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Community and Q&A

Double stud walls, again

Elizabeth Hager | Posted in Energy Efficiency and Durability on

I’m sure many of you are really tired of discussing wall sections so I apologize ahead of time. I have read lots of the blogs and discussions on here about walls, insulation, double-wall construction, etc. I’m designing a house in South-Central Pennsylvania (Zone 5 near Zone 4) and am having difficulty with the wall. My initial wall section was a 2×6 wall filled with R22 Roxul, sheathed with plywood, followed by 2” of R8 Roxul Comfortboard, a rainscreen and fiber cement siding. When we met with the builder, he looked at the wall section, said it was a “solid” wall, that it would be expensive, that we might as well build two walls, and that he didn’t think the insulation or siding guys would enjoy it—long screws, finding stud edges, window trim, etc. After more research, I don’t think R8 out the outside is sufficient anyway. So I’ve been redesigning the wall section to try to address constructability and cost while optimizing the wall.

I think the builder would prefer to just meet the Code requirement by putting insulation in the cavity. The builder’s solution to energy efficiency is to offer SIPS. While I admire the energy efficiency of SIPS, I think they are an over-engineered system that is too dependent on perfect installation techniques. I’d also like to avoid using foam for a variety of reasons. We are not trying to create a Passive House, but at least a pretty good house. (this is not yet fully defined, but better than code is a start.) I don’t think placing insulation in the stud cavity alone is going to meet our expectations. The builder seems opposed to adding insulation on the exterior, so it seems to me that a double stud wall is our option. This would make the siding and window installation more like the builder’s team is used to.

I started with Lstiburek’s ideal wall combined with some Swedish framing. See Image 1. Since the house will have a concrete floor, there’s 4” of foam at the edge that needs to be covered by something. (I had originally shown the insulation angled back to the wall to avoid that, but the builder didn’t think that was needed—I had shown 2” insulation, and he said the trim and drywall would cover it.) So I put the interior wall over the slab edge insulation (sort of like the one in Dan Kolbert’s recent post), and made the exterior wall the load-bearing one. I like the idea of having a resilient air barrier on the inner side of the interior wall as well as the idea of a service cavity. But this also means that, in terms of efficiency of material, we’re “wasting” a structural material there. There’s also a very difficult air sealing situation at the second floor joist. The wall has a max R value 45 with R15 inside the sheathing and R30 outside.

So, I angled the slab edge insulation, and made the interior wall the bearing wall. See image 2. This required making the foundation wall 10” instead of 8”. This seems to use material a little more efficiently (above ground), reduces thermal bridges, and seems like the air barrier details would be easier (located at the plywood on the inner side of the interior wall. However, the walls would have to be built separately with the inner one being fully completed and air sealed before the outer one. The air barrier at the windows might be problematic, assuming that the windows are placed in the outer wall. The wall also has a max R value 45 with R15 inside the sheathing and R30 outside.

On this last one, I have shifted more toward Dan Kolbert’s wall, eliminating the plywood at the inner side of the interior wall, the exterior wall as the bearing wall, and using 2 5.5” Roxul batts. See image 3. This is an attempt to make the wall easier to build. Does this work in climate zone 5? Is cellulose necessary to make this work? Would it be necessary to put an air barrier on the inside?

I would welcome any feedback. I’m particularly interested in constructability issues. What are your thoughts about tapering the insulation at the slab edge? Are there other suggestions for a cost effective way to get at least R30 walls?

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Elizabeth,

    They are all interesting variants, and it was fun to be able to see the design progression.

    As a builder I'd much prefer option three. You end up with simple sequence of construction requiring no unusual techniques, and all the elements of conventional construction. What you lose over over option two is the buried (protected) air-sealed sheathing, and easier to insulate rim joist. But that comes at the cost of it being hard to insulate, and having no exterior sheathing.

    The question as to how resilient these assemblies are without the moisture buffering of cellulose is an open one. Perhaps the presence of a rain-screen cavity mitigates any risk. I simply don't know.

    I'd consider adding a layer of plywood under the bottom-plates to tie the walls together, although your builder may prefer to build the inner-wall later and use gussets.

    Do you need that deep a wall in zone 5?

    1. Elizabeth Hager | | #2

      Thanks for the comments. No I don't need one that deep and I have another variation where I've reduced the space between the walls to 1.5". I didn't want to include too many variations that weren't substantively different. Maybe I should just make the builder happy and do a 2x8 wall with zipR. I think that has resilience issues as well though.

