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double-stud wall with vapor-barrier on exterior-side of interior-wall

buildzilla | Posted in Green Building Techniques on

considering a double-stud wall with an integrated-service-cavity as described here:

i also read below from here:

> In taller walls, it’s important to connect the inner and outer studs for additional strength as well as partition the bays every second bay – to make dense-packing of the double stud cavities easier to reach proper density and maintain quality control.

so… it seems like if you place something like intello on the outside of the inner-wall, that you are giving up the ability to (easily)

(a) use gussets to join the inner and outer walls for increased support, and
(b) partition the bays to facilitate dense-packing

anyone have experience with an integrated-service-cavity that can comment?

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  1. maine_tyler | | #1


    I remember commenting on this exact question in a recent thread. Was that your thread? I am just repeating myself if so.

    It would be nice to hear from the pros that build double-studs all the time (I built a one-off). I don't think 'they' typically use the integrated service cavities.

    I did build an integrated cavity with intello. The cellulose installer I used didn't seem to have too big of a problem with the lack of bay partitions, but they definitely said they prefer to blow against insulweb. My intello had been in place for enough time that temperature cycles made it a bit stretchy, and with 24" centers, I had significant bulging. And yes, I absolutely did have the intello drum tight at install-- it simply responds to temperature fluctuations (I watched it go from tight to loose, to really tight, to loose...). Whether this was the only reason it bulged, or whether it is inherent to intello, I don't know. I do think insulweb is inherently stiffer and obviously lets air through which is nice for the dense packing process.
    And of course you have to go around and clean/tape all the blow-in holes (including slits in the corner to allow for proper air-flow during packing).

    While it allowed my electrical boxes to be entirely within the air barrier, and it provided some additional protection to the intello, I'm not sure it was really worth it.

    Don't forget that window bucks will connect inner and outer walls regardless. How tall are your walls?

    1. buildzilla | | #6

      hi tyler,

      i recall u responding to another thread on double-stud walls, but it never gets old :)

      you provided some additional bits of detail here which i appreciate.

      good point on the window-buck connections, there is at least those.

      in my case there will be some tall ceilings like 9' and 10' in spots,
      but also a great room with some two-story walls.

      even if those are balloon-framed, i'm guessing would need some blocking/partitioning cause that would probably be impossible to dense-pack as one large space.

      so in retrospect, you would skip the service-cavity, maybe gusset/partition and just intello over the outside of the inner wall detailing around electrical-boxes?

      i guess if you did that, you could even use insul-web first, do the dense-packing, and then put intello over the insul-web if intello wasn't ideal as a dense-pack webbing...

  2. Expert Member
    Michael Maines | | #2

    If your insulator is dense-packing a thick wall (~12") without partitioning bays, I seriously doubt you are getting enough density to avoid settling.

    I design a lot of double-stud walls and I have tried various methods but currently spec aligned studs with plywood gussets at each stud. I use the sheathing as the primary air control layer so it's not critically important to have the interior be airtight, so a service cavity is not necessary.

    If you want a service cavity anyway, I recommend (and in the past have designed) a double-stud wall with cross-strapped 2x3s at the interior, inside a vapor control membrane.

    If you want fewer layers and don't mind doing some unusual (but effective) details and out-of-sequence work, check out the Sugar Bush House truss system:

    1. maine_tyler | | #3

      >"If your insulator is dense-packing a thick wall (~12") without partitioning bays, I seriously doubt you are getting enough density to avoid settling."

      In my case, I had 10.5" walls, minus 3.5" for the inner wall, so 7" of pack. Having felt the install, I have confidence the wall is packed well, but that doesn't mean its the best design. I was actually concerned about the lack of bay partitions, and so while I am confident the wall is packed well, I probably wouldn't do it the same again.

      1. Expert Member
        Michael Maines | | #4

        Ah, 7" is pretty small so I can see that it would work fine. With deeper bays you don't get as much friction holding the insulation in place so you need to get close to 4 pcf, vs. the industry standard of 3.5 pcf (and the lower densities I sometimes see on job sites).

