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Is a 2×8 wall better than a 2×6 wall with exterior insulation?

Mixed_Beans | Posted in General Questions on

I am working out the wall assembly four out new build and I am trying to figure out whether it is better to go with a 2×6 wall or a 2×8 wall. Here is what I got for both

Home is in climate zone 5 

2×6 wall (from inside to out)
– Gypsum board/sheetrock
– 2×3 service cavity
– 2×6 framing with dense packed wood fiber (~ R-20)
– Zip sheathing
– 5.5 inches of wood fiberboard (~ R-19)
– Rain screen 
– Cladding (Vertical aluminum on 1st floor and horizontal Shou Sugi Ban wood on second floor up to peak)

2×8
Basically the same assembly but without the 2×3 service cavity. In this case we would maybe use some 1x strapping (maybe with the 2×6 wall too) for the service cavity.

– Dense pack wood fiber in the 2×8 (~ R-27)
– 4 inches wood fiber board on the exterior (~ R14)

I think the 2×8 with thinner exterior sheathing might be more manageable form a construction standpoint especially when it comes to attaching the rain screen and cladding. My understanding is that having a vapor open insulation on the outside also gives me some leniency with the amount on the outside although  I think, in both scenarios, my rations are good. 

Thoughts?

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Replies

  1. DennisWood | | #1

    I'd be tending towards the wall with a service cavity just based on the ease of dealing with air sealing with this type of wall. There are far fewer penetrations to deal with when using a service cavity, and you have more flexibility with respect to having plumbing (code permitting) on an outside wall. Having done a commercial project with this kind of wall, I'd 100% do it again on a residential.

    1. Mixed_Beans | | #10

      I saw somewhere that the strapping could serve as a service cavity. All that really needs to be ran on the exterior walls would be wiring and maybe plumbing and gas.

  2. canada_deck | | #2

    Are you going to have a vapor barrier? Will you be able to avoid all plumbing in the exterior wall? Have you considered t-studs? When I see wall assemblies like this, I also wonder what % of walls will be taken by windows as there are diminishing returns in making the walls so thick if there are going to be a lot of windows anyway.

    1. Mixed_Beans | | #11

      I believe were only looking at the kitchen with plumbing in the exterior wall all but i suppose we could avoid that to a certain extent

      There are a decent number of windows. It sounds like I need to run a model to see what we actually need. I was designing this off the 10 20 40 60 rule which is why the values I listed are on the high side.

      As for t-studs I had considered them at one point. I forget why I passed. Might have been availability at the time but things are likely different now. I might just look at them again. Thanks for the reminder.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Mixed_Beans,

    I know I bang on about this every time the subject comes up, but I don't think service cavities make sense unless there is actually going to be something in them. At best they should be used where services are concentrated.

    The living room wall to my left has two electrical boxes that needed air-sealing. If it had been detailed so the sheathing was the primary air-barrier, it would have none. How would empty interior stud wall help that assembly?

    1. Mixed_Beans | | #12

      Fair point. It'll mostly be wiring and I figured that would be easier to deal with if it wasn't in the wall cavity. The strapping might be all I would need in either scenario(2x6 vs 2x8)

      1. Expert Member
        MALCOLM TAYLOR | | #14

        Mixed_Beans,

        I don't see what strapping buys you? Wiring isn't a problem, it's only boxes that cause air-leakage - and as I said, if your primary air-barrier is the sheathing, it doesn't matter much.

        1. Mixed_Beans | | #21

          I was thinking if I have dense-packed wood fiber/cellulose then I shouldn't run wiring in the cavities. If there were ever an issue with the wiring or additional wiring was needed then a service cavity would make the wiring easier to get to. Similar to what I've seen up here in New England where they add strapping to the ceiling. I suppose some type of conduit would be an option if the wiring were to be in the cavity.

  4. walta100 | | #4

    The preposed walls sound great but my guess is either will be so expensive to build that they will never recover the extra cost to build them over the code min wall of cavity R20 + continuous R5

    Consider making the time to build a BEopt computer model and build the house with the lowest cost to build and operate.

    You may find this thread interesting.
    https://www.greenbuildingadvisor.com/question/zone-5-exterior-insulation

    Walta

    1. Mixed_Beans | | #9

      Thanks. I played with the software a bit but early on in the design process. I think I need to fire it up again and see what i can get out of it assuming I use it correctly.

