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Green Building Blog

A Case for Double-Stud Walls

Straightforward construction, common materials, and low embodied carbon are some of the benefits of this well-insulated assembly

As a small-volume builder who mostly does renovations, I have not built that many new houses in my career. But those I have built, have all been thoughtfully designed and constructed, with serious attention paid to energy-efficiency and resource conservation. Since we rarely build more than one new house per year, and often have a year or longer between new house starts, I have time to re-think my wall sections between each build. Despite that, I keep coming back to the method we’ve used since we built our first high-performance home, now over a decade ago—double-stud walls filled with dense-pack cellulose insulation, which is exactly what we did on the home shown here.

The ideal wood-framed wall assembly is debated constantly on GBA and elsewhere. I am well aware of the “cold sheathing” concern that comes up when double-stud walls are modeled in WUFI and other energy modeling software. I have previously described this as the Yeti of building science—it is much discussed, yet rarely seen.

We returned to our first double-stud wall house several years ago. The clients wanted to turn a window into a door leading out to a small balcony. Fortunately for our research purposes, the window was on a wall that was not only north-facing, but faced the water as well. If there was anywhere on the house where we should have seen moisture problems, it was here. Nevertheless, we saw zero signs of condensation on the sheathing.

On several more recent houses, we have buried Omnisense wireless monitors in the walls and in the ceiling assemblies to measure temperature, relative humidity, dew point, and wood moisture equivalent. The batteries in these devices are long-lasting—we have yet to have a monitor die on us and have collected data over many annual cycles. Again,…

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29 Comments

  1. Vivian Girard | | #1

    This article is making an interesting case for double-stud walls.

    When it comes to vapor barrier, recent literature on the topic and my personal experience make me think that vapor diffusion through walls is a negligible issue in most of the country. Rain water and air leakage (which can carry far more moisture) is what I worry about. A useful illustration on this is found in this Matt Risinger video. https://youtu.be/FXXgjvOJcYI?t=312

    There is a minor typo toward the end: "We will often block off a bay every 8 inches to 10 inches" I think feet instead of inches is the proper word.

    If someone could provide a rough number for quality dense-pack cellulose (cost per square foot of double wall or per cubic foot of cellulose installed) that would be greatly appreciated!

    1. User avater
      Michael Maines | | #5

      Vivian, 8 ft to 10 ft is correct. Each stud bay is open to the next one, so when you try to fill one bay it spills over into the adjacent bays. So every few studs you connect the inner stud and outer stud, typically with "netting," the air-permeable fabric that cellulose installers use.

    2. User avater GBA Editor
      Brian Pontolilo | | #6

      Good catch Vivian.

      I updated the text.

    3. Dan Kolbert | | #13

      Thanks, Vivian. I think it's too hard to know if unit costs are relevant in different places. Even my different insulation subs have different prices. But I'd count on somewhere between $3-5/sf for 12" thick dense pack.

  2. Malcolm Taylor | | #2

    Dan,

    Great article. I've been looking forward to it for ages. That house you built with Kaplan/ Thompson is one of my favorates. A couple of small questions I'd be interested in hearing your opinion on:

    Is there a reason you mount your windows outboard of the cavity strapping? Builders here in the PNW were split about 50/50 when rain-screens first came out, but all mount the windows first now. It makes head-flashing back to the sheathing easier.

    How important do you think the moisture buffering of the cellulose is? Would you be comfortable doing similar walls with batts?

    1. User avater
      Ben Bogie | | #7

      Hey Malcolm, I can chime in on some of this...

      We don’t always bring the windows out flush to the plane of the cladding but on this house it was the look the client/architect were after so that’s the direction we went. This avoids the need of extension jambs one both sides of the window but does add a bit more work for properly integrating the flashing layers. Stay tuned to GBA and FHB for an upcoming article on the methods we used on this particular project.

      As far as using batts in a double stud assembly- I’m cautious of that approach. I think the reason that moisture profile modeling of double stud walls misses the mark is that it doesn’t take into account the 3D hygric buffet capacity of cellulose. No other readily available insulation can mimic this phenomenon of absorption and redistribution. Blown in wood fiber will likely perform the same when it becomes more available here in the US.

      If you’re going to use a batt type insulation then mineral wool would be my choice but you need to pay extra attention to it having no voids at the sheathing interface as well as an inboard vapor retarder like Siga Majrex.

      1. Dan Kolbert | | #8

        What Ben said. And I actually wrote a piece about that house a decade ago - https://www.jlconline.com/how-to/insulation/building-a-tight-house_o

        1. Malcolm Taylor | | #14

          Thank you Ben and Dan.

        2. Malcolm Taylor | | #26

          Dan and Ben,

          One more related question: If the double walls are isolated - say the inner-one is sheathed on the outside as an air-barrier/vapour retarder - do you think some combination of using batts and cellulose would then make sense? Sorry if this is sort of leading things into the weeds a bit.

