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Safe-Buildable-Double Stud Wall Revisited

Cody Fischer | Posted in Green Building Techniques on

Hoping to re-start a conversation about improving on Lstibureks ideal double stud wall. This is a ridiculously long post…but it does contain 5 specific questions that I’ve called out in bold.

Several great threads on this topic linked below that have sadly died out:

Lstiburek’s Ideal Double-Stud Wall Design

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

https://www.greenbuildingadvisor.com/question/double-stud-wall-midwall-air-barrier

https://www.greenbuildingadvisor.com/question/can-someone-explain-the-science-behind-a-mid-wall-vapor-barrier-on-a-double-stud-wall

I am building a 4-story passive house apartment building in Minneapolis (6A). We are doing a double stud assembly with cellulose because we also have embodied carbon reduction targets.

We have settled on a load bearing exterior wall to simplify the thermal layer details at the foot/stem-wall transition. The load bearing wall is 2×6 for structural reasons. Like everyone here…we are weighing material cost, buildability, and durability/failure risk.

Kevin Zorski’s wall (attached – also see second link above) was our initial preferred approach because it seemed to balance buildability and risk…kind of a mid-point between Lstiburek’s wall and Ben Bogie’s FHB double stud wall.

Though more buildable than Lstiburek’s wall, it presents a few costly drawbacks in our application over the baseline presented by Ben Bogie:

1) ($ + Material) Hangers for our 18″ trusses are quite expensive
2) ($ + Material) Using hangers will require an 18″ wide double layer plywood ribbon around the entire building perimeter at each floor to serve as a nailer for the truss hangers
3) ($ + labor) An extra mobilization of our HVAC subcontractor to install and air seal ductwork through the exterior stud wall before the interior stud wall goes up.
4) ($ + material/labor) – having both exterior sheathing (DensGlass) and interior sheathing (plywood) of exterior stud wall

We are waiting to hear back on hygrothermal modeling of this assembly to determine whether locating the air/vapor barrier 5 1/2″ into the 15″ wall assembly is enough in our climate zone to meet PHIUS+ requirements. I will post those results.

The cost and buildability drawbacks above keep bringing me back to trying to make Ben Bogie’s assembly work for a multifamily building built to last 100+ years.

The assembly below is a tweak on his, with the addition of a smart vapor retarder.

Exterior to interior: (15″ total depth)
– Cladding
– 1×4 rainscreen
– 1/2″ plywood taped at seems with Siga Wigluv (air barrier #1)
– 2×6 load bearing exterior stud wall
– 2×4 interior stud wall
– netting between two stud walls and on interior face of interior stud wall to sub-divide
– dense packed cellulose
– Certainteed MemBrain (Air & Vapor Barrier #2)
– USG EcoSmart GWB

Many will point out that it is better to put the MemBrain at the exterior of the interior stud wall to 1) ensure continuity at install (i.e. no tricky outlet boxes, drywallers being carless with sawzalls, etc) and 2) protect it from future maintenance work / tenant wall hanging nails.

That said, the sequencing is much simpler and more cost effective putting the MemBrain on the interior of the interior stud wall.

In weighing the risk/costs of this path, I’d like people’s thoughts on some scenarios

Scenario-1: The vapor barrier is compromised with nails from hanging pictures.

Question-1: How much risk (moisture) does a nail sized hole allowing humid air / vapor intrusion during cold months into a wall with 15″ of cellulose really pose given the hygric properties of cellulose? Does your answer change over 100 years?

Scenario-2: At install, the detail around an electrical box is poorly executed. As gypcrete is being poured, vents are accidentally left off and humidity in the building gets astronomically high (winter months).

Question-2: Can the assembly recover from this one-off super moisture accumulation event followed by years of seasonal moisture similar to Scenario-1?

I would love any thoughts on questions 1 & 2 above!!

Assuming one answers: (Q1) It is risky, especially over 100 years and (Q2) would likely result in mold. That leads us to putting the vapor retarder on the exterior of the interior stud wall.

I prefer to use MemBrain instead of plywood to reduce material cost and embodied carbon.

