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Community and Q&A

Wall Assembly – Validation or Comments

jmfb_k7 | Posted in Energy Efficiency and Durability on

I am looking to finalize plans with the architect before I go out for bids, building a ranch home with a walkout basement (due to the sloping lot) on the WI/IL border between Milwaukee and Chicago (technically climate zone 6, but just over the border it is 5).
This will be a forever home, so we want it to last 500 years (keep it in the family) :). Was originally hoping for ICF but the cost was just not in the cards, so it will be stick frame. So looking for the longest-lasting wall assembly I can afford which led me to the perfect wall, a la the remote or persist wall style (from what I read doublestud may be cheaper -although not sure with where wood prices are today actually – but that remote is a sure thing if you get it right).
We also want to be energy efficient so we are hoping for total wall R of 25 or more (minus window impact).
My architect wants to put interior insulation in as well to keep us under 4″ of foam on the exterior. I am for some reason super-afraid of moisture issues, so was hoping to use a self-adhered rubberized membrane to cover the sheathing and keep all of the outdoor moisture outdoors. With interior insulation I have to be weary of indoor humidity then, and so I was looking at the SIGA Majrex as a way to keep indoor humidity out of the wall without creating a vapor barrier sandwich – plus I can use this as a ‘netting’ to blow in cellulose to the cavity. I’d then strap 2X horizontally to give me a 1.5″ service cavity for electrical, and not have to worry about puncturing what would essentially be a fully airtight frame.

In all, the layers from out to in would be:
-Vinyl Cladding
-Rainscreen (thinking of ripping 3/4″ PT plywood to 3 inch slats, but would non treated 1X3 do OK instead?)
-2″ GPS (graphite EPS for max sustained R)
-Another 2″ of GPS with seams overlapped (total 18-19 R)
-Grace Perm-a-Barrier or Henry Blueskin
-7/16″ OSB
-2×6 Studs 24 o.c. advanced framing (dense packed with cellulose, total 19.8)
-Majrex Air/Vapor barrier
-2X3 or 2×4 horizontal strapping
-1/2″ drywall
-Latex paint

What concerns me most is REMOTE says 2/3 of the R should be on the outside, but elsewhere it seems I could get by with 50/50 and I think this would technically meet code – I’m just hesitant that we’ll get colder winters over time or something where I end up regretting the extra R’s of the interior. Is this just overkill? Would it be more cost-effective to go 6″ of GPS on the exterior (only R27) and not worry about the interior insulation or air barrier?

I would love any and all opinions and insights.
-Thanks, John

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  1. creativedestruction | | #1

    Unless you plan to keep constant high interior humidity during the winter, you would be better off omitting the Siga membrane to allow as much interior drying as possible from the primary air barrier/vapor control, especially with it being an essentially perfect one in a self-adhered membrane on OSB. The 2/3, 1/3 rule of thumb comes from Canada. You'll be in a better range for zone 6 with your proposed continuous insulation over 2x6 cellulose. Mid-wall air barriers like this are great because they almost always stay dry above the dew point whether you're heating or air-conditioning by being insulated inside and out.

    I would opt for untreated 1x4 for the rainscreen. 1x3s like to split near the ends. Last thought: I can't say I've seen a vinyl cladding product made to last 500 years, nor one I'd want to look at for even 5 but that's personal preference. The biggest issue is it can't usually span unsupported between furring strips. Better to go with a wood or metal product up for the long haul.

    1. jmfb_k7 | | #2

      Thanks for the input Jason.

      My wife has several fish tanks (which I am currently investigating on how to make airtight, or at least not humid somehow - seems like they should be able to dehumidify and put that heat back into the water, but until I get there they are basically evap humifiers), and requires we run a fan-powered humidifier in the kids rooms if they are sick, so controlling indoor humidity will be a challenge. I'm looking at leveraging a CERV for ventilation which seems keenly focused on humidity, and a standalone dehumifier in the basement, so that may do the trick but I was hoping to be safe vs sorry. I like your perspective though so I appreciate the comment.

      I was thining vinyl would be cheaper now so I could do something nicer in 5-10 years (I just want the shell to last 500), but I'll re-evaluate that too. Much appreciated.

  2. Jon_R | | #3

    > I could get by with 50/50 and I think this would technically meet code

    This significantly exceeds code, even in zone 7. Which means that you could simplify the interior side with no Majrex and no strapping (just drywall and low side of Class III paint).

    Consider taped plywood plus DrainWrap. More resiliency if exterior water ever makes it that far.

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