      1. walta100 | | #23

        I am convinced it would be impossible to teach yourself how to use the BEopt software.

        The training videos on YouTube make the software useful.
        https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d

        The software needs you to input real world cost per square foot cost numbers from actual bids without them it is using pre covid averages that are totally wrong today.

        I agree with JollyGreen that the window and siding guys are going the charge more with exterior insulation.

        Walta

    2. jollygreenshortguy | | #19

      I'm not convinced the true cost of exterior insulation is always fully considered. It considerably complicates door and window installation, especially in regions where contractors are not very comfortable exterior installation practices.

  5. rockies63 | | #5

    I take it the only reason you're needing a service cavity is for electrical, and the depth of the cavity is to accommodate electrical outlet boxes (no plumbing pipes within the exterior walls)?

    Why not save a ton of building materials and do a 2x6 top and bottom plate with 2x4 studs that are offset? The inner set could be 24" on center for the drywall and the outside set 16" on center for the sheathing, This would eliminate a lot of thermal bridging through the studs and you could weave the electrical wiring through the studs so you wouldn't need the service cavity. For the outlet boxes, they can be airsealed and still have insulation behind them.

    1. Expert Member
      MALCOLM TAYLOR | | #6

      rockies63,

      2"x6" @16" yields a whole wall value of R-17.3.
      The staggered studs with cellulose gives you R-20.1

      Numbers courtesy of Robert Riversong.

      1. Mixed_Beans | | #13

        Malcom. Are you saying the the 2x4 staggered stud would be better? I failed to mention I was looking at 24 OC for both assemblies.

        1. Expert Member
          MALCOLM TAYLOR | | #16

          Mixed_Beans,

          The staggered stud wall gains you about R-3. It complicates the framing, and means you can't use batts. It wouldn't be my first choice unless you maybe used wider plates to make the gains significant.

          1. nlbailey | | #17

            Is there a reason you couldn’t do batts? Couldn’t you use unfaced for studs closest to exterior and faced for interior?

        2. Expert Member
          MALCOLM TAYLOR | | #18

          nlbaily,

          With 2"x4" studs on a 2"x6" plate, you don't have two complete stud bays. Every stud bay is interrupted by a stud.

  6. Patrick_OSullivan | | #7

    I don't understand these wall assemblies in climate zone 5. I doubt they will pencil out on an economic basis, and unless you have already identified the perfect contractor, to most of ones you will meet, 4+" of exterior insulation will likely seem like the structure was designed by an alien civilization.

    When there is that much of a gap between the furring and the wall, the connections between the two become like little beams and unless you're going to do a Larsen truss (https://www.greenbuildingadvisor.com/article/all-about-larsen-trusses) type wall, the fasteners need to be a little beefier than normal to resist bending. It all just becomes rather finicky.

  7. andy_ | | #8

    Just some quick addition...
    1/2" drywall
    + 2 1/2" cavity
    + 5 1/2" stud
    + 1/2" sheathing
    + 5 1/2" wood fiber
    + 3/4" rain screen
    + 3/4" siding
    = 16 inches.

    16 inches. SIXTEEN. INCHES. ...in Zone5.
    That's a thick, expensive, complicated, time and resource consuming wall. Do you think you'll see a return in your climate?
    I'm all for building above the code minimum but there is a very real drop in effective R value as the thickness increases in walls. There are much simpler and less resource intensive ways to hit the sweet spot of performance, comfort, and aesthetics even if you do have unlimited funds to reinvent the wall.

    1. Mixed_Beans | | #15

      Good points. I was starting with the 10 20 40 60 framework but it is sounding like that is overkill. What would you suggest?

      1. andy_ | | #28

        Are you building a certified PassivHaus®©?
        Maybe I'm being a bit cynical lately but as cool as it is to get a plaque that says you don't burn a lot of fuel to heat your home, is it really the best use of limited resources to reach that goal?
        Build a simpler wall. Choose one that is climate appropriate and buildable by your local builder. Air seal and insulate it well. Put some solar on the roof and install a couple minisplits. You'll spend less, actually get it built, and in the long run will have a smaller carbon footprint (and bigger bank balance) than building an over the top passivhaus.