    2. buildzilla | | #5

      the integrated-service-cavity would co-opt the inner-wall so like tyler says elsewhere in this thread, the area being dense-packed would only be the outer-wall (either 2x4 or 2x6) and the gap (3" in my case), so the dense-pack-ability situation is somewhat mitigated, but...

      your comment about treating the sheathing as the primary-air-barrier and it's subsequent impact on the importance of the airtightness of the interior-vapor-barrier leaves me puzzled.

      i was going to use wrapped (eg mento) seam-taped cdx-plywood sheathing at the exterior which could presumably be considered a primary-air-barrier.

      the information i've digested makes me think that it's still important to mitigate condensation-risk with a vapor-barrier (eg intello) to minimizing the movement of vapor from inside the envelope to the cold-sheathing.

      this information (assembly drawings, installation videos, etc) shows interior vapor barriers treated with meticulous care, as illustrated by service-cavity strategies that allow it to dodge many penetrations by the trades, and non-trivial efforts to keep it continuous (even around joists between floors) with equal care to the external air barrier.

      the first part of this post asserts the following:

      > You know the air barrier must be continuous, without interruption around the entirety of the conditioned space - inboard of the main insulation layer, thereby keeping the conditioned air within the conditioned space, optimizing efficiency and minimizing the chance of moisture damages.

      so it implies that an interior-primary-air-barrier is the goal, but could be subjective and could also be marketing angle to sell more tape and stuff.

      just so i'm clear on your position, u r suggesting that the trades go at the vapor-barrier-free walls first (this also allows for gussets/partitioning between the walls as desired), then apply the vapor-barrier, but just cut-n-tape around things like electrical-boxes, and then dense-pack away?

      1. Expert Member
        MALCOLM TAYLOR | | #7


        The main reason to have a dedicated service cavity is to maintain the integrity of the primary air-barrier (I guess a second reason would be the ease of renovating?). If that air-barrier is on the exterior, any interior membrane is primarily functioning as a vapor-barrier, meaning its location isn't as important, and neither are penetrations and sealing, as vapor-barriers act as a percentage of the area they cover.

        Foursevenfive are right: you do want a secondary air-barrier to stop interior air getting into the assembly, but even an imperfectly sealed interior membrane and the drywall are more than adequate for this function.

        Given that, is there any reason not to insulate the service cavity? That would allow you to make the walls thinner by reducing the size of the gap between the two stud-walls, and if future disruption of the cellulose when renovating was a consideration, you could install the membrane on the exterior of the inner-wall and insulate those stud bays with batts.

        1. buildzilla | | #8

          hi malcolm,

          the fact that exterior-primary-air-barriers greatly reduce the value of service-cavities is an epiphany for me.

          another testimony to gba q+a and fine folks like yourself and mr maines for helping me gain that light-bulb moment.

          attempting to maintain the service-cavity was piling up the trade-offs like interfering with gussets and partitions, creating difficult membrane routing and sacrificing extra dense-pack space.

          knowing that i can gain all that back with limited downside is priceless.

          when you say:

          > vapor-barriers act as a percentage of the area they cover

          does that mean that something like a slice in the barrier is almost meaningless, and it would literally take missing barrier on entire fractions of the wall to be a concern?

          if that's the case, what is your thinking about how to approach tricky bits like the parts of the wall between floors where the joists hit the rim?

          would option-2 from below feel like overkill?

          1. Expert Member
            MALCOLM TAYLOR | | #10


            Yes that's right. A hole or a slit in an air-barrier can make it completely ineffective. A vapor-barrier with a hole that was 10% of it's surface would still be 90% effective.

            The Foursevenfour details are to maintain the integrity of the air-barrier. I you aren't using the membrane for that you don't need to incorporate all that tricky stuff.

            Michael knows a lot more about double stud walls than I do. I've never need able to justify building one in our temperate climate.

    3. buildzilla | | #9

      thanks michael, very helpful.

      since you have good experience with double-stud walls, can you share a few anecdotal details?

