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Community and Q&A

Interior Air Sealing and ZIP Sheathing

CarsonB | Posted in General Questions on

I see a lot of discussions on the importance of air sealing.  Air tight drywall.  Air tight electrical boxes.  Foaming/caulking between sheathing and framing members in each stud bay.  How important is any of this when your air layer is your external taped zip sheathing? Aside from what’s “best”, if you were building the house yourself, would you bother for example foaming the bay perimeters when using zip or is the zip sufficient?  Is it mostly for air quality or is the concern wind washing insulation with interior air?

thanks,
Carson

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Replies

  1. Expert Member
    Zephyr7 | | #1

    You only “need” one air barrier, but it’s better to have some redundancy. This is why it’s common to use the exterior sheathing (Zip or otherwise) as a primary air barrier, taped and sealed, and to still install airtight drywall on at least the exterior walls. The air tight drywall helps to limit the possibility of moisture problems in the walls, and also helps with overall air tightness as a secondary air barrier.

    Think of the sheathing as your primary air barrier, and the other layers (drywall, etc) as extra insurance. The added cost is minimal so it’s usually worth the little extra detailing to seal up the drywall too.

    Bill

  2. Scott Wilson | | #2

    Here's a great air sealing trick. If you wanted, you could add some horizontal blocking between the exterior wall studs and then screw through the end interior wall stud and exterior drywall layer to anchor the interior wall a bit more (in addition to the nails through the top and bottom plates into the ceiling and floor joists.

    https://www.youtube.com/watch?v=qZ71h8DUPqg

    1. CarsonB | | #4

      Thanks Scott, sounds like a good detail to pass to the framer. I’m always worried as an owner I’m being annoying and over micromanaging details, but I’ll continue to see what I can get away with:)

    2. Tyler Keniston | | #6

      How does that partition wall attach? Add a connector plate or piece of ply between top plates maybe? You could still add ladder blocking I suppose and maintain the slot for drywall, but he doesn't show that...

      1. Expert Member
        Malcolm Taylor | | #21

        Tyler,

        Unless the connection is above the top plate the drywall won't slide through.
        I've seen this described a couple of times and don't see much of an advantage.

        - Interior wall junctions are useful as stops for the drywall, so you don't end up with butt joints in the rooms - as will occur is you run the drywall continuously behind them.
        - You also can't run plumbing, electrical, or other services from the interior to the exterior walls. Something that happens frequently on rough-ins.
        - If y0u are using an interior vapour/air barrier, this makes things more, not less complicated, as there is no practical way to pull long sheets the whole length of an exterior wall, or seal them behind the interior ones.

        1. Expert Member
          Akos | | #23

          The best way I've found to do this by putting a 6" strip of 3/4" ply behind the first stud of the interior partition wall. This now gives you something solid to screw the drywall to but also gives continuity to the air barrier across the partition while still allowing for insulation behind.

          1. CarsonB | | #25

            In my situation the drywall slides behind timbers. I believe the standard practice is to put a strip of 3/4 plywood behind the timber and the drywall then butts up against this. Is this hopeless to air seal or would caulking this joint behind the timber be sufficient? Timbers are notorious for shrinking and breaking caulk joints.

  3. Jon R | | #3

    Experience with SIPs in Alaska showed that an interior air barrier can be vital. Even if you can reduce condensation/sorption mid panel (as SIPs do but Zip doesn't), the joints can fail due to interior air moving close to the cold exterior, depositing moisture and then returning to the interior.

    While we know that air movement moves a lot of moisture and many recommend interior side and/or both side air barriers, I haven't seem good data. I wouldn't build with T&G interior and unfaced fiberglass (very leaky on the interior side). But cellulose might be close enough to an interior side air barrier.

    Energy Star requires interior side in Zones 4-8:
    https://basc.pnnl.gov/resource-guides/continuous-air-barrier-exterior-walls

    Significant disadvantages to not doing both:
    https://www.buildingscience.com/documents/reports/rr-0403-air-barriers/view

    BSC: "low density fibrous insulations should be covered by air barriers in areas subject to air movement".