        1. Mixed_Beans | | #29

          Pretty Good House would be closer to what we are aiming for. No need for any plaques. Just a comfortable house that doesn't use much energy and ultimately if we could get to net neutral or even positive that would be great.

          1. andy_ | | #31

            I really like the concept behind the Pretty Good House. One of the things that I wholeheartedly agree with is "a PGH goes above code until it stops making financial sense."
            I'll admit that I too have been sucked in to all sorts of details and extras that I saw on here or other parts of the internet. An observation from my Father in Law who is a retired house builder after seeing the plans for my first house...
            "You're going to overdo it on your first house, include too much of your wish list and try to make the best house ever. It will take too long and be over budget. You'll realize you could've lived without a lot of it. Your second house will be cheap and quick but you won't like it either because some of the things that made you proud on the first one aren't there. Your third house will have the right balance."
            Try to design that third house first.

          2. Expert Member
            MALCOLM TAYLOR | | #32

            Andy,

            That's very good advice.

  8. jollygreenshortguy | | #20

    Lots of good comments already but I'll add my 2 cents.
    Taped exterior sheathing for air barrier - This is easier to make continuous because there are fewer obstructions compared to interior.
    No need for service cavity especially if you've been able to move most of the plumbing to interior walls.
    I know continuous exterior insulation is very popular, for good reason, but it does add considerable complication and expense to the door and window installations, especially if the contractor is not familiar with it.
    I'm attaching an image of 2 wall sections I did as a comparison. In your case a variable perm membrane wouldn't be needed in zone 5. The left option achieves continuous insulation while the right option doesn't. But the whole wall results are nearly the same. The left option requires 2 steps in both insulation and framing, although you could also do this option with blown insulation instead of 2 layers of batts. Also, the 2x horizontal strapping can be done with offcuts and does not require as much effort as building an entire second wall. The right option has only 1 step for insulation and 1 for framing, which should save some labor. But the framing materials will be a bit more expensive. The framed walls will be heavier to lift but for framers using jacks it would make no difference.

    As always, I don't guarantee my numbers 100%. This is not my area of expertise. But I believe they are useful for comparative purposes.

    1. Mixed_Beans | | #22

      Thanks for sharing. It's a little hard to read the image when blown up but I think I see what you are getting at. Looks like you are doing the continuous insulation on the interior? Is it actually continuous with the strapping?

      I'm not too worried about window and door installation with exterior. Besides seeing enough videos on this topic the window manufacturer actually comes on site to work with the carpenters to get the window detailing right.

      1. jollygreenshortguy | | #25

        Yes, I'm sorry about the quality of the image. The image I uploaded was large and clear but apparently GBA downgraded it considerably. Maybe these will be more clear.

        1. Mixed_Beans | | #30

          Yes. These are great. Thanks for sharing.

      2. jollygreenshortguy | | #26

        Yes, the insulation is continuous except where the vertical studs cross the horizontal strapping. I haven't calculated the area but it must well exceed 90% of the wall surface.
        It's great that you are fine with the door and window installation. I'm just pointing out that the extra effort and skill involved is an additional cost.

  9. Ryan_SLC | | #24

    I think 2x8 should be the "green" answer to insulation.

    Mineral wool in R23 or R30 costs similar to R15. Or hopefully the new wood insulation I keep reading about. Or fluffy Cathedral R38.

    That's the answer. Exterior foam just seems so inefficient to my non trade person's experience because of the planning + detailing, and shear amount of money it takes for those two items. AND they are still ASTM Smoke and/or fire concerns.

    1. andy_ | | #27

      "Mineral wool in R23 or R30 costs similar to R15. " ????
      No it doesn't. Not if you're buying more than a couple bags.

      1. Ryan_SLC | | #33

        It is hard to even imagine where exterior foam material, detail, and work makes the mineral wool cost differences even meaningful. A grand? Two grand. Done in a day by a non expert. Safer. Going to last regardless of work quality almost. Sounds greener, cheaper, and higher R value.

    2. jollygreenshortguy | | #34

      In terms of greenhouse warming potential (GWP) mineral wool is almost identical in impact to EPS foam, for a given R-value. Cellulose insulation, blown or batt, would certainly be a greener option. Mineral wool is quite energy intensive to fabricate.

