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R-value of Staggered Stud Wall

kenmoremmm | Posted in Green Building Techniques on

Could someone help me figure out the effective wall R-value for the following wall assembly?

Staggered 2×4 studs: 16″ OC both faces
2×7″ plates (a 2×8 ripped down to 7″)
Roxul batts between cavities (R-14 per 3.5″)
2″ Comfortboard on the exterior (R-8)

Thanks

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Replies

  1. MarkMac | | #1

    I use this online calculator. https://cwc.ca/design-tools/effective-r-calculator/ It took a few minutes to understand how it works but then it’s straight forward.

    Using the closest option to what you described would probably be the double 2x4 with 2.5” gap and R32. I selected 7/16 OSB and 16” studs. It suggests R-27.3. If you start selecting too many other options, there may not be a wall assembled worked up as it pulls from a database of thousands of designs.

    If you chose double wall, don’t choose any rigid as they don’t have any wall designs like that. However the rigid is pretty much the same between nominal and effective values.

    So add R-8 to the above and you have approx R35 as effective.

  2. Expert Member
    Zephyr7 | | #2

    Without doing the math for the real whole-wall R value (I’m just in my car at the moment between jobsites), you look to have two layers of 3.5” batts plus R8 exterior continuous insulation. These would all add to get up to about R36. Note that 3.5” of mineral wool is usually R15, so you might want to double check your R14 number.

    The studs do cut into the overall effective R value for both sides of the wall, but since neither sides stud goes all the way through, thermal bridging is greatly reduced. Obviously the exterior continuous insulation doesn’t have any issue with thermal bridging. Put that all together and I’d put you at a conservative R34 whole wall value.

    Bill

    1. Trevor Lambert | | #7

      Does it actually even matter if the studs are staggered? If you look at the two walls as separate assemblies, each one is a standard wall with a framing factor. The combined R-value should be simply the sum of the two assembly r-values. I think the only difference with staggering the studs is more evenly spreading out the heat loss, but in terms of total heat loss it's not clear how it would be any different.

      1. Expert Member
        Zephyr7 | | #9

        If the studs are right against each other (think "two 2x4s lined up to be the same size as a 2x8", then you have thermal bridging, just with a thicker wall assembly. If you leave a gap, the gap needs to be at least the width of a stud for a "true" double stud wall to perform like a staggered stud wall. Basically you need to keep the thermal bridging of the studs down as much as possible. If you build a staggered stud wall with 2x4s that is twice the depth of a 2x4 wall, then you have half of the insulation in each "side" of that wall assembly, and each half acts to limit the thermal briding of the other half.

        It's not quite linear, so not quite what you'd think in terms of moving the studs making no difference. Allison Bailes wrote an article about this, although I don't think he specifically mentioned double stud walls. The principle is still the same though.

        Bill

  3. kenmoremmm | | #3

    Does that 2.5" gap assume that it's insulated? If so, there's a big difference there.

    And yes, I poked around that website too, but it didn't seem like they had "staggered stud" as an option. I think the double 2x4 vs a staggered stud are vastly different in terms of concept.

    1. MarkMac | | #5

      pdf here shows the calc for the double 2x4 design they're quoting values for. That's about as close to what you describe as I've found on their site. I was evaluating a similar design but cannot seem to justify the gains of the double wall, vs the additional cost for my area.

  4. Jon R | | #4

    Accounting for wood studs and more thermal bridging in the baseplates (than studs) and using R14 , I estimate around R31. You can get a much more exact number using THERM.

    I'm curious - you wouldn't be better off with a standard 2x6 wall with strips of Zip-R or nailbase and inset mineral wool batts to the exterior of the sheathing? Good total R (say R30+) value, warm sheathing, lots of inward and outward drying, minimal foam, batts are much cheaper than Comfortboard and it forms a rain-screen. Meets 2021 R702.7 code with a smart Class II on the interior (and probably with a Class III on the interior).

