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Newly Installed Acrylic Tub Creaking

bb399 | Posted in General Questions on

My newly installed acrylic tub makes creaking noises stepping around and into and out of the tub. I did a Google search and found that this is caused when an acrylic tub is not set in mortar. This was confirmed when the contractor came over, cut the wall behind the shower/tub, and showed me the bottom of the tub. It was installed on four feet, and I believe the plumbers/contractor used adhesive glue to secure. There in no mortar securing the tub.

My question is how best to solve this problem. I’ve seen multiple videos with contactors using foam to stop the flexing/sound. Is this the best course forward? If so, what would be the best kind of foam to use? It looked like around ~3-4 inches from the subfloor to the bottom of the tub. I’ve read some articles that state to use ridge foam and then closed cell spray foam above the ridge foam. I’ve also seen videos that just used the spray foam. 

My contractor will fix, but he hasn’t heard of the spray foam solution. This makes me a bit nervous (it seems like a somewhat common fix), so I’m looking for advice on the best way forward with advice on materials to use.

I’m tempted to have him take out the tub and set a new tub in mortar properly. I completely renovated the bathroom, and I’m worried the tub will fail in a few years.

Thanks in advance.

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  1. bigred | | #1

    Pull the acrylic tub and install a cast iron one. Lowes sells a Kohler cast iron alcove tub for $400. In the long term, a cast iron will last forever, the acrylic will start to show scratches and look dingy in just a few years.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Ours is going on 25 years. I use gel-gloss on it every six months or so. Maybe I'm just not very picky?

      1. bigred | | #6

        Did you have children grow up in the same bathroom? When my son left for college I redid his bathroom and replaced a very dingy acrylic tub (that did flex quite badly) with a Kohler cast iron (about 18 years ago now). Tub is solid alike a rock, and looks as good as new almost 20 years later. Back then only paid $200. Not sure what the cost of an acrylic, but for $400 and a truly lifelong tub, just seems to make the most sense. Now all that said, I DIDN"T install it myself, they are really heavy.

  2. pjpfeiff | | #3

    I believe when mine was installed the plumbers just packed mortar (or actually drywall compound, I think) under it after after was in place rather than lowering it into a bed of mortar. It seems to have worked well.

  3. capecodhaus | | #4

    The cast iron tub is a good option at that price and I like using Kohler products in the bathroom.

    I would have installed that same cast iron tub in my own home when I remodeled recently but its too heavy for myself to handle alone and help is hard to find living deep in the woods. I do my own plumbing, carpentry and drywall/tile etc..

    I used a full bag of mortar under my Sterling acrylic tub made by the Kohler Company. A full bag wet will fill a 5 gallon bucket basically, without that, the tub will be cracked the first time anyone stands in it.

    If you go with an acrylic/fiberglass tub check carefully for defects in the finish before installing. I had to open up 3 different boxes at the big box store to find an undamaged one. If using Ferguson plumbing supply, your results will vary.

    I hand pick only, as deliveries will become damaged goods.

  4. Expert Member
    Michael Maines | | #5

    If you can reach, use canned spray foam. Fill the tub with water first. (From the "lessons learned the hard way" file...) Don't fill the entire space, but place it in blobs so it can expand laterally as well as vertically. Setting in mortar is a better solution but I don't think it's worth pulling or replacing the tub to do so.

  5. onslow | | #7


    Second on Michael's suggestion to fill the tub with water. The power of foam is more than you might think. I once created some window problems by bowing side jambs before the advent of low expansion foam. The blob placement will also help head off a problem with air trapped in the boxed ribs used to stiffen the underside. I would not recommend using the latex based foams despite easier clean up. I have found them to lack the level of crush resistance of urethane foams.

    If you do go with a new tub and set it in mortar, apply the same multiple mound approach or tall rows to avoid humping up the bottom and to ease the setting in. In the same category of learned lessons - don't mix it too stiff and make one big blob thinking it will squash out nicely. I had to dance around for several fretful minutes in one tub upon discovering how low my pounds per square inch really is. At least the mortar didn't all flow toward the drain cut out, which is why I mixed it tighter than usual.

  6. Expert Member
    Akos | | #8

    My tiler is the old school type of guy that does everything 100%, checks every detail and never takes shortcuts. He sets tubs in spray foam, so I would trust it to hold up.

    You want to use dense foam, so only use low expansion foam. Depending on how much space there is, you might need a hose extension to reach the area by the skirt.

  7. Patrick_OSullivan | | #9

    One tip (forget where I picked this up) if you install a tub in mortar (which overall is a great idea)... put a sheet of poly down on the subfloor and then put the mortar on top of that. If you ever have to remove it (e.g. changing a tub to a shower), you (or some poor soul in the future) won't spend time chipping mortar away from the subfloor, and likely damaging it in the process.

    1. bigred | | #10

      That is a brilliant idea.

    2. tommay | | #11

      Plus the fact that the lime eats away at the which case it's best to install some "cement" board first, if doing so....

    3. Expert Member
      NICK KEENAN | | #12

      A guy at a tile store told me they do that. They change up their displays every six months or so. By putting down sheeting when it's time for a new display they can take up the old one with no fuss.

  8. bb399 | | #13

    Thanks everyone for your replies.

    I wanted to give you an update on the progress in the hopes it may help others and to ask a few additional follow up questions/advice.

    My contractor removed the first row of tiles and realized the plumber hadn't screwed in the integral tub flange into the wall but instead but the screw 'on top of' the flange, which was rubbing against the tub. He fixed that issue. This was also a slight creaking near the front left side of the tub. He cut a thin piece of the wall away and the sound stopped. He is going to caulk it.

    The contractor's plan for the bottom of the tub is to put 2 2x4s underneath near the legs for additional support. He is nervous about the spray foam (even though multiple people have recommended it to him) because he used a bit of the non structural stuff during an earlier fix, and the tub was rubbing against it, making additional noise. I said that as long as he uses two-part foam and fills the whole cavity, it should work. However, he wants to try the 2X4s. We aren't going to secure it but leave it for a few days to check.

    What are your thoughts on this 2X 4 'solution'? I want to make sure that the 2X4s won't cause additional issues, like cracks down the road if the tub keeps 'hitting' the 2X4s when I step into it.

    I've attached a few photos (badly taken, sorry) to show the 2x4s.

    Also for some context, I live in a walk up condo in an old building, so I picked an acrylic tub because I thought it would be easier for my contractor to carry and install. I now wish I had just ordered a cast iron and had him deal with getting it up the stairs. Lesson learned.

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