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Community and Q&A

Insulating a steel drop-in bathtub

Daniel Herskowitz | Posted in Mechanicals on

We are installing a steel bathtub. Does it make sense to put insulation under it? What kind would you recommend? Thanks!

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  1. Kye Ford | | #1

    Holmes on Homes always did the spray foam method. Get a couple cans of maximum expanding foam and start spraying under the tub area then set the tub in the foam before it sets. Many tub manufactures recommend setting tub bases in concrete but that can make for a cold bath in the winter. I've been opting for the foam method lately and the only tub users in our house "kids" seem not to be complaining about the water getting too cold too quick as compared to previous houses with tub bases with concrete.

  2. Expert Member
    Malcolm Taylor | | #2

    If you do use foam make sure you fill the tub with water first. Even then the foam can lift the tub as it expands and then you have a real mess on your hands.

  3. Aaron Phillips | | #3

    I would also recommend using the spray foam on the underbelly of the tub. For two main reasons. 1 it helps keep the water warmer longer and 2 it helps reduce noise in the room below (if there is one below)
    A couple questions for your though. Any particular reason you are going with a steel tub and not cast iron? I ask for several reasons which a few can be found in the "new tub: cast iron or steel?" forum found on this site. Additionally, is the tub going over a framed floor or directly on concrete?

    One thing to note, regardless of install you want to make sure you don't alter the natural slope to the drain.

  4. Daniel Herskowitz | | #4

    Thanks for your answers!
    We are using a steel tub because do to space constraints it is an unusual size: 54". The only tub we are finding in this size that is affordable is a steel one.

    It's seems like the consensus is foam. We are trying to find a foam alternative so I was thinking of just stuffing it with mineral wool. Do you see any negatives with this approach?

  5. Expert Member
    Dana Dorsett | | #5

    I prefer blown cellulose for this application, on all sorts of grounds. With spray foam it's glued in place, even if it DOESN'T lift up and create other issued during installation. If the tub needs to be mechanically supported in the middle, rigid EPS works just fine, then fill the rest with blown cellulose.

  6. Aaron Phillips | | #6

    I don't see any negatives with mineral wool, unless you are placing it directly on concrete. If that's the case, water can be wicked up through the slab which will most likely cause mold down the line (this will depend on what your basement is like in general). If you are placing on a framed floor then I don't see any issues.

    Coincidentally I just read a short article in the fine homebuilding magazine (No. 241, pg 30) that says it's making a comeback. Mineral wool would obviously provide good insulation and good soundproofing. This would be my second option of choice in this application next to spray foam.

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