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Community and Q&A

Cast-iron bathtub install

Jamie_K | Posted in General Questions on

Hi, there.  We were thinking of doing a Kohler cast iron alcove tub.  If we are thinking about doing a tub/shower surround w Wedi board above tub (under tile), is there anything special we need to put in before tub?  Or just tub properly set tub on new slab on grade concrete floor up against 3 sides of wall (2 ft concrete stem wall exterior side and wood framed walls other 2 sides)?

I don’t think we want insulation anywhere anyways against 2 foot concrete stem wall do we – if anything insulation wise, rigid foam only but not fiberglass?  Don’t want to create a moisture trap.


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  1. GBA Editor
    Martin Holladay | | #1

    Installation instructions are on the web. The link can be found on the web page below (click "Installation and Care Guide"), or simply open the attachment.

    Kohler cast-iron alcove bathtub

  2. Jamie_K | | #2

    Thanks, Martin! And they talk about stud pocket - anything different for the exterior concrete stem wall one side of tub will be up against?

    I've read a lot of your articles on barriers and think we have good wall designed for wood framing above concrete stem wall; but hadn't thought of that 2 ft concrete stem wall around the perimeter in general. Do you recommend treating it like a basement wall article you wrote quite a while ago, where you would either do no interior insulation there or rigid foam only on the interior side of that 2 ft of concrete?

    Thanks a bunch! And my name finally attached to me on my profile :)

  3. Expert Member
    Dana Dorsett | | #3

    Foundation insulation is a requirement in the IRC in US climate zone 3 and higher, assuming it's a stem wall on a crawl space rather than slab-on grade. If you aren't in US climate zone 1 or 2, follow the IRC's prescriptive for your climate.

    For foundation walls (unlike fully above grade mass walls) the IRC makes no distinction between interior insulation vs exterior. If continuous insulation (no studs) by zone the minimums would be:

    climate zone----------Rvalue


    4A & 4B_____________R10

    5 or higher & 4C______R15

    If it's a slab on grade it's R10 slab edge insulation to a depth of 2 feet for zones 4 & 5, or to a depth of 4 feet for zones 6-8.

    1. Jamie_K | | #5

      Thanks, Dana! Both our house and shop will be slab on grade. We are starting with shop, where slab on grade with 2 foot stem walls were poured - pressure treated wood then wood framing above that. Maybe slab on grade edge insulation is not required for shops (non living quarter buildings)?

      For house though, typically up here (borderline 5/6 zone) they put slab edge insulation on inside not outside (how I've seen suggest): will inside slab edge insulation be best here?

      Since 2 feet of concrete shows above grade (or above our slab) indoors before the wood framing continues above, I just didn't know about insulation against that interior concrete or not. I thought I say an article in archives for basement walls that says no interior insulation against concrete basement walls or only rigid foam insulation if insulation used (and was thinking I should treat it like that)?
      (We were going to use paperless backed fiberglass insulation on wood framing above, but I'm thinking that would not be wise to put on that bottom 2 feet against the interior concrete.)

      I'm assuming paperless backed fiberglass insulation in wood framing behind Wedi or Schluter board in shower is okay?

      Thanks a bunch! I really like learning all this! :)

  4. user-2310254 | | #4

    If there will be other trades on-site prior to occupancy, be sure to protect the tub using a cardboard liner or similar product. The enamel finish is tough, but it can be damaged or stained if abused.

    1. Jamie_K | | #6

      Ah - thanks, Steve! Good idea - thank you, thank you! What do you mean by cardboard liner - what department or where would that usually be located in like a Home Depot say?

      1. GBA Editor
        Martin Holladay | | #7

        A big piece of cardboard. Pick up a free box at your local appliance store -- anywhere that sells refrigerators.

      2. user-2310254 | | #8


        Sorry for the slow response. Martin is right. You can use a piece of double-walled cardboard or scraps of rigid foam. There are also manufactured products such as these:

        Also be sure to drain the tub after the plumbers perform their leak test.

  5. user-36575 | | #9

    Another place to get large pieces of cardboard is your local bike shop. Almost every bike they receive is shipped in a large cardboard box, so they've usually got plenty of boxes.

  6. Jamie_K | | #10

    Thank you Martin, Steve & Andrew :)

  7. Expert Member
    Michael Maines | | #11

    I recommend using a fluid-applied protective coat. Tubs take a beating from drywall, tile, plumbing and electrical work. Cardboard helps, but grit still gets under it, and tradespeople always step either on the edge of the tub or inside the tub. Here's one product:

    1. user-2310254 | | #12

      Pro-tect looks like a great solution, and it is a lot cheaper than hiring a tub refinisher.

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