      1. Expert Member
        Akos | | #4

        I don't think you are in cold enough climate to need to worry too much about condensation in walls. Cellulose is nice but sometimes it is very expensive to install, going with plywood instead of OSB would buy a bit of extra margin.

        The one issue with your #3 assembly is that you'll have some big gaps where the studs end with 5.5" batts. Might be better to go with 3 layers of 3.5" batts or go with load bearing 2x6 walls with interior 2x4 or 2x6 wall without a gap but studs offset. The R value difference is not enough to worry too much about unless you are trying to hit an energy target like passive house.

        The bonus of the 2x6 load bearing walls is that you can move your rim board in a bit and have room for 2" of rigid insulation on the outside.

        I think your builder is probably right on the 2x8 + ZipR. There is nothing wrong with that stackup, you would need an interior vapor barrier. ZipR is easy to install incorrectly especially if the subs overdrive the nails. Make sure your builder is familiar with it and can guarantee a quality install.

        1. Expert Member
          Malcolm Taylor | | #5

          Akos,

          I missed that the wall uses just two layers of insulation. It definitely needs a separate one for the gap.

          1. Elizabeth Hager | | #7

            Thanks for the feedback. That was my original choice. Then someone else suggested that 2 layers of thicker insulation would result in less handling. I'm also considering switching to cellulose, just concerned about the quality of the installation.

  2. GBA Editor
    Brian Pontolilo | | #3

    Hi Elizabeth.

    I agree with Malcolm. And I trust Dan Kolbert's expertise. He's been building and monitoring these walls for a while.

    Beyond the moisture buffering quality, cellulose is also a more environmentally-friendly option that mineral wool insulation.

    If I were building these walls, I'd air-seal all six sides of the cavities and I'd probably use a smart vapor retarder as the air barrier on the interior. A ventilated rainscreen is a must.

    This project has tapered slab edge insulation, like you are suggesting: Lessons From a Practical Passive House. They found, through their energy modeling, that it worked. But note that the bottom edge is thicker than it would have been if they had not tapered it.

    Also, notice that they also insulated the inside of the bottom plate to break a thermal bridge that runs up through the footing. That won't be necessary with your double stud walls, because there will be continuous insulation between the bottom plates.

    1. Elizabeth Hager | | #6

      Thanks for the link--that was very helpful.

      My only question regarding Dan Kolbert's wall section is the different climate zones. I don't have enough experience in this area to fully understand the effects of moving his wall section to my climate zone. However, since his climate is colder than mine, performance here in zone 5 shouldn't be any worse? Is that a reasonable generalization?

      Regarding air-sealing all six sides: is this because of the mineral wool or just good practice regardless? I've read some of the debate about one vs. two air barriers and haven't come to a conclusion. However, two seems safer. I'm also concerned that a thin air barrier behind the drywall would be prone to damage. Could this be effectively placed at the inner side of the interior wall? Extending the inner layer up to the second floor deck also seems labor-intensive.

      I'm concerned that creating a relatively complex wall will result in poor execution since the construction team is not that familiar with high performance construction practices. I would like to keep this as simple as possible to optimize the chances of success. What are your thoughts on the thicker single stud wall and zipR sheathing? Am I exaggerating the complexity of a double-stud wall and 6-side air-sealing for a traditional construction crew?

      1. GBA Editor
        Brian Pontolilo | | #8

        Hi Elizabeth.

        The beauty of a double-stud wall design is that you can simply adjust the space between the walls to get to the target R-value. Everything else can stay the same (within reason). Dan is in a colder climate zone, so the assembly will be less risky for you (although Dan might say that it's not risky at all as long as the details are executed well). So, your work is to determine the R-value you are trying to hit.

        I've heard the arguments for one air barrier vs. two vs. air sealing all six sides of the assembly. I believe the latter is a worthwhile approach for two reasons. First, after rain or snow melt, the most likely and damaging way that your wall will get wet is moist air getting into the assembly and condensing on a cold surface. That air could potentially come from many places. Also, while dense packed cellulose has some air sealing potential, other insulation like fiberglass don't perform well when there are convective loops happening in the wall. Finally, air sealing all six sides is not difficult or expensive to do. It's cheap insurance.

        I think the 2x8 wall with ZIP R-sheathing is a perfectly fine wall and this situation is exactly why they developed that product. Did you see yesterday's feature: Working with ZIP R-Sheathing

        If you go this route, it is still important to get all of the air sealing details right, to include a ventilated rainscreen, and to have either the right R-value of ZIP R or an interior vapor retarder. According to the 2018 IRC (code), in zone 5, R-5 for 2×4 walls and R-7.5 for 2×6 wall would give you adequate condensation control. You'll have to calculate that for a 2x8 wall. For more on that, see: Calculating the Minimum Thickness of Rigid Foam Sheathing.