      1. do you typically do inner or outer load-bearing?

      2. in pgh terms, do you think 10" thick is sufficient for cz-6?

      3. are 2x4 @ 16oc typically sufficient from a structural engineering perspective?

      4. what size/quantity of gussets do you spec per stud connection?

      5. assuming service-cavities aren't required for air-tightness purposes, should i still be thinking about strapping inboard of the vapor-barrier for a smooth sheet-rock install, or does rolling and/or thick enough sheet-rock mitigate that concern?

      1. Expert Member
        Michael Maines | | #11

        You're welcome.

        1. I prefer to make the outer wall load-bearing but I have done projects with the inner wall load-bearing, including a new house that was recently completed and one under construction now--it's the best way I've found to do cost-effective, high-performance walls on a slab with exterior insulation. I use a thick mudsill and plywood gussets to support the outer wall. A licensed engineer signed off on the completed project. There is more moisture accumulation in the outer wall so that's another reason to make the interior wall load-bearing. But it just seems simpler to me to make the outer wall load-bearing. Some builders build the entire exterior then come back to frame the interior; others build both walls at the same time.

        2. Yes; in fact we are about to start a new home near Portland, ME with 8" double stud walls, and two near Augusta, ME are almost complete, but that's the thinnest that I think makes sense. 10" is fine. In most cases the extra cost to get to 12" is not large and R-40 is a nice round number, so that's what I usually aim for, but if cost or space is an issue then 10" (R-36) is just fine.

        3. Yes, except for a few situations: when the wall is supporting two floors and a roof, such as a walkout basement with a 2-story home above; when there is a lot of glazing or tall walls that need extra lateral support; or in seismic zones they may be needed but I don't have experience with those. Table R602.3(5) shows the basic rules:

        4. It depends whether the gussets are supporting the exterior wall or not. If the outer wall is structural, the gussets are just to help keep the studs straight, so a 3" x10" rip is fine. When the inner wall is structural, I like 24" tall gussets, but that's based on intuition, not math.

        5. Excellent question. Strapping the interior is preferable, especially if you're attempting to dense-pack directly against the vapor membrane, but I think a better approach is to have the insulators lip-stitch Insulweb which reduces the R-value very slightly but allows for insulation bulging the Insulweb, then install the vapor membrane separately. With a little rolling that should work without strapping, but you will likely still get a few drywall screw pops. Running a layer of strapping should eliminate the risk of screw pops, but it's extra time and labor. (My co-conspirators Dan Kolbert and Ben Bogie, who built a double-stud house I designed just before building the house featured in this article, think that the vapor-control membrane isn't necessary, and they use data loggers to prove it. But I prefer the added safety and performance of using a membrane.)

        1. Expert Member
          MALCOLM TAYLOR | | #12


          I understand the need for gussets when one of the walls lands on the foam gap, or has poor bearing on the stem-wall, but I'm a it confused as to why they would be necessary otherwise when we don't worry about the straightness of single exterior or interior walls?

          1. Expert Member
            Michael Maines | | #13

            Malcolm, I have a feeling that you get better lumber there than we do here. A high percentage of spruce 2x4s will not stay straight. I don't require the gussets when the exterior wall is bearing; it's what I tell builders when they balk at framing with 2x4s instead of 2x6s which stay somewhat straighter. Before drywalling interior walls on my own projects I go around and straighten crooked studs with a planer or strategically placed saw cuts. Some builders use engineered lumber. Or we all deal with uneven walls if stud-straightening is ignored.

          2. Expert Member
            MALCOLM TAYLOR | | #14


            I sounds like we are luckier than I knew.

        2. Tim_O | | #16

          Do you typically spec something fancy/expensive like Intello when doing a vapor membrane or stick to something more cost effective? I guess when it comes to vapor variable membranes, there isn't a huge variety of prices.

    4. buildzilla | | #15

      are you saying that the gussets can double as partitions for bays, or would that explicitly involve strips of insulweb the entire length of each stud across inner and outer walls?

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