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Community and Q&A

Walkout basement shower assembly

joge2468 | Posted in General Questions on

We’re in the process of completely redoing the walk out basement of a 1940’s house. The old slab has been removed and we’re going back with 4″ of gravel, 2″ of EPS, vapor barrier, and a heated 4″ slab. The location for the new shower has us a bit stumped…

At some point, the basement was added by digging out. So, we’ve got the original poured foundation wall (front elevation) to about 5′ below grade terminating at a 2″ slab creating a crawlspace floor. The crawlspace then steps down, with a CMU retaining wall, to about 8.5′ below grade.

The side walls step down four times to the daylight back of the house. These sidewalls become half brick, half framing where the crawlspace steps down. It is in one of these corners that we intend to build a shower. So, the shower will have 39″ of masonry topped by 5′ framed wall. The question is, how do we build this shower?

Proposed: 3/4″ Kerdi board adhered directly to masonry to serve as both insulation and tile backer. Rockwool in stud bays above grade fronted by kerdi. Will this work? Is kerdi a vapor barrier that will trap moisture in the rockwool? If moisture travels through the masonry walls, will kerdi protect the tile/grout? Thank you in advance for your help.

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  1. Expert Member
    NICK KEENAN | | #1

    The exterior basement wall behind the shower should be built like an exterior basement wall: waterproof, airtight, vapor tight and insulated. Then the wall of the shower itself should be waterproof.

    There is some disagreement over whether the walls of a basement should be vapor permeable. I believe they shouldn't be and that only impervious insulation should be used. If the rest of your basement relies on permeability for vapor transmission then the area behind the shower needs to be treated differently and assume no transmission.

  2. CMObuilds | | #2

    I wouldn't integrate the shower into the basement insulation strategy. Insulate the basement, then build the shower. I don't know what climate you are in, but if I used the Kerdi as insulation and my tile backer I think my shower would be quite cold.

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