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Can Expanding Spray Foam Be Used Under a Bathtub?

When building or renovating a bathroom, you may wonder how you can insulate under your bathtub or bring stability to your tub and keep it from flexing and moving.

An easy way to solve these problems would be to use expanding spray foam. You can definitely use it under a bathtub, and it’s relatively inexpensive and easy to use.

But you must make sure to use the correct spray foam for your problem and to use it properly.

Using expanding spray foam improperly or using the wrong one for your application can cause you many problems down the road.

expanding foam

Spray Foam Use Cases

Insulation Under Bathtub

Expanding foam is known as a good insulator, and many contractors use it. But installing the spray foam can sometimes be a complex job.

This will depend on your bathroom and, more importantly, how you gain access underneath your tub. Most of us will have a crawlspace to gain access underneath our tub.

But for those who don’t, cutting a hole in the drywall can give you access, but you must cut a big enough hole to insulate the entire tub. Repairing all that drywall, though, will likely be more costly and time-consuming than your other option, pulling the tub.

Odds are you will have some type of crawlspace. Simply apply the spray foam to the tubs underside and all the seams where the tub meets your floor and wall.

Just make sure when applying the expanding foam, you leave space for the foam to expand.


Always avoid plumbing and electrical conduits when applying your foam, as you may need to access them in the future.

Bathtub Support to Prevent Flexing

Even though there are other options to support the bathtub to fix flex, expanding spray foam is still a contender.

When using spray foam to fix flex, you hopefully have a crawlspace. If not, there are other ways to get access underneath your tub, as discussed in the insulation application section.

Fixing flex does not require as much access as insulation. You can even drill a hole where you have flex and apply the foam that way, but I always prefer seeing the space in which I am applying the spray foam.

  • Begin by filling up your bathtub with water to weigh it down.
  • Then fill the empty space underneath your tub with spray foam.

You must be very careful not to overdo it when applying your foam.

It helps to know how much the foam will expand before using it.

If you spray too much foam under your tub, you can cause it to rise off of your floor. If this were to happen, you would have to remove all the foam and start the process over again.

Miscellaneous Bathroom Uses

While you are in your bathroom using spray foam on your tub, you can do a few other things to get the most benefit out of the foam you purchased.

You can apply expanding foam to the seams underneath your sink to stop or prevent leaking water.

If your showerhead is loose or wobbling, applying a little foam around where your showerhead comes out of the wall can shore up that problem.

If you are remodeling or building your bathroom and have access to the interior walls, then spraying them with foam can add a sound dampening effect to the room.

Choosing The Right Expanding Spray Foam

Choosing the proper foam for your specific needs means everything.

There are many types of spray foam to choose from, such as high expansion, high resilience, dry fast, closed-cell, and polyurethane, just to name a few.

If you’re unfamiliar with foam, it is pretty easy to choose the wrong one.

For Bathtub Insulation

When choosing spray foam for insulation, you need to look at its R-value.

The R-value indicates how well it resists heat and how well the foam insulates. The higher the R-value, the better insulation you will have.

On the high end, you can buy a foam with an R-value of R-7 per inch. That is the best insulation you can get from expanding foam.

If you’re not dealing with extreme weather conditions, then foam with an R-value around six should do the trick.

Besides R-value, make sure you are buying a closed-cell foam that can handle moisture.

For Fixing Bathtub Flex

When looking for a spray foam product to fix your bathtub flex issues, you need a high expansion foam.

Unless there is very little space underneath your tub, an inch or two or less, let’s say, then a low expansion foam would be a better choice.

High expansion foam can grow up to 200 to 300 times larger than its liquid state. So keep that in mind and do not use it more than necessary.

Even though closed cell foams are best for insulation, they prevent moisture, making them handy for any bathtub application.

Both low and high expansion foams are available as closed-cell products.

Expanding Spray Foam Alternatives

Alternative Insulation

When it comes to insulating your bathtub, expanding foam is your best choice, but there are other options out there.

You can use fiberglass insulation which can usually