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Community and Q&A

Air barrier location in a double stud wall

John Ranson | Posted in Energy Efficiency and Durability on

I’m considering an R-40 double stud wall for Rochester, NY, zone 5, with the following construction:

Shiplap siding,
1×3 furring (rainscreen),
Felt (WRB),
Fiberboard,
Loadbearing 2x4s or 2x6s filled with cellulose,
Plywood (air barrier),
Gap filled with cellulose,
Non-loadbearing 2x4s filled with cellulose,
[Smart membrane (maybe)],
Gypsum,
Paint (Class III vapor retarder)

I’ve seen a number of designs that put the plywood on the exterior surface of the inner wall. Located instead on the interior surface of the outer wall, will it be insulated enough to be protected from moisture buildup?

Here are my thoughts: Code lets you use a Class 3 vapor retarder if approximately 1/3 of the whole wall R-value is exterior rigid foam. Now, assuming an R-14 (whole wall) 2×6 exterior wall, my design also has 1/3 of the R-value outside of the plywood. Unlike rigid foam, my wall can additionally dry to the exterior. So it seems to me that 2x6s on the exterior should be safe.

However, it’s not clear to me if the 2x4s would be okay. The plywood isn’t quite as well insulated, but it can dry to the exterior.

Any thoughts on this?

–John

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Replies

  1. Malcolm Taylor | | #1

    John,
    The build-ability of the assembly depends on a couple of things. Is it a one storey house? Does it have a framed floor or a slab?

  2. John Ranson | | #2

    I'm planning a single story over a basement with 8" foundation walls. The foundation will be rectangular.

    I was thinking of framing the floor inside the outer wall to get a continuous air barrier to the sill plate. I would use blocking instead of a rim joist to minimize the footprint of the floor. A 2x4 wall, 1/2" plywood and 3" of bearing for the joists is only 7", fitting easily on the foundation. With a 2x6 wall, I would need the sill and outer wall to overhang the foundation by an inch.

    --John

  3. Malcolm Taylor | | #3

    You've obviously already thought all this through. Unless we are both missing something, this sounds like a good, build-able wall. The sequencing of framing is going to be a bit unusual, but that just requires a bit of forethought.

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