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How risky is a single layer nailbase SIP over a single stud wall with sealed sheathing?

JustHousing | Posted in Energy Efficiency and Durability on

In the quest for a cost-effective, durable super-insulated wall, we’ve typically done double stud walls with dense pack cellulose, with the air barrier/vapor retarder on the inside behind the sheetrock (polyamide), and sometimes had the exterior sheathing (plywood or fiberboard, or a combination thereof) taped as the air barrier. We’ve also designed walls with a single stud wall and the exterior sheathing (typically OSB) sealed as the air barrier, and two layers of rigid foam sheathing to the exterior, with furring strips over the WRB over the foam, fastened through to the studs. This “single stud wall with foam sheathing” has the benefit of a rigid durable air barrier outboard of the wiring (and in some cases some plumbing and/or heat distribution), and the air barrier is pretty resistant to future tampering by homeowner renovation. However, builders in my area are not fond of installing two layers of foam followed by furring – it is a lot of labor time and hard to keep things square and plumb outside. I’m wondering if a single stud wall with one thick SIP nailbase panel is a good idea. This concept lets builders get a house framed in, and allows the air barrier to be carefully detailed and tested. If the nailbase panels are installed with sealant and minimally expanding foam between the joints, or maybe with a spline panel joint, how worried should I be about shrinkage or movement over time? I don’t want to lose the continuity of the thermal sheathing layer. Feedback? I’m in climate zone 7, so am thinking about a 2 x 4 wall with dense pack cellulose (roughly R-12) and a 7 3/4″ nailbase panel (R-28).

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think there is anything risky about what you suggest. The only issue is cost.

    Here is an article about a house with a wall assembly that resembles the one you are contemplating: Passivhaus on a Budget.

    The Claudia King / Lindsey Tweed house in Falmouth, Maine, used a similar system. Read about it here: More Job Site Visits in Maine.

    This approach has also been used for retrofits. Here is a link for more information: A Deep Energy Retrofit Using Nailbase Insulation Panels.

  2. JustHousing | | #2

    Thanks for the links, Martin. I'd been looking at the DER article, but wanting examples of new construction. All you sent helps. Our builder is providing a cost comparison of the single wall with nailbase SIPs vs double stud. We'll have an estimate soon. I love double stud wall construction, and prefer using dense pack cellulose to rigid foam, but I've become fond of the idea of getting a rigid air barrier "away" from the interior, and no one I know wants to put OSB to the inside of the double stud wall and then add a "service cavity." Not cost or space-competitive. We've thought about framing a stud wall as described, then doing the Larsen trusses to the outside, with dense pack from the outside (as I've seen others do), but around here with so much winter construction, insulating with dense pack from the OUTSIDE just doesn't make sense. So I'm looking at options, and this is looking appealing if, as you say, the cost works out.

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