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Appropriate treatment of shower/tub walls adjacent to dense-packed cellulose?

sfriedberg | Posted in General Questions on

Climate zone 4C marine, planning for new construction.

Planned typical R-26 exterior wall section: fiber cement siding, 1X rainscreen furring, 3″ 8pcf rock wool, taped sheathing (primary air control and WRB), 2X4 load-bearing wall with 3.5pcf cellulose, 1/2″ gypsum board.

How should interior high-humidity areas like showers adjacent to this exterior wall section be handled?

Current plan for bathroom walls adjacent to the exterior is to net the 2X4 load bearing wall for cellulose, leave off the 1/2″ gypsum board, and frame additional 2X plumbing walls. Specifically adjacent to the shower/tub, frame a 2X4 plumbing wall (no waste lines in this specific area, so no need for 2×6), 1/2″ cement backer board, an applied pan membrane, and tile. However, we were not planning to tile or extend the membrane all the way to the ceiling, much less across the ceiling. The wisdom of leaving the upper part of the walls (and the ceiling) permeable to water vapor, especially in proximity to cellulose, has been questioned.

The shower areas are not saunas or full-on steam rooms (for which full-wall plus ceiling membrane and tile are often recommended), but I don’t have enough experience to judge risks from vapor drive in this situation. Assume long hot energy- and water-wasting showers. While the exterior wall section is theoretically capable of eventually drying to the outside, that’s not something I want to depend on given the local 6-month soggy season.

The shower/tub area is open to the rest of the bathroom, which will be exhausted via HRV.

Among the options are:
1) Don’t worry; the cellulose can handle the vapor passing through the non-tiled portions of the walls.
2) Extend the membrane all the way to the ceiling.
3) Use rock wool batts rather than dense pack cellulose in these areas. (Although that obviously doesn’t affect the quantity of moisture infiltration at all.)

I am assuming the cellulose-retaining net is no vapor retarder at all (not even “smart”).

Suggestions? Recommendations?

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  1. Dana1 | | #1

    With a rainscreen the sheathing still dries toward the exterior, even if it's pretty slow during the months when the outdoor temps are 40F and the dew points hang in the high 30s.

    Something like 2-mil polyethylene in the steamy shower rooms would work. At a vapor permeance of about 0.15 it's a very tight class-II vapor retarder, but still not a true vapor barrier.

    Cellulose would be more protective of the sheathing and structural wood than rock wool.

  2. sfriedberg | | #2

    Place the polyethylene between the gypsum/cement board and the plumbing wall studs? And should it extend the full height of the wall (including the area with membrane on the interior of the cement board)?

  3. GBA Editor
    Martin Holladay | | #3

    I vote for a vapor barrier all the way to the ceiling in the tub/shower area. It should go between the cellulose netting and the plumbing wall studs. Any kind of polyethylene or vapor-impermeable membrane will work.

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