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Double stud wall question

AdamHorkay | Posted in General Questions on

I am planning on building a new house in Indianapolis, IN (southern part of climate zone 5) and want to make it very green but also want to keep it on a relatively low budget. I am considering a double stud wall assembly listed from exterior to interior below:

Gapped Rain Screen Cladding (0.34)
1x Furring and Air Space (1.00)
1/2″ Huberwood Zip System Sheathing with Taped Joints (0.62)
2×3 Advanced Framed Wall @ 24″ OC Dense Packed with Cellulose (9.25)
2.5″ Cavity with Dense Packed Cellulose (9.25)
1/2″ OSB (or plywood) with Taped Joints (0.62)
2×6 Advanced Framed Wall @ 24″ OC Dense Packed with Cellulose (20.35)
1/2″ Drywall (0.45)

Based on my research this wall should achieve an R-Value of 42.73 including air films. Is this wall assembly a good choice or is it over kill for my climate zone? Is there a certain R-Value at which point there is more insulation than needed?

Is there a better energy efficient wall assembly that breaks thermal bridging for my climate zone than the double stud wall listed above? 

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Replies

  1. Jon R | | #1

    I suggest entering "double wall" into the search box.

    You need to at least follow the recommendations below (and code). Personally, I'd go even further and use a high external to internal perm ratio (eg, plywood sheathing), a smart vapor retarder (like MemBrain) and double air barriers (internal and external, well tested with a blower door).

    How much insulation makes sense is highly dependent on costs and how long you intend to live there.

    https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

  2. Expert Member
    Malcolm Taylor | | #2

    "Is there a certain R-Value at which point there is more insulation than needed?"

    Yes and it isn't just dependant on climate. The ideal R value of walls also depends on the R value of the other building components, especially the amount of glazing you have. It is something that can be modelled fairly well during the design phase.

    Two things in your assembly worry me a bit:

    Open cladding changes the function of the rain-screen gap and WRB by not providing an effective first layer of protection against bulk-water intrusion. If you are set on using it for aesthetic reasons i'd suggest using a more robust WRB than just the taped Zip.

    You don't often see 2"x3"s on a building site for a few reasons. They are appreciably less strong in bending that 2"x4"s, cost about the same amount, and are often hard to find in the usual lengths you need.

    1. AdamHorkay | | #3

      Thanks Malcolm!

      As for your first concern, I got the idea for an open cladding rain screen from Eric Reinholdt's Long Studio* (see Youtube link for video) which used a 30lbs building felt (perm 2) as the WRB with Vaproshield around openings and corners. I wasn't sure if the zip system was robust enough in order to achieve that detail. What would you suggest in this case?

      For your second concern, I got the idea for 2x3s in an article** from Joseph Lstiburek where he suggests that. After a little further investigation it was pointed out that one should not bother and just go with 2x4s instead.

      Also, do you suggest plywood or OSB for sheathing material?

      *https://www.youtube.com/watch?v=y6LisafZQrE

      **https://www.greenbuildingadvisor.com/article/lstibureks-ideal-double-stud-wall-design

      1. Expert Member
        Malcolm Taylor | | #4

        Adam,

        I don't know much about detailing open cladding because our code here in coastal BC doesn't allow it. I would err on the side of caution and use a WRB specifically designed to hold up to UV exposure and repeated wetting over time. Something like: https://vaproshield.com/products/wall-wrb-ab/revealshield/installation

        As far as your stack-up, as Jon suggested, I'd be inclined to have a more permeable sheathing on the exterior and less so one on the inside. So perhaps substitute plywood for the Zip and paint either the inner-sheathing or drywall with a vapour-barrier paint.

        1. GBA Editor
          Brian Pontolilo | | #5

          Benjamin Obdyke and Pro Clima also have WRBs for open joint siding:
          Benjamin Obdyke
          Pro Clima

          I've also known of builders using ZIP sheathing with an additional layer of felt to beef up the WRB and disappear behind the open joints.

  3. Jon R | | #6

    Would be interesting to see moisture analysis of open gap siding. While the gaps would allow in some driven rain, the gaps would also provide more drying. Who knows where it balances out.

    Don't overlook the beneficial effects of large overhangs.

    1. Expert Member
      Malcolm Taylor | | #7

      I think open-cladding is a completely different animal than what we usually think of as a rain-screen. It may be quite effective in some climates, but the detailing needs to be completely different if you are trying to stop bulk-water at the WRB, rather than at the cladding.

      To me it's more akin to a clad building which has an additional moisture deflector outside it. So the inner-cladding needs to be something that can keep out water, wind, insects, and UV damage. That's not something conventional WRB's (especially building paper) is designed to do.

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