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What do you think of this wall makeup?

Adam Nickerson | Posted in Energy Efficiency and Durability on

I’ve spent countless hours researching (new to energy efficiency,) and I’ve finally decided on a wall design for a new construction house I’m building. Curious as to what you think.

Location: CT (Zone 5)

1/2″ Gypsum Board/Latex Paint
2×2 horizontal strapping with Roxul ComfortBoard 80 (1.5″, R6)
2×6 wood studs 24″oc with Roxul R23
7/16″ ZipSystem, taped appropriately
(2) layers 1.5″ foil faced Polyiso, outside layer taped
1×3 strapping
Hardie Board siding

Effective R-value of both Roxul products: r23.42
Putting an R value of r5/in on the Polyiso: r15
R value of gyp, sheathing, airspace: r2
Total effective R-value: 40.42

I appreciate your help. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    As long as you know which of these layers is your water-resistive barrier (WRB) -- I'm guessing that maybe you will use the taped Zip sheathing as your WRB -- and as long as you have a plan to integrate your window flashing with your WRB, your assembly will work.

    There may be less expensive ways to achieve your goal, but your wall assembly meets the rule of thumb (for this type of wall) that at least 27% of the wall's total R-value needs to come from the rigid foam layer in Zone 5.

    For more information on these issues, see:

    How to Design a Wall

    Combining Exterior Rigid Foam With Fluffy Insulation

  2. Adam Nickerson | | #2

    Thank you very much for your response, Martin.

    I was planning on the taped Polyiso as the WRB/air barrier, and the taped zip sheathing as a back-up. I believe it's correct that approach would eliminate the need for a house wrap.

    Based on my knowledge, I assume a cheaper route would be to replace the Roxul with dense-packed cellulose? I like the idea of the 2x2 strapping on the interior to lessen the thermal bridging of the studs, while also helping with drywall installation as I'm designing the house with advanced framing in mind (which seems great for exterior components, but throws off interior 4x8 panels significantly.) Since I'd be blowing the insulation in myself, I can't be confident I'd get a consistent pack given my lack of skill, so I'm budgeting for the Roxul with consistency and ease of installation in mind.

    On that subject: would installing the ComfortBoard on the exterior (perhaps on top of the Polyiso?) and leaving the 1.5" space between the 2x2 strapping as airspace do anything of benefit? I went back and forth on that after reading about the "Perfect Wall" where it states R-values should be equally distributed on the inside and outside of wall sheathing.

    Thanks again for your time and effort.

  3. Brendan Albano | | #3

    If the taped foam is the WRB, you could probably just do taped regular OSB for your sheathing/air barrier and presumably save a few bucks.

    On the "perfect wall" note, it's not that R-values should be equally distributed, but that they should be distributed as appropriate to your climate zone. As long as greater than 27% is in the exterior insulation in Zone 5, you're good on that front.

    You might get more bang for your buck by skipping the strapping and 1.5" roxul on the inside and throwing an extra 1.5" of polyiso on the outside instead. But I'm not experienced enough to have a good gut sense of what costs more than what!

  4. Adam Nickerson | | #4

    Thank you, Brendan.

    Your suggestion to just go with regular OSB was also one I thought of, so I'm glad you brought it up. I'm leaning toward doing that, as you're right, the cost vs benefit most likely won't make sense.

    I do still like the idea of the strapping, though, somewhat mainly for thermal bridging purposes. If my math is correct:

    The surface are of one (1) 10' stud (1.5" side) is 180 square inches of bridging.
    If 2x2 strapping is installed horizontally at 24"oc, that cuts the bridging to 11.25 square inches (ignoring the top & bottom plates, no change there.)

    And it helps with drywall hanging, which I hate with burning fiery passion, so even if I'm wrong about the thermal bridging benefit, I think I'd still have to strap!

  5. Brendan Albano | | #5

    If it helps with drywall hanging, that's great, and if you're doing the strapping anyway, you mind as well insulate it, and it does add R-value.

    Your exterior foam already is ensuring that you have no thermal bridges the inside to the outside. I think at this point, you should be thinking about effective R-value more than thermal bridges. 1.5" of polyiso has a higher effective r-value than 1.5" of mineral wool between 2x2 strapping.

    That said, the difference is marginal (a few R at most), and if the strapping aids constructability, then great!

    For comparison, take a simple 2x6 wall @ 24" OC, mineral wool cavity insulation, and 4" of exterior polyiso (2x 2" layers, only 1" extra foam from your wall):

    This link says that mineral wool in a 2x6 is effective R-19.11. 4" of polyiso is R-20 at the R-5 value you're using. That's R-39.11, throw in your gypsum, your sheathing, and your air films, and you've got basically the same R-value as your wall, but in a slightly simpler assembly (ignoring the drywall hanging benefits of the strapping).

    It's up to you which is more valuable!

  6. Adam Nickerson | | #6

    I just may be sold on that. Thanks! Seems like a very solid way to go; cheaper, and as close to the "perfect wall" I could get without brick veneer.

