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