealant around the perimeter framing, then put up the panel (plywood sheathing or drywall depending on what I'm doing)."

        I get that. And in a new build like mine, with both sides of the framing still open, I'll do both sides of the framing. But specifically on the point of seams between framing members -- would you run a bead of sealant between, say, the top plate and the crown plate (or between doubled studs), on the exterior or interior face of that seam?

  10. jberks | | #23

    James,

    I think you just want someone to tell you you are right.

    You are right.

    I think sealing between stacked lumber is a practice of air sealing that came from renovations, where having access to the entire exterior isn't available. I'm just guessing, but hey, it sounds like it fits.

    I have in the past sealed between stacked studs, in a renovation. But if I was to do a new build, where I have 360 access around the house and have no overhangs and doing an airtight wrb like a roll on or zip, then I wouldn't.

    There you go.

    For what it's worth from some random guy on the internet.

    Jamie

  11. Robert Opaluch | | #26

    Armando said "I go to the jobsite to train Builders and Subs on proper air sealing. I’ve always believed that if I have details on my plans, I increase my chances of getting it done right."

    People don't remember every detail from training. Even if they did, they make mistakes, skip steps, are in a hurry and can get sloppy, or get distracted and miss things when they are not paying attention. And they may not pay attention to every detail in plans, or know or remember exactly how to follow "the best laid plans of mice and men..."

    So Armando, Zephyr and others suggest: The backup plan is duplication. Or "triplication", or whatever. The job won't be done perfectly, wood moves as it dries and gets whacked during construction, tapes may not seal perfectly, ... and those fallible construction workers aren't perfect. Lots and lots of potential things can go wrong, with thousands of construction details to be done quickly.

    The costs of that duplication of effort might not be much in material and labor. Zephyr noted: "It's a lot more difficult to come back to seal something you missed later, so my thinking has always been to do a little extra when the wall is open and it's easy, to avoid having to come back and fix anything later."

    Zephyr also pointed out that air can find its way through the building envelope through some convoluted paths we hadn't considered. Another reason for overkill when sealing just about any path air could take, even if its not in the sheathing or drywall sealing planes.

    My comment is just a duplication of effort, just in case some of these details were missed. ;-)

    1. Expert Member
      MALCOLM TAYLOR | | #27

      Robert,

      " The backup plan is duplication. Or "triplication", or whatever"

      I guess my question is: what's the "whatever"? If an interior and exterior air-barrier can't be trusted, how far do we continue along this road of further layers? Four? Five?

      This strategy of multiple redundancies also seems to be limited solely to air-sealing. I guess you could argue we do have secondary barriers against water infiltration in the form of underlays for roofs, and WRBs on walls, but we leave it at that, and expect them to work. This idea that air-sealing is so important and difficult that it needs more than a primary and secondary layer surely needs some justification beyond we just don't trust them. If that's true then we should probably rethink how we are doing this task, not just add more layers we don't trust.

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