    And, since I'm quite set on strapping, I'll simply use 1x3's on the interior as well, giving me a more comfortable drywall attachment area, less insulation removed from electrical boxes (3/4" is 3/4" after all) and still taking care of my thermal bridging obsession.

    I like it!

    1/2" gyp/latex paint
    1x3 horizontal strapping (no insulation)
    2x6 wood studs with Roxul R23
    7/16" OSB, non-zip, but taped
    (2) 2" layers Polyiso, outside layer taped as WRB/air barrier
    1x3 vertical strapping
    Fiberboard siding

    So glad I asked.

  7. Timothy Tucker | | #7

    Keep in mind that there can be more to comfort in a house than just effective air sealing and energy efficiency.

    Example: depending on the location of the house, if noise is a concern (near a busy road or industrial plants, etc.), a double wall system could help to keep things quieter inside.

    In your example, rather than 1x3 strapping, putting an interior 2x4 wall with a small spacing between that and the outside framing would both reduce the amount of sound transfer and allow you to do all your interior wiring with no impact to insulation.

    Similarly, there reductions in noise transfer between rooms using staggered 2x4 studs spaced 24" on center on a 2x6 with a dado down the middle (vs traditional 16" on center 2x4's).

  8. Adam Nickerson | | #8

    Thankfully, the location is very rural Connecticut, so sound isn't much of an issue. Country mouse living at its finest.

    I'm intrigued by the between-room framing. I understand the 2x6 plates; I've not seen it done with a dado, however. I assume that breaks the sound transference even more?

  9. Expert Member
    Malcolm Taylor | | #9

    You seem to have a good handle on how you want to build. The one thing that I don't understand is how horizontal strapping will help you hang the drywall. Are you planning to hang the boards vertically?

  10. Adam Nickerson | | #10

    Hi Malcolm,

    No, standard horizontal. I want to frame using the advanced techniques, with my stud corners as my 2' dimensional base. Doing it that way places the end of a sheet of drywall between studs on the "corner cavity" walls. I thought the strapping would fix that. I know there's a product available for this purpose (which also tapers the butt joint which is nice,) but weighing the pros and cons of adding additional blocking or studs vs strapping, it seemed the latter would be the best option.

    That said, I've not framed like this before and it's very possible I'm not understanding something; any setting-me-straight would be greatly appreciated.

  11. Expert Member
    Malcolm Taylor | | #11

    Sorry Adam, I don't understand at all. It might just be me though. Your exterior wall spacing never gives you regular dimensions for drywall sheets, but in the rare cases where the room is longer that 16 ft, you just either cut the sheets or use a backing board. Strapping the walls seems an awfully labour intensive way to avoid something I've never heard anyone see as a problem.

  12. Adam Nickerson | | #12

    From what I understand about Advanced Framing, the two-stud corner is pretty important. In framing like that, using drywall clips solves the attachment issue for the corner, however, if you begin the sheet 5.5" (for 2x6 construction) in from the first stud, the end of your 8' sheet is 5.5" past the 96"oc stud mark. I've modeled an example and attached.

    For skilled drywall hangers, an extra cut to get things back on track for every other wall would be considered a framing error they now have to fix with an additional drywall cut. Knowing how particular they are (and rightfully so (don't hate me. I need you,)) that wouldn't go over well.

    After modeling, I realized the same issue appears with the exterior sheathing (on the walls perpendicular to the interior drywall issue.) So no matter the wall, with two-stud corners you'll be either cutting a piece of sheathing or drywall, leading to a bunch of 5.5" waste pieces which may be difficult to reuse elsewhere.

    Backing boards would certainly work, but they'd have to be used at every butt joint. Actually, the sheathing cutoffs could be used for this, but it'd still be a huge pain from the hanger's perspective.

    I can justify strapping the interior easily. Larger base for attachment, strengthens the wall, staple them initially and use longer drywall screws to fully attach, substantially less thermal bridging surface area from drywall to wood cutting down on not only bridging but sound transmission as well, and less insulation removal to accommodate junction boxes (for Roxul R23, that's a savings of slightly over R3.) But, strapping BOTH sides of every exterior wall would not make me love life to any satisfactory degree.

    Am I completely missing something? It seems the two-stud corner creates far more hassle than it's worth. And yet, I still want them...there must be a way?

  13. Expert Member
    Malcolm Taylor | | #13

    You have over-thought this. All framing, Advanced or not, pulls its dimensions from the exterior corners so you never have a problem with the exterior sheathing. The drywall boards are ordered long enough to span the entire room, or if the wall is longer the two sheets, are joined either on a stud or with a spline board. Boarding crews cut down almost every sheet. Like every job, you are going to end up with a pickup truck bed of scraps that can't be re-used.

    Please don't take this as criticism, and sorry if I have belaboured this, but people building for the first time often worry about things that aren't really problems - and this is one of them. Strap if you want, for any number of other reasons. But if your drywall boarders show up and you explain you did it for them, they will be bemused.

  14. Adam Nickerson | | #14

    I'm very often guilty of over-thinking, and I take no offense. I appreciate the reality check!

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