  5. Forrest Stanley | | #6

    I agree with Zephyr7 The value is going to be about R34 for a simple straight section of wall. But, Adding the extra lumber that will be required for headers, cripples, corners, etc. for a whole house will bring it down quite a bit (probably to around R31 or so). The R34 is for a wall with gaps between the studs and the comfort board or sheet rock. If you go to the trouble to insulate those gaps (and it's a lot of labor time) the R value will increase to around R36. Still, this is a creative solution to increasing the R value of the walls without much increase in the area of the foundation and roof. However, this still does nothing to address the cold sheathing/moisture problems inherent in high R value walls.
    The real difference between double wall construction and staggered stud is that the double wall method allows for unlimited insulation thickness between the walls at no increase in lumber costs for the wall lumber. The only way to address the cold sheathing/moisture problems is with an air/moisture barrier set in the middle of the wall (no more than about 1/3rd of the total thermal resistance of the wall from the warm side) - or thick insulation (usually a foam) outside the sheathing . GOOD LUCK!

    1. Expert Member
      Malcolm Taylor | | #8

      Forrest,

      "The only way to address the cold sheathing/moisture problems is with an air/moisture barrier set in the middle of the wall (no more than about 1/3rd of the total thermal resistance of the wall from the warm side)"

      Can you flesh out the logic behind that? I have seen interdictions about having the vapour barrier no more than 1/3rd of the way from the interior, but that's to stop it from becoming the first condensation surface. I didn't understand that to have anything to do with whether the sheathing was cold or damp.

  6. kenmoremmm | | #10

    Sorry, I'm not understanding the logic here.

    How do you take two walls that are nominally insulated to R-14 each, have no extra insulation between them, and end up with R34?

    Per the attached file, a basic 2x4 wall @ 16" OC with R13 is effectively R10. This does not include framing factor (windows, doors, headers, etc).

    So, if you use the logic of a staggered stud wall being two * standard 2x4 wall, this gives you an effective R-20 wall. Then add in your R-8 exterior insulation to give you a total of R-28. What I'm not sure is how the bridging elements (since they're effectively removed for the studs) changes it.

    As to the question of why not do 2x6 + R19 + 2" exterior: you'll see from the attached that this gets you an R-23 using case 2a and substituting rockwool for XPS (2" vs 1" as well). In my ideal world, I would go with 2x6 + 4" of exterior comfortboard, but the numbers for materials are spendy and I'm certain the labor is much higher considering the annoying detailing of 4" of squishy exterior surface to hang cladding on.

    My debate at this point in time is whether to go with the stagged stud + 2" comfortboard, or ICF with 4" thick EPS both sides (R-35 assembly). I think from a cost and performance standpoint, the ICF wins. Environment standpoint is debatable, but I'm less concerned with that.

    1. Expert Member
      Akos | | #12

      ICF with two times 4" EPS is nowhere near R35. The thermal mass buys you a bit (more if you are in a desert climate with big temperature swings), but not that much.

      Staggered stud walls are more work to build than a double stud without the option of spacing them apart for extra insulation.

      If you are looking for a simple ~R35 wall assembly, I would go for 2x8 24" OC with HD batts plus exterior 2"polyiso (3" EPS or R12 ZipR).

      The other option is to go for 9.5" I joists for studs and use batts or dense pack and no exterior rigid. The nice benfit of I-joists is the narrow web between the flanges significantly reduce the thermal bridging without having to deal with exterior insulation.

      Double stud wall doesn't start making sense unless you are looking at 12" thick (R40 plus) or so walls.

  7. Forrest Stanley | | #11

    Malcolm:
    The IRC and other building codes often require a calculated thickness of insulation to be attached to the outside of the sheathing. This is designed to keep the interior surface of the sheathing warm enough to preclude condensation of interior moisture which slowly migrates through the insulation to the inside surface of the sheathing. Similar calculations can be done for an air/moisture barrier placed inside a double stud wall( if the barrier is a good one the interior moisture can't get to the sheathing).
    So your right, at that point we don't care about the temp. of the sheathing because there's not enough moisture to condense. In the above example wall there is nothing to prevent the migration of moisture through the insulation to the interior surface of the comfort board, which I guess constitutes sheathing. If the design temp. is low (say -15 f (-26 c)) the inside surface of the comfort board will be way below the dewpoint given commonly expected temperature and humidity levels inside the house.

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