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Renovation with interior poly vapor barrier

jnarchitects | Posted in Green Building Techniques on

We are working on a renovation including an addition of an existing house in Zone 5.

Existing wall: 2×6 @ 16″ o.c. w/ fiberglass and an interior poly vapor barrier.

We would like to improve the R-value of the assembly, but will not be removing the plaster from the interior face of the majority of the walls. This means we do not have the opportunity to remove the vapor barrier.

As a result, do we lose the opportunity to add exterior polyiso, because of the resulting vapor barrier sandwich?

Any suggestion for adding exterior insulation that would have an appropriate permeability given the interior poly?

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  1. jnarchitects | | #1

    Mineral wool board...?

  2. GBA Editor
    Martin Holladay | | #2

    You've answered your own question, evidently. More here: Wrapping an Older House with Rock Wool Insulation.

  3. jnarchitects | | #3

    Thanks for the link...Sometimes you just have to ask the question to find the answer.

  4. jnarchitects | | #4

    There's more to the story...another portion of the house is timber framed w/ sips. If we wrap the 2x6 portion we were going to wrap the SIPS as well. They are 30yrs old and barely R-20.

    If we are forced to mineral wool on the 2x6 portion, any reason not to stick with mineral wool on the sips areas and just put the WRB against the OSB?


  5. GBA Editor
    Martin Holladay | | #5

    Mineral wool will also work on the exterior of SIPs.

  6. Expert Member
    Dana Dorsett | | #6

    Using unfaced EPS on the exterior also works. It's more vapor retardent than mineral wool, but not nearly as vapor retardent as XPS or the typical facers on polyiso. Type-II EPS is between 3 & 4 perms at 1", which is more vapor permeable than dry plywood or #15 felt.

    If you use a high-perm housewrap there is no real problem with putting up to 3" (R12.5) on the exterior a which point it's still over 1-perm, and only somewhat more vapor retardent than 2 layers of exterior latex paint. At R12.5 it would roughly double the whole-wall R value of the assembly (assuming R19s and 16" o.c. framing), and would be sufficient for dew point control on a 2x6 wall even in climate zone 6, even if the interior vapor barrier were breached. You'd be fine from a dew point POV at 2" /R8.4 in zone 5, and would be running roughly 2-perms, which is more than enough drying capacity for the sheathing.

    EPS is about the same R/inch as stone wool panels, and a fraction of the weight and cost. It doesn't have the fire-proof aspects of stone wool- it'll light off nicely if you put the flames to it, but under fiber-cement or stucco it would be hard to get it going- a forest fire big enough to ignite it through that type of siding would already have taken your house via the roof & windows.

    If you went with even lower density Type-I EPS you'd have even higher permeance, and could go up to 4", which would come in at about R15. But it's more easily damaged in handling (especially in thinner sheets).

    If you're doing 2-3" in a single layer, use a ship-lap version to thermally break the seams. EPS will usually shrink a small amount over 2-3 decades, and a ship-lap seam joint will preserve most of the performance over time. If multiple layers of 1-2" thickness, overlap the joints by a foot or so, for both better air tightness, and thermal breaks over any future shrinkage.

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