      1. Ryan_SLC | | #35

        But like I said before, not only cost+time, per R value.

        I would imagine a 2x8 wall with R30 mineral wool or even fluffy R30C (or compression tables of R38/R38C) is above needed R value or expected R value of outside foam with R19 in walls with no effort, no hard detailing, no tape failures, no pest damage, better in every metric for money, detail, skills, material, time, and decreased failure potential. No foam recycling needed. No future questions on re cladding. No long fasteners.

        2x8 walls also forces us to revisit ceilings. Cathedral ceilings that are built to code at 2x8. That's too thin for anything but massive furring, which is questionable. With minimum vent channel and R38C, still not possible even furring 2x4 down. Spray foam is the only solution.

        Change ceiling code and we've effectively killed off the need for spray foam, almost ever. No more adherence, cracking, resheathing concerns. No more nasty spray foam.

        And you get a stronger building out of it.

        2x8 all they way original poster, then you can totally skip the outside insulation and end up with a higher R value. No way does it cost more money too if you're not doing the work.

        1. Mixed_Beans | | #41

          So you are saying if I get enough insulation in the wall exterior insulation is not needed? You would still have a thermal bridging issue wouldn't you?

          1. jollygreenshortguy | | #47

            "In theory" you would have a thermal bridging issue. But it's the whole wall R-value that determines your energy loss. The "issue" is basically non-existent. The wood framing has an R-value of between 1 and 1.5 R per inch. It's not as if you are going to feel cold spots or get condensation in the wall. You will experience no difference in comfort and for a given whole wall R value, no difference in energy consumption either.

            My comment about cellulose, to which Ryan responded, was just meant to point out a reason to consider cellulose rather than mineral wool. In terms of R/inch they are similar. But cellulose has other benefits, namely less environmental impact and a greater ability to absorb and release moisture should you ever get any in the wall. It's called "hygric buffering". It's a bit like a sponge in that if some moisture should get in the wall, it will spread throughout the cellulose at a low concentration and then evaporate away over time, instead of being held in a denser concentration in one spot, more likely causing damage to adjacent surfaces.

  10. rkymtnoffgrd | | #36

    I would have to agree with the previous comments about complexity and expense for zone 5. I'm guilty of overcomplicating my zone 7 build, and I regret some of what I attempted to do and assumptions I made that I could find tradesmen that would be willing to build it reasonably. But if a super high-performing wall is your goal, your among good company here...
    However.... I would go no further than 2x6, 24"OC framing, 2" rigid subslab, continuous subslab plastic taped to bottom of Zip R3 wall, tapped over/inside of top plates, taped interior to continuous osb underside energy heel trusses b/4 interior walls go up. Be cognizant to limiting celling penetrations, install an HVR, and your looking at very tight, substantially efficient, comparatively inexpensive, relatively conventionally constructed house. Insulate with fluffy of your choice...

    1. Expert Member
      MALCOLM TAYLOR | | #37

      rkymtnoffgrd,

      Lessons learned maybe, but the pictures you posted of your build yesterday look good!

      1. rkymtnoffgrd | | #38

        Thanks Malcolm, here's an example of one complication in my wall assembly and roof design... Look at the upper uninsulated portion of my cabin. This is well above the R-70 insulation in the attic, yet to match the plain of the siding, this will require vertical furring strips and horizontal blocking to be installed prior to the siding going at 16' up in the air.... I assumed contractors could easily adapt to such things. This is not the case, contractors flat out told me they don't build like that... Fact is, I'm building it myself now, one weekend at a time, and there are many things I would have done different.. From the roof line, to the truss design, to how the foundation is insulated... Many times, form did not meet function...

        1. Expert Member
          MALCOLM TAYLOR | | #39

          skymtnoffgrid,

          It's good to be clear eyes about the decisions you make, and during construction I spend a fair amount of time feeling unrelentingly negative about every error. That feeling persists once I'm finished, and for a while they are all I see, but thankfully that quickly disappears, and you realize living in the space that they are pretty inconsequential. I bet that happens with you too.

          1. Expert Member
            DCcontrarian | | #44

            I find it much harder to live in space that I had something to do with building.