          1. Stephen Watts | | #27

            To extend this question, what are thoughts about blown-in high density fiberglass with "smart" air barrier/vapor retarder? There are some portions of the country (e.g. northeastern Oregon) where dense pack cellulose is not available, but blown-in fiberglass is.

  3. User avater
    Eric Whetzel | | #3

    I would add to Malcolm's question, wondering about Rockwool batts in particular. I used them on my house and found them straightforward to install. A good option if you don't have access to, or confidence in, a local dense-pack cellulose installer, so maybe Rockwool batts are a good option for double stud walls?

  4. Aaron Beckworth | | #4

    Dan,

    It seems common for thick straw bale walls to have splayed window openings to allow more light into the room and to reduce glare. With your 12 inch thick double stud walls, does the deep angular window opening reduce natural lighting in any noticeable way?

    I'd appreciate a photo of a finished window from the interior of a double stud wall, if anyone would be so kind as to post.

    Thanks,
    Aaron

    1. Dan Kolbert | | #9

      I don't think it's a significant issue. Here are some pix from various projects.

      1. Aaron Beckworth | | #16

        The 12 inch thick window opening does not seem as overwhelming as I had imagined it might, even for this lone hung window. This window in particular and the thickness of jamb and sill have a similar look you'd find in old adobe homes here in the SW. Very nice, thanks for sharing.

    2. Dan Kolbert | | #10

      2

    3. Dan Kolbert | | #11

      3

    4. Malcolm Taylor | | #15

      Aaron,

      Some pictures Martin posted of alternatives in a similar discussion:
      https://www.greenbuildingadvisor.com/question/framing-splayed-window-openings-with-double-frame-walls

      I find that, perhaps counter-intuitively, they somehow make the windows feel smaller.

  5. Dan Kolbert | | #12

    4

  6. User avater
    TIM MCCARTHY | | #17

    Great Article Dan..do you use any type of vapor control layer on the inside of the double stud wall, and what is your typical WRB on the exterior?

    1. Dan Kolbert | | #18

      Hey Tim - no, nothing on the inside but drywall. I tend toward vapor-open these days.

      And we like the self-adhering WRB's - SIGA Majvest, Blueskin, etc. With rain screen.

      1. Malcolm Taylor | | #19

        Dan,

        How easy are the self-adhered WRBs to install?

        1. Dan Kolbert | | #20

          Pretty easy. Ben and crew try to do it on the ground and strap the walls for siding, so you skip the staple step.

  7. Brad Walker | | #21

    Thank you for these details in the article and the comments on how this assembly is performing in the field. Maybe Dan or someone else can answer my question. Where are you connecting the 2x pieces you use to connect the two walls? Does it connect the bottom plates or the studs? I'm looking at a similar wall assembly and slab foundation for my house build.

    We are also curious about what most builders do for sliding doors on this isolated slab situation where the foam is not covered by the interior wall. Do you cut a seat in the stem wall and the foam and cover with something? Looking for ideas on what works well and looks good.

    Thanks,
    Brad Walker

    1. Dan Kolbert | | #22

      Hey Brad - it doesn't really matter where you nail. I think stud is easier than plate, but I can't think of any reason why it matters.

      And there are various things you can do at doors - thin the perimeter insulation in those areas, recess it and put some other material in there, or hold to the interior and do extension jambs to the exterior.

  8. Kye Ford | | #23

    Dan couldn't agree more about false concerns for "cold sheathing". I have never run into any of the problems that people are so concerned about. It just doesn't show up even taking apart poorly constructed osb clad buildings years later.

    Anyway can you explain why you prefer plywood vs zip sheathing?

    1. Dan Kolbert | | #24

      It's not a huge preference, but one nice thing about plywood (or solid wood) over OSB is that, when it gets wet, it's permeability increases. Also the baseline perm rating of plywood is higher than zip.

      But we've built several houses with Zip (including the first one, mentioned in the article) with no apparent problems.

  9. Dan Kolbert | | #25

    FMI - here's a video that ProTradeCraft shot last year with Ben and the rest of the crew on double wall. https://www.youtube.com/watch?v=b6xOWrmUvls

  10. Jim Roncace | | #28

    I'm curious about the interior ceiling construction. You state that you install OSB as an air barrier. Is this necessary? Wouldn't air sealing the drywall provide similar performance?

    What do you install below the ceiling OSB? Are you installing strapping and then drywall to create a service cavity for electrical wiring?

    Thanks.
    Jim

    1. Dan Kolbert | | #29

      Hey Jim - yes, we strap down. 2 perpendicular layers of strapping.

      We do it to create a robust assembly that is difficult to damage and can be tested before drywall. We’ve done membranes but they’re easily damaged in that location.

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