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Insulation and vapor barrier considerations for two differently insulated walls in the same bathroom

BarnyardBuilder | Posted in General Questions on

I’m renovating an attic into a bedroom and decided to add a small full bath in zone 4 lower Delaware.

Of the 4 walls of the bath, two are existing 2×4 walls in the attic. The other two will be new and in the new room space.

One of two existing walls is a knee wall for which I have insulated with unfaced ROXUL R-15 between the studs and 1 inch dbl faced polyiso on the exterior (outside envelope attic facing ) back of the studs.   The other wall is essentially an exterior wall of the house with the interior side in a bedroom. The exterior of that wall is in the old attic space that will be the new bed/bath space.  This wall from the inside out has 1/2-inch drywall, blown-in compacted fiberglass and 7/16-inch exterior OSB sheathing in place.

I plan on framing a new 2×4 wall against the two existing walls to run plumbing but am not sure how I  need to insulate that new air space in relation to the existing insulation in both walls.  I plan on using something like Dens Armor Drywall around the shower, regular drywall over the rest .  The kneewall seems straightforward but the OSB sheathing raise a flag.

Comments welcome and thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    Your question is unclear. If you add an additional 2x4 stud wall against an existing 2x4 exterior wall or an existing 2x4 partition, you have created a thicker wall -- either 7 inches thick, or even thicker (if you choose to leave a space between the two bottom plates).

    This thicker wall can be insulated. You'll end up with a higher R-value than you would have had with a simple 2x4 wall.

    1. BarnyardBuilder | | #2

      Thanks Martin.

      Yes, this could use some clarity. Ill give it another try. The knee wall is clear to me. Its the other existing wall with OSB on it. My concern is that by placing a new 2x4 wall against the OSB and covering with the sheetrock described above, will I create an area ripe for mold behind the new sheetrock in the existing OSB covered wall? I know OSB is permiable and am thinking any moisture will dry to either side of the double wall but am not sure. Or do I need to take the OSB down?
      Hopefully this is clearer than MUD.

  2. Jon_R | | #3

    Your question comes down to how to build a good, OSB sheathed double wall. GBA has some articles on this (available with the search box at the top).

  3. Expert Member
    Peter Yost | | #4

    Another variable is the drying potential of the OSB to the exterior; will the cladding be installed directly against the WRB on the OSB or will there be a vented space between the two?

    Forgiveness within this assembly can come from either or both directions...


  4. BarnyardBuilder | | #5

    Thanks all. Ill look at double walls Insulation practices.

  5. BarnyardBuilder | | #6

    I looked on Insulating Double Walls...Still dont have a solution relative to OSB on an interior wall after conversion. Sorry if Im just not getting it. . Maybe posting a pic will help. The OSB wall shown in the pic is the one in question. On the other side of it is a conditioned bedroom that is part of the original house. The OSB wall faces the attic Im turning into a bathroom currently unconditioned ( I referred to it as exterior because essentially, it is). I dont want to open this wall if I dont have to nor take the OSB down and so I would place a new 2/4 wall against it to run plumbing then dry wall. Can the OSB remain or do I have an issue with moisture from the bath because of the OSB permeability ? If I use plain drywall in the bath would the moisture from the bath ultimately dry to the interior of the bath?

    Thanks for your help and patience

  6. Jon_R | | #7

    So you will end up with an interior wall between two conditioned spaces that has OSB in the middle of it? Properly exhaust the bath moisture and there is only brief vapor drive and condensation risk. Not a problem.

    1. BarnyardBuilder | | #8

      Thanks Jon. Makes sense. Found similar response from my insulation guy.

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