            I generally try to avoid drywall for that reason, while I may not be any worse at it than the average guy, I'm much more critical when I know where the bodies are buried. And drywall is just never perfect.

          2. Expert Member
            MALCOLM TAYLOR | | #46

            DC,

            I love coming back to a project after some time has gone by. I'm left with the feeling: Did I really build this?

        2. rkymtnoffgrd | | #51

          My build does bring me great pride, and is a great success thus far (it will be another year working weekends at this pace), this picture says it all... last Thursday nite, pulling into the cabin at 9PM after being gone since Sunday, 7 degrees outside... I'm not hooked up to power yet, no thermal input besides the sun, no exterior insulation on the south side (not finished yet), the concrete floor is not stained slate black, the secondary air barrier (drywall) is not installed, taped, sealed, or painted, and yet the cabin maintains a temp of 45-65 degrees inside with outside temps between -5-25 degrees all week... I did install osb under my trusses and taped sealed to outside zip... so the build is very tight without drywall. The slab is just now stabilized its been increasing 1-2 degrees every week since I finished the R70 cellulous in the attic four weeks ago. Its truly remarkable to walk into such a warm space on such a cold day with no power whatsoever... I wish that for Mixed Beans....

    2. Mixed_Beans | | #42

      I've been hesitant to do foam anywhere but under the slab. Do you think you the wall you propose is going to be enough for climate zone 5? I'm probably going with cellulose or wood fiber in the cavities. That is about R-20. Anoth 3.6 with Zip-R 3 is an R-Value of 23 which is effectively much lower especially with the thermal bridging is it not?

      1. rkymtnoffgrd | | #49

        I had to go with exterior wall insulation on my build due to condensation and the minimum ratio I felt necessary to ensure against it in zone 7. I would not change that for my build, but if I was building a little lower on the mountain in zone 5, (Im in a dry environment and not required by code), then I would really simplify my wall assembly to something more conventional, but still adamantly focusing on a super tight envelope, (air infiltration trumps insulation) but otherwise shifting those dollars to upgrading the window package. Dollars better spent in my opinion. 2x6 and ZipR3 is very conventional and as efficient I would let my walls be in my dry environment for zone 5. Zip R3 provides some need thermal break, is easy to handle/install, no brainer to complete seal and flash, but thin enough that condensation can still be controlled. Id even eliminate the rainscreen (GASP!!) and settle for repainting my concrete siding every 10years... But this is advise from a guy who is 3x over budget and can not longer afford a contractor... I'm worn down from 2 years of fighting with them and have found it more rewarding and pleasurable to build it myself and take the time to do it right.
        I actually had one contractor look at my wall assembly, and chuckling asked," Insulation is on the outside?!" As if my goal was to heat the outside...
        That said, most of my issues was with the wall assemby, the concrete siding and properly sealing/flasing corners, windows etc... Its was just totally foreign to contractors around here... I had another guy, after pondering my wall assembly said, "Wait, So, I'm basically framing and insulating the inside, then I'm insulating the outside, then framing again on top of that, so I can then install siding? And the outside framing is done on ladders and scaffold?!!"

  11. rockies63 | | #40

    I thought continuous exterior insulation was now a code requirement in zones 3 to 8?

    1. Expert Member
      DCcontrarian | | #45

      Even the 2021 code allows R30 in 4 to 8 and R20 in 3 without continuous. If you choose to go the continuous insulation route, it doesn't have to be exterior. It can be interior or between the two parts of a double-stud wall.

    2. jollygreenshortguy | | #48

      The IRC 2024 will be pushing insulation levels back DOWN to 2018 code.
      There is always the option of calculating the U-value of the wall assembly. So if you are in zones 6-8 you can take that approach if you want to avoid exterior continuous insulation. And there is always the option of doing your continuous insulation on the INTERIOR, by means of a double wall or something similar.

      1. Patrick_OSullivan | | #50

        > The IRC 2024 will be pushing insulation levels back DOWN to 2018 code.

        I've only seen the change down as far as ceilings/roofs, not walls. But if I missed it, happy to be corrected.

  12. Mixed_Beans | | #43

    If you are having fun picking my wall design apart feel free to chime in on my roof design. It's a doozy too. Surely over engineered and expensive.

    https://www.greenbuildingadvisor.com/question/is-this-a-good-roof-assembly

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