    1. CarsonB | | #9

      Thanks Tyler, either my search skills are getting worse or I'm losing track of everything I've read. Unfortunately, both of those links seem to be along the lines of "maybe?".

  4. Tyler Keniston | | #7

    >"Foaming/caulking between sheathing and framing members in each stud bay."

    Anything that happens at the exterior sheathing plane (even if it's the 'interior' of that plane) is really your exterior air-tight layer; it's still exterior to your insulation. So there really wouldn't be much sense in using taped zip sheathing (assuming a proper job) and then also caulking/foaming the perimeter of the bays where framing meets sheathing.

    If you're wanting the potential/alleged benefits of an interior air barrier, that needs to be something like ADA or a membrane interior to the insulation.

    Note also wind washing is—as it sounds—specific to wind from the exterior. It's not the same as interior convective currents.

    As others have said, the answer might depend a bit on what your insulation is. Something like dp-cellulose will get you a tighter assembly through and through (less convective looping) than pink fb-batts. Whether convective looping is a problem seems to be an open (unanswered) question, as far as I can tell. It seems unlikely that it's a 'significant' problem in most cases (perhaps more so with a thick wall in a cold climate?)

    My unscientifically backed posit would be that interior convective looping would be more likely as pressure differentials increase between 'significant' interior pathways. So perhaps it makes sense to ensure the bottom and top of your drywall (where it meets floor and ceiling) are tight, if nothing else.

    1. CarsonB | | #10

      insulation is mineral wool batts in 2x4 studs (exterior rigid foam exterior of sheathing). I guess that's better than some of the less dense fluffy insulation? Top and bottom of drywall doesn't sound bad. So you suggest ignoring plumbing/electrical because they aren't enough to drive significant convection through the wall?

      1. Tyler Keniston | | #12

        With batts of any kind, installation is key (avoiding voids. cutting precisely around obstructions).

        Really, I'm just speculating that mid-wall penetrations (if not very large) may not be as important as leaks that are further apart vertically, or larger overall. From there, it's just a matter of how high up the fruit tree you want to pick—making sure to knock the lowest hanging fruit off as you go.

        Beyond that, I won't be that helpful in advising what is 'enough.' I personally tend towards over-doing; so If I were doing my own work, I would likely take the time to do at least a cursory seal of all penetrations. But if I had to dictate to a contractor/sub how far to go—and pay more for them to do it— the decision may be a bit tougher. But as Bill suggests, this generally shouldn't be a huge cost adder.

        Most important will be to get the primary air barrier (your taped sheathing and ceiling/roof barrier), as well as all it's transitions (to foundation, to ceiling), well detailed and executed.
        More 'maybe' for ya ;)

        1. CarsonB | | #17

          "But if I had to dictate to a contractor/sub how far to go—and pay more for them to do it— the decision may be a bit tougher. " Exactly. Some of these details, like going through and caulking the top and bottom sounds like something I could do myself after hours. Details like california corners, tying zip to the ceiling air barrier, Scott's suggestion of offsetting the interior partitions, etc. are more difficult as it's a matter of what I can get away with and properly communicate to the builder/subs.

          1. Tyler Keniston | | #24

            >"tying zip to the ceiling air barrier,"

            You may have some other plan for connecting your ceiling air barrier with your primary zip, but just to be clear— because it sounds like you're talking about this primary connection—this is a detail where continuity is very important and 'fussing' is definitively worth it. (assuming your ceiling is your primary air barrier)

            There's multiple ways to do that of course, which is maybe what you're getting at.

            edit: Just noticing your statement about exterior insulation. That helps reduce the risks of convective looping, for what it's worth. Also, not sure if you're planning on blower door tests, but that can help verify the robustness of your primary.

          2. CarsonB | | #26

            We are working with a local energy program that gives blower door tests, as well as thousands in incentives. The air seal connection to the roof from what I’ve read is to leave a large zip tape at the top plate, which can be taped to the ceiling membrane (likely intello).

