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Staggered double stud wall on 2×8 plates

sayn3ver | Posted in General Questions on

Is it worth the trouble to do a double stud 2×4 wall staggered on a 7.25″ or even a 7.5 ” plate vs a 2×8 wall?

 With either plate, is the air gap detrimental or is 1/2″  rigid  insulation needed between?

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  1. user-723121 | | #1

    I have used the 2x8 staggered double stud wall, R-30 in a 7 1/4" space. There is no air gap, insulation will be continuous for the 7 1/4" depth.

  2. maine_tyler | | #2

    Are the 2x4 studs aligned? Wouldn't there be a 1/4" gap?
    *edit, somehow I missed the word staggered.

    What climate zone are you in? My first impression is that there's little point to a double stud wall that thin.

  3. Expert Member
    Michael Maines | | #3

    The staggered-stud wall will perform a little better than using 2x8s as studs, but it's not hugely different, as a 2x8 is about R-8.7 on its own which isn't terrible as far as thermal bridges go. Framed 16" o.c. a 2x8 wall with cellulose insulation will be about R-24.5.

    Arguments against framing with 2x8 studs are that the wall are heavy to stand up, a single 2x8 might cost more than two 2x4s and that 2x8s likely come from older-growth timber than 2x4s, so they may not be as environmentally responsible to use.

    1. sayn3ver | | #13

      For the detailed breakdown

  4. Expert Member
    Akos | | #4

    If the studs are in-line, there is a very slight increase in assembly R value as it adds about an R1 to the studs, this brings up the lumber portion of the assembly from ~R8.4 to R9.4. If you put 1/2" polyiso inbetween it bumps the lumber portion up to R11.4.

    To put this in perspective:
    -2x8 R24 Assembly
    -2x4+2x4 with air gap R24.5 Assembly
    -2x4+iso+2x4 R26 Assembly
    -staggered 2x4 R29 Assembly

    Staggered stud walls are a pain to insulate with batts unless you insulate the outside stud bay first before framing the inside, I would only go for that if you are dense packing.

    1. Expert Member
      Michael Maines | | #5

      How did you calculate the R-29 staggered 2x4 studs? If both inner and outer layers are at 16" o.c., I get R-24.53, using the parallel heat flow method. Oh, is that including polyiso?

      1. Expert Member
        Akos | | #6

        I used R4.2/inch for HD batts. The framing fraction is probably a bit too optimistic at 20% but I get a stud only assembly R value of 26.5. Add in sheathing/drywall/siding and air films for the R29.

        The plyiso was only used a 1 1/2" wide spacer between the studs for the in-line stud example.

        1. Expert Member
          Michael Maines | | #10

          Got it. I assumed R-3.6/in for cellulose or regular fiberglass, and I didn't include sheathing, drywall or air films.

    2. sayn3ver | | #14

      Was planning on building the wall with the exterior 2x4 and 2x8 plates conventionally.

      Once the building was dried in I would add the offset studding on the inside to essentially having two layers of r15 batts that didn't line up except at the two doors and two windows on the structure (it's a detached shop/office).

      My thoughts are the prints I have on hand are designed around a single 2x4 wall so strength isn't an issue

      I just figured eliminating some thermal bridging may be worth more than just framing with 2x8. But due to interior layout of the stairs I cannot really afford to lose any more additional interior space to say 2x10 or larger plates.

      I'm restricted to the exterior dimension already due to town zoning.

      1. maine_tyler | | #15

        What's the reason for single wide plates? If you're setting studs later, you'll have to toe nail...

        I wold either just use separate plates or look at something like a Mooney wall

        1. Trevor_Lambert | | #16

          I was thinking the same thing. I'm planning on doing that, but the difference is I'm using the studs for structure and the outer studs will hang over the slab, necessitating a solid connection of some kind. Shared plates seems like the simplest option.

        2. sayn3ver | | #22

          I mean I could add the offset interior studs at the same time.

          I suppose it doesn't matter when I take the time to layout the interior studs, just thought it might be easier for a non framer to layout and toenail the interior studs after. It's really just holding the drywall and insulation.

          All my electrical will be piped in emt surface mounted.

          Just to clarify, I would
          Be using 2x8 plates and 2x4 framed 16oc. These would be laid out as if I was building a single 2x4 wall. The interior 2x4 studs would be offset side to side from the exterior run, except at doors and windows.

          The theory is you'd have two 2x4 walls with the studs covered by r15 insulation on either the cool side or the warm side of the wall.

          I'm not opposed to just building two 2x4 walls separately. Just throught it would be stronger sharing the top and bottom plates.

          These are going atop an 8" cmu stem wall so I have 7.625" of block to work with.

          Honestly typing this out I probably should just do that and run 1/2" foam strips between the studs and plates.

  5. plumb_bob | | #7

    Michael, I have a hard time seeing how the double stud wall and the single stud 2x8 wall are comparable, as the the thermal bridging is eliminated with the double studs, except at the plates. On cold mornings I can see the frost ghosting on 2x8 walls but not double stud walls.
    As far as labour, 2x8 walls are fast to build but brutal to lift by hand, so a machine is often used. Double stud walls are obviously more work, both to frame and insulate.

    1. maine_tyler | | #8

      Double stud with only 1/4" between studs?
      * edit, sorry I missed the OP reference ti staggered.

    2. Expert Member
      Michael Maines | | #11

      Plumb_bob, "thermal bridging" is one of those fuzzy terms that doesn't mean anything unless you define it. A 2x8 is about R-8.7 so it's not a terrible insulator.