Installation Hypothesis: Prior to stud wall being tipped up, insulation subcontractor would…
– staple cellulose netting / fabric
– apply acoustical caulk to bottom of bottom plate & top of top plate
– MemBrain cut with 8″ excess on top and bottom of wall and on wall ends
– MemBrain stapled to wall through acoustical caulk
– Wall is tipped up by framer
– At the corners, fold the two pieces of MemBrain from adjoining walls together and stapled them inside a stud, then tape the joint.
– Cut hole through MemBrain/netting and blow in dense packed cellulose
– Patch all holes

This seems incredibly complicated, slow ($$) and requires both the framer and insulator to be working on the wall at the same time and coordinating the prep and tip up of each section of wall. I can’t imagine getting fair subcontractor pricing for something that sounds this complicated.

With the MemBrain installed and the exterior plywood sheathing taped, I also don’t know where the air goes during dense packing. Waiting to tape the exterior sheathing seems insufficient.
Question-3: Is this a problem?

This approach also has no way to sub-divide the wall to allow for quality dense pack installation. With the MemBrain on at tip up, its not possible to install netting at intervals between the two stud walls.
Question-4: Do we need netting to break up a 11.25″ deep wall of cellulose?

If we just used plywood instead of MemBrain…
Question-5: Is it sufficient to tape vertical seams, tape top/bottom plate seams before tip up, apply a bead of acoustical caulk to bottom of bottom plate before tip up? I’m then at a loss on what we do to keep the vapor barrier continuous at the top plate.

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Replies

  1. Stephen Sheehy | | #1

    For our double stud wall, the builder framed the interior wall on the slab, stapled a Siga membrane to the exterior side and left a big enough flap on top and the sides that the membrane could be stapled to the bottom chord of the roof trusses. Once the interior wall were erected, holes were cut to allow installation of dense packed cellulose.
    What I think was a big benefit is that the interior 2x4 wall allowed wiring and plumbing to be mostly inside the air barrier and we can hang pictures, etc. without compromising the barrier. We did insulate that space with fiberglass batts after rough wiring/plumbing.

  2. Expert Member
    Akos | | #2

    Vapor diffusion is based on surface area, so a 95% vapor barrier blocks 95% of the vapor flow. Small holes or discontinuity in it make very little difference.

    The reason you see vapor barriers meticulously detailed is they sometimes also serve as the warm side air barrier, which case holes matter. Even then, nail sized holes are such small leaks that I would never worry about it. For device boxes, go for vapor tight ones. These have a gasketed flange that seals up against the drywall, hard to mess up and requires no extra work.

    I think a mid wall vapor barrier is a great academic idea, it is just not possible to fit into standard workflow. Any way you look at it, you would have to have both framers and insulators out twice. For a special project that is possible, but does add cost.

    Form the studies I've seen on standard thick walls, the issue seems to be using a low perm exterior sheathing such as OSB. Even OSB can be made to work provided the cladding is vented. If you want to improve it use a more permeable exterior sheathing such as CDX or gypsum. Simpler than trying to figure out the mid wall vapor barrier.

    I think the bigger question of a 100 year wall is not only one that will hold up but also one that can be easily repaired and brought back to original performance.

    For example, even when I'm extra careful, whenever an old wall with a vapor barrier is opened up, it is next to impossible to get the vapor barrier sealed up again. There are inevitable tears and holes where the drywall is cut.

    I think for this reason, instead of working with membranes, an assembly that uses sheet goods is better in the long run. You want to be able to open a wall, fix the inevitable water leak and close it back up without effecting the performance of the wall.

    Thinking more along these lines, I think a thick wall built with I-joists with OSB as the warm side air/vapor barrier would work better. It might also make sense to cross strap the wall with 2x3 on edge over the OSB similar to a Mooney wall for a service cavity (this can be left uninsulated or R8 rolls).

    A bit more material cost but only one wall to build and can be easily dense packed since each bay is already separated.

  3. Jason S. | | #3

    Forgive the 'question with a question' but do you really need that thick of wall for an apartment building? How many units? Multifamily tend to be internal load driven. Your biggest challenge in hitting passive house targets will be the mechanicals, be it unitized or communal or some hybrid. I would keep the walls even simpler and spend the money elsewhere.

    1. Cody Fischer | | #4

      We'll make the walls as thin as the model let's us. We haven't gotten that far yet in the design process. Just kicking off DD right now with energy modeling being an iterative part of that in the coming weeks.

      This is a 4-story, 23 unit, 20,000SF building. We'll be using one Minotair per unit with inline auxiliary heat for heating, cooling and fresh air.

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