  5. Expert Member
    Akos | | #8

    With taped exterior sheathing, you don't have to go nuts with your warm side air barrier, but you do want one.

    You just want to avoid the usual uglies like no drywall behind tubs on outside walls.

    If you are installing a vapor barrier, focus on getting that tight (caulk around perimeter, tape all seams) and don't worry about drywall details. If it is just drywall simply caulking around the perimeter is good enough.

    1. CarsonB | | #11

      thanks Akos, no vapor barrier and using some sort of membrane sounds fussy and expensive. Caulking only the top and bottom of the drywall, or also around penetrations and where the drywall attaches to the studs?

      1. Expert Member
        Akos | | #13

        In my area a vapor barrier is required by code, taking a bit of extra care with it is very easy.

        By perimeter I mean, along top and bottom, at each end of the wall and around any openings such as doors and windows. There is no point at every stud or end of sheets.

        It is still a bit fussy work and if your installer is not used to it, or worst, doesn't believe in it, you might have to babysit it or do the caulking yourself. Also watch they cut the sheets to the right height, some times the bottom is cut too short and doesn't even reach the bottom plate (it will be covered by baseboard is not an excuse).

        A bit of canned foam around any major penetration doesn't hurt either, but you should not have any plumbing in exterior walls.

        Air tight electrical boxes add only a couple of extra dollars to the build and require no extra labor from the drywall installer.

        1. CarsonB | | #15

          "Air tight electrical boxes" - Something like this? https://www.homedepot.com/p/Carlon-1-Gang-22-1-2-cu-in-New-Work-Non-Metallic-Vapor-Electrical-Tight-Wall-Box-FN-23R/100131883?modalType=drawer. I don't believe I've ever seen something like that in stock locally.
          I also see air sealing boxes like this one, https://foursevenfive.com/lessco-utility-box/, but that seems to be designed for attaching to a membrane.

          1. Expert Member
            Akos | | #20

            The first one (Carlon). They work well. I also use them in interior partitions where I want to limit sound transmission.

            Generally, for any special material you want, make sure to order it to the job site ahead of time and don't rely on the sub to handle the logistics.

          2. CarsonB | | #28

            Given the supply issues I've been seeing, ordering everything ahead of time probably not a bad idea right now...

  6. James Howison | | #14

    The concept of air-tight drywall seems impossible in any area that has expansive soils. Maybe fancy foundation systems help (voids, waffles, super-deep piles), but everyone here seems to expect constant cracking in the drywall. Has anyone tested the longevity of air-tight drywall (to both movement and homeowner/electrician penetrations?

    1. Jon R | | #18

      Or the long term performance of various other types of air sealing?

      Going way off track here, but IMO, underground roofs that isolate surface moisture from foundations would be effective in minimizing differential foundation movement.

    2. Expert Member
      Malcolm Taylor | | #22

      James,

      Something is going badly wrong in homes where the drywall is cracking - expansive soils or not.

  7. Jon R | | #16

    If one doesn't believe that cooling creates enough pressure for significant looping in walls, there are often room-to-room pressure differentials (caused by HVAC systems) pushing air sideways through the walls. 3 pascals is quite significant compared to typical indoor/outdoor differentials.

    Installing MemBrain and air tight electrical boxes might be wise insurance. Hopefully it will remain somewhat effective after the drywall installation creates holes and rips. Or just go with cellulose.

    1. CarsonB | | #27

      Air tight boxes are only about double in cost in bulk so shouldn’t be bad. We are using intello for the ceiling, but with labor involved that’s a fairly expensive cost upgrade for the walls. I’m thinking that if I can’t trust me/subs to get the drywall detailing right then that’s probably more so for a wall membrane. Would the membrane be more straightforward? Do you tape around penetrations before adding drywall or do the same caulking/spray foam as you would with drywall?

  8. Expert Member
    Deleted | | #19

    Deleted

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