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Double Stud Walls with air gap in between and batt insulation?

alex_coe | Posted in General Questions on

For my wall design I’m leaning toward a 2×4 double stud wall with a 5/8″ gap between the frames and rockwool batt insulation. There will also be 2″ of exterior R8 Rockwool.

Is there any problem with having an uninsulated 5/8″ air gap between the stud frames? Convection looping?

I’m thinking to use the gap between frames as a service cavity for wiring and perhaps plumbing. Is this ok to do or problematic in any way?

My plan is to first frame the exterior wall frames and get the building closed in, then insulate and attach vapor barrier… then move on to framing the inner wall frame of the double stud assembly.

If all of this is ok to do then that preserves the integrity of the vapor barrier via its protected placement and also makes for easier wiring and plumbing, as well as less compromise and detailing time with insulation.

Combined with the exterior R8 rockwool I feel like this would be a great design for me, but that air gap in the wall is an unknown element.

Cheers & thanks 🙂

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  1. creativedestruction | | #1

    The biggest value add of a double stud wall is the opportunity for a thermal break and continuous insulation between the two framed walls. That's not to say what you're proposing would have issues, but it seems like a missed opportunity. Consider the air and vapor control location(s) carefully as it relates to your climate.

    If it were my choice, I would do either exterior mineral wool or a double stud assembly with a sufficient insulation gap, not both.

  2. Expert Member
    Michael Maines | | #2

    I agree with Jason; I'd skip the exterior insulation and add more space between studs, and fully insulate the cavity.

    If you want to stick with your plan, though, it should work. Convective looping happens when there is an air space and a temperature differential. Buried in the wall as yours would be, I don't think you would get convective looping.

    Another problem with introducing a gap between insulation layers is that those interstitial spaces can lead to moisture accumulation.

    You mention vapor barrier--what is your climate zone, why would you want a vapor barrier instead of a vapor retarder, and at what location in your assembly would you put the barrier/retarder?

  3. alex_coe | | #3

    Hey guys, thanks for chiming in :)

    These walls are sitting over a first floor ICF. If I pull the interior stud frame inward to create a larger gap I lose valuable floor space with a building that's only 20' wide. If I add it to the exterior I lose nothing. It just gets sandwiched between the sheathing and rainscreen while I install the rainscreen.

    Then there is the cold sheathing condensation risk. If I have a vented rain screen with vapor permeable exterior insulation the sheathing will be golden, and if I choose to use OSB (for cost or sound isolation benefits) I can do it without worry. Add a smart vapor retarder into the wall assembly, protected between the stud frames.. and it seems like performance and lifespan will be excellent.

    I think the overall R value of the wall via wrapping the building with continuous R8 Rockwool followed by R14 + R14 in the double stud walls should work well in my climate zone and that any thicker would be diminishing returns in the balance of cost/labor/performance/floor space.
    Installing the insulation in this design with minimal compromise & detailing is so simple. I'm not keen on having to hire someone to do dense pack cellulose or the way it would interfere with my plan to place the vapor retarder between the frames. I LOVE the idea of running services in front of the SVR.

    I'm in climate zone 5. Here's the full wall assembly, from outside to inside:

    Cement board siding
    1x4 vertical strapping creates a rain screen vent (and clamps exterior rockwool in place)
    2" R8 Comfortboard 80 continuous exterior insulation
    OSB or plywood sheathing
    2x4 stud frame 16"OC insulated with R14 Rockwool
    Certainteed Membrain smart vapor retarder
    5/8" air gap for service cavity
    2x4 stud frame 24"OC insulated with R14 Rockwool
    5/8" drywall

    For me this feels like a good balance between performance, thickness, labor, materials, simplicity, low risk and longevity. My entire building footprint is 20x40, so inches matter.

  4. creativedestruction | | #4

    Again, nothing inherently wrong with that wall assembly in terms of performance or drying potential, but it's uber heavy on the labor side.

    Simple trade, if you add another layer of exterior mineral wool and use 2x6 advanced frame with the sheathing sealed as your air barrier and vapor control, the whole stud cavity is fair game for running services. More floor area, better thermal resistance with more continuity, better airtightness and fewer trips around the house. Drilling holes for electrical is a lot quicker than framing a second wall and sealing up a membrane.

  5. alex_coe | | #5

    I like it. I modeled the assy here and it says R29 ext insulation is required to skip the interior vapor control in CZ5:
    I thought R16 ext insulation would be enough to not be risky there, so I was surprised at that.

    Another option would be to use a single layer of exterior rockwool - 2x6 wall w/R22 - SVR - then horizontal 2x2 insulated service cavity. This may be the best way to go for my top floor.

    One benefit a double stud wall provides though is the ability to do an independently framed ceiling which is not coupled to the floor joists above for sound isolation. There's no need of this for the top floor of course.

    1. creativedestruction | | #6

      You only need R7.5 exterior ci for adequate sheathing condensation control in Z5. Code and that calculator might also dictate that you include an interior "class 3" vapor retarder. Latex paint on gypsum board qualifies. That part is more from outdated thinking pushed into the code than science.

      1. alex_coe | | #7

        Ahh yes, class III vapor control is all that's required there and latex paint takes care of that.
        If I only do a single layer of R8 exterior rockwool plus R22 in the 2x6 wall that gives me an effective R-Value of 27. Probably worth going a bit thicker isn't it? Good to aim for at least R30 effective?

        If I did 2"+1.5" (R8+R6) exterior rockwool that would land me at R31.5 effective

        1. Expert Member
          Akos | | #8

          With expensive insulation and assembly you propose, you very quickly hit the rule of diminishing returns:

          In zone 5, an R20+ wall is most of your energy savings. The only time it is worth to shoot for more is that you are insulating with cheap materials and there is little extra cost for the R value (ie dense packed double stud wall).

          I would say even the 2x6 24" OC plus R5 is good enough. At that thickness at least you can hang the rain screen strapping with standard framing nails, which is much faster than dealing with long screws. Also keep in mind that mineral wool is slightly squishy, you will need to do a bit of tweaking to keep your siding flat. I would try a mockup first to get a hang of it.

          With less than R7.5 of rigid, you will need a warm side vapor retarder. 6 mil poly is fine for this but you can also go with faced batts or one of the variable perms membranes. Since the mineral wool is vapor permeable, your wall still has plenty of drying capacity towards the exterior.

          1. alex_coe | | #9

            Akos.. thanks for that link. It looks like it's not worth going more than a single layer of R8 Comfortboard 80 on exterior. For my top floor I'll just do a 2x6 wall with R22 and then R8 exterior. For the separate suite below it I'll probably stay with the double stud though for maximum sound isolation between the 2 units.

            Comfortboard 80 is an 8lb density vs the usual 2lb squishy interior rockwool batts, so using either 2.5" wide strips of treated 3/4" plywood or 1x4 lumber as strapping (haven't decided between those yet) the compression is minimal and easily managed with screws. Checking with a string line and adjusting where needed will seal the deal. My cost on R8 Comfortboard 80 here is decent as well at $1.50psf .

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