      The usual way to calculate assembly R-values is by parallel heat flow, so you don't consider the three-dimensional aspect; you assume that heat is going straight through the assembly at every point. Using that approach, it doesn't matter whether the studs are offset; the only difference is that (2) 2x4s are 7" while a single 2x8 is 7 1/4".

      In reality there is some three-dimensional heat flow but it's not easy to calculate and won't change the results significantly.

      1. maine_tyler | | #12

        OK now I'm confused. When you say offset in your latest post Michael, do you just mean 'gapped'?
        Because an 8"ish stagger would certainly change the linear parallel heat flow calc. since each stud would have 3.5" of insulation in series with it rather than another 3.5" of stud.
        Still, in the grand scheme, perhaps not monumental...

        1. Expert Member
          Michael Maines | | #18

          Tyler, unless I'm missing something obvious, which is possible, that's what the math says for offset studs. I know that instinctively you would think that offsetting has an impact, and I'm sure it does, but not as large as you might think for the reasons I noted above.

          1. maine_tyler | | #21

            OK. Yes I agree the actual difference is very small.

            I was being literal. When using the 2d parallel heat flow equations, technically the math does differ. For just the stud cavities (excluding common elements like plates, jacks, etc.) I get R 21.88 in line and R 23.7 for off set. ( that's using R3.7/in insulation and R1.25/in wood. Assuming also 7 inch thickness all around).

            But the therm model proves that is an over simplification. I'm honestly a bit perplexed by those results, but i guess with the common framing elements added in, and maybe the 3d elements (?) it makes sense.

          2. Expert Member
            Michael Maines | | #23

            Tyler, a 2x8 is 7.25" so that's what I'm using for calculations, with 16" o.c. studs, whether offset or in line.

            For offset studs, using your R-values, I get R-4.375 for the studs and R-13.875 for the insulation in line with the studs, for a total of R-18.25 where there is framing, which makes up 3" of the 16" spacing, or R-3.422. The remainder of the cavity has R-26.825 insulation making up 13" of the 16", or R-21.795. R-3.422 + R-21.795 = R-25.22.

            Compare that to aligned studs: 7" of framing is R-8.75 plus 1/4" of R-3.7 insulation is R-4.625 for 1.5" of the 16" cavity, or R-0.434. The remaining space is 7.25" x R-3.7/in = 26.825; 14.5" of the 16" makes that R-24.31. R-0.434 + R-24.31 = R-24.74, or less than R-0.5 difference from the offset version.

            If you haven't already, check out my comment #19 below.

          3. maine_tyler | | #24

            Thanks Michael,

            I did see post #19 and the therm model Kohta links to is the one I was referencing. I actually commented in that thread on how I didn't understand how that could be but, honestly, doing the linear heat flow calcs has helped me to better understand.

            I think I was reading you too literally (or incorrectly). When you said, "Using [the parallel heat flow] approach, it doesn't matter whether the studs are offset; the only difference is that (2) 2x4s are 7" while a single 2x8 is 7 1/4"."

            When you said 'it doesn't matter,' I thought you meant that the math inputs were identical, not just that the results were very close. I agree they are very close.

            In post #21 I did use 7" instead of 7.25" for simplicity, so here's my math for 7.25":

            ---------- Staggered

            ---------- Aligned

            I'm not totally following your calcs. Carrying decimals to a high degree of accuracy matters a lot here (I did the calcs all at once on the calc so no rounding losses). Your first result is maybe (?) close enough for that to be the difference. On your second calc I'm not so sure. Are you adding parallel R values?
            Either way, it's certainly not a practical difference!! Cheers

            *A bigger difference would be seen if wood had a lower R value and/or if the insulation had a higher R value (especially the former). So in the extreme case of metal studs, it would make a significant difference!

      2. Trevor_Lambert | | #20

        Thermal bridging is a term that probably would best be forgotten., not that there's any way for that to happen. It gets abused a lot.

        What constitutes a bridge? An ideal bridge would be R0, a very close approximation of which might be steel (R0.003). A material with R3.5 is considered a good insulator. Where along those lines do we draw the line between bridge and insulator? R1 may seem like it's closer to R0.003, but really it's closer to R3. This becomes obvious if you convert the R-values to their proper representation of heat transfer, the U-value.

        Perfect Bridge U=infinity
        Steel U=333
        2x4 lumber U=1
        fibreglass, cellulose U=0.28
        Perfect insulator U=0

  6. maine_tyler | | #9

    Have you considered a Mooney wall?

  7. plumb_bob | | #17

    Ok thanks. I do not do the thermal calculations so I find the subject quite confusing at times. My thinking aligns with Tyler's as above, if you replace 1/2 of the 2x8 with insulation there will be much better performance.

    As always, I appreciate the discussion and insight.

    1. Expert Member
  8. sayn3ver | | #25

    Following up:

    Still waiting on pricing and the utility to relocate the existing gas service to the house to proceed.

    I am still considering a staggered frame wall for the exterior over using plain 2x8's. Attaching a quick screenshot of what I am thinking. It doesn't eliminate thermal bridging at the plates, rim board, doorways or windows.

    It is cheaper than using all 2x8 as two premium 2x4's is still a few dollars cheaper than an equivalent 2x8. I believe it would improve the thermal performance of the wall assembly for not much extra effort. two layers of r15 rockwool which in my experience is nicer to work with than r30 batts and more rigid. I'm still partial to framing the interior studs after the structure is up and dried in as it will make insulating easier.

    This is for a heated garage shop with office in the attic (think cape cod or bungalow 1.5 story). The garage space will be kept 55F/60F during the winter. The upper office area will be separated thermally and have its own entrance and HVAC.

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