        Also, you may find this informative as you consider assemblies: The Four Control Layers of a Wall.

        And it sounds like you may want to consult with a local designer, builder, or consultant with some more expertise than your builder has on high-performance walls.

        1. Elizabeth Hager | | #12

          Thank you Brian. Yes, I read yesterday's post about ZipR and I've read the 4 control layers. I'm not sure that the builder would be happy about thicker insulation/longer nails. He seems to be avoiding exterior insulation...the cheapest way to reach code is to fill the cavity. For better energy performance, his solution is SIPS. I'm really tired of hearing builders and realtors complain about the changes to the energy code being expensive.
          I think you may be right about a different builder...the issue is that he is the only builder for the lot we have tentatively selected. We may need to keep looking. Thanks for confirming.

  3. Expert Member
    Rick Evans | | #9

    Elizabeth,

    I acted as GC for my own double stud wall house a couple of years ago. It was a first for all parties involved. The beauty of the double stud wall is its simplicity: You stand the outer wall and then you stand the inner wall. The framers had no issues with it. They actually liked having the plywood connecting the top plates when it came time to roll the roof trusses. They are usually balancing on 2x6 top plates two floors up. Our walls allowed for 12" "Sidewalks", as they put it.

    When Brian referenced air sealing all six sides of each cavity- I believe he as referring to batts only. Most batts have little in the way of density compared to dense pack cellulose or spray foam. Therefore, air can pass through them creating a phenomenon called "convection currents". The convection currents can erode the R-vale of the batt considerably. You won't have to worry about this if you opt for dense pack fiberglass or cellulose.

    1. Elizabeth Hager | | #13

      Your experience is good to hear. We had considered acting as our own GC but the issue became one of financing. The bank requires a GC who builds multiple houses per year. My education is in architecture but my experience is all academic (teaching at the community college level) so reading about the experiences of others is very helpful. and architecture school never got this technical. I won't get started about architectural education. :)

  4. Doug McEvers | | #10

    I would look for a builder that is currently well versed in building durable, highly efficient homes. They are available, trying to train someone who could care less about the final outcome is a waste of valuable time.

    Consider a a double wall with a common 2X8 plate, staggered studs with 2 layers of R-15 insulation. For your climate you should dry to both sides in my opinion.

    1. Elizabeth Hager | | #15

      Thanks! This confirms my feeling about the contractor. I have been frustrated by at best having to pay for his learning curve and at worst not getting what we are looking for.

  5. Dan Kolbert | | #11

    Hey Elizabeth - a friend just alerted me to this thread. I'm so glad my article helped - there's a fuller version coming out in the next Fine Homebuilding (shameless plug).

    Have you looked for a cellulose installer? I don't know how common they are in your area. Otherwise I agree with Brian's advice -- would definitely use a variable-permeance membrane like SIGA’s Majrex, CertainTeed’s MemBrain or Pro Clima’s Intello. And not a bad idea even with cellulose.

    With taping the plywood seams and a little extra care at windows, doors and transitions at foundation and roof, it's not that hard to get down to 2.0 ACH50 or less, which should be safe. Obviously work a blower door test in before insulation if you can.

    I know you didn't ask, but if your concerns are environmental, I'd urge you to reconsider fiber cement siding. Check out LP Smart side or just wood clapboards.

    1. Elizabeth Hager | | #14

      Thanks Dan. The contractor we have been working with is not particularly communicative so I'm not sure about his cellulose installer. I will look into that more. I would prefer a more collaborative approach so perhaps a different contractor is the answer.
      I have briefly looked at LP SmartSide, but will reconsider that. Regarding wood clapboards, my husband is focused on durability and doesn't want to paint. However, the rainscreen should make that less of an issue. Thanks for the suggestion.
      I look forward to seeing the fuller version in the next issue.

      1. Dan Kolbert | | #16

        The contractor sounds like a dud. Either find someone with experience with how you want to build or at least is interested in learning. It's very risky to increase insulation and do a half-assed job with the details.

  6. Dick Russell | | #17

    Elizabeth, if you combine the ideas of your diagrams #2 and #3, you'd have the inner wall frame over foundation so it can bear the load of at least the upper level floor. That lets you terminate the upper floor joists and rim board at the inner wall, letting you insulate that rim board as part of the lower level wall cavity. As in your diagram #2, the tops of the wall frames are tied together by the subflooring of the upper level. As for tying together the bottom plates, as Malcolm suggested, another thought would be to have the concrete contractor place a second row of anchor bolts. Also, as already suggested, I'd go with plywood sheathing, taped WRB, top and bottom vented rainscreen gap, and "smart" interior vapor retarder, which could be detailed as interior air barrier. The resulting structure would look much like that of my house, completed nine years ago.

    Building a PGH type of house isn't all that hard to do, or all that expensive either. All the effort is really in just the exterior walls. After the bulk of the framing was completed, I asked the lead framer on my house project how much longer he thought it had taken his crew of four to build and stand the interior frames of the exterior walls. He thought about that, then told me about a day and a half. Most everything after that was basically the same as for a conventionally built house.

    When I got into the process of getting our house built (central NH, CZ 6), I did all my homework well ahead of time, specified all the critical exterior frame construction details and had the drawings produced, then found a local builder very interested in doing something quite different from anything he had done before. I sat down with him and went over all the drawings and critical details of what would be different, particularly air sealing, so construction sequence would let us achieve our goals. I didn't tell him how to do framing, and he understood and trusted my knowledge of how to get what I wanted.

    I was living in the cottage on the lot during construction, and I had just retired, so it was easy for me to stay on top of things. After hours, I'd walk around the project, doing my own inspecting. Occasionally I'd find something that had been missed in the way of air sealing, I'd pass this on, and the crew would fix it. There were numerous DIY things I found I could do, like patching tears in housewrap. Another thing I did, which you might consider, was to cut and fit strips of foam board between the bottom plates of the inner and outer wall frames. It was easy to do, and it made me feel better about the later blowing of cellulose to have a foam strip between the cellulose and the poly-covered concrete.

    I do hope you can find a builder who can build the house you want, not just what he's familiar with. Remember, it's your house, not his. If he's smart, he'll realize building yours gives him valuable education for his future projects.

    1. Elizabeth Hager | | #19

      Thanks for sharing your experience! A day and a half more is not much.
      The idea of supporting the second floor on the inner wall and the roof on the outer wall is an interesting one. I wonder if the building department would approve that? It was my understanding that one or the other wall had to be the bearing wall.

      1. Expert Member
        Malcolm Taylor | | #20

        Sharing the load-bearing between the two walls means each of them needs structural lintels and something (usually sheathing) to resist shear.

  7. Clarke Olsen | | #18

    What about this: 2x6 wall @ 2' oc with Roxul, 1/2" foil faced foam board w/taped joints, horizontal 2x4s @ 2' oc, drywall. All wiring in last space, using 4x4 boxes w/mud plates.
    Putting drywall vertically eliminates but joints.

  8. Dick Russell | | #21

    Malcolm, when you said: "Sharing the load-bearing between the two walls means each of them needs structural lintels and something (usually sheathing) to resist shear."...
    did you mean resistance to racking sideways? With the tops of the upper level floor joists tied together with the rim board, and the tops of those joists and the rim board secured laterally to the outer wall frame by the upper level subfloor, that entire floor system is constrained laterally by the sheathing on the outer frame. Have I missed something? Anyway, the engineering company who did the framing drawings for my home approved the plans for assembling the upper level floor as described.

    1. Expert Member
      Malcolm Taylor | | #22

      Dick,

      On two of the variants that were posted, the outer wall was fairly unrestrained. Depending on where you are, relying on the subfloor won't be sufficient. As your project shows, It's not something that can't be done, just another thing that adds constraints (like not overhanging the foundation) and needs addressing.

  9. pplp | | #23

    you might find this an eye opener. Looking at doing a variation on double stud but going back 80 years to where buildings were commonly erected with no wall sheathing but only had let in angle bracing , or strong tie strapping at the corners and near openings in the wall. Doing this as a double stud wall with the inner wall as the support a layer of air barrier sheet then i joists / larsen wall with this cavity filled with dense pack. On the exterior of the I joist another layer of air barrier. Eliminating the sheathing entirely but only a thermal connection between the I joist and the inner 2x wall. or you might consider something like in this wall https://foursevenfive.com/blog/the-double-stud-wall-simplified-low-cost-high-performance/

  10. T Carlson | | #24

    The third one would be the easiest to build, the first one has 2nd floor load path issues that are a concern and should get flagged in permitting.
    Also a stem wall is usually 6”, why pour thicker if you don't need to.

    That said, trying to inset foam without having a wall sitting on foam, you can pour a 10” wall and stack a 1’ high 6” form on top to give you 6” of bearing and a 4” inset. Would have to dig an extra foot down, no big deal. I do this when doing zero entry houses and have the floor system sunk into the foundation with full basements but it would work in this double stud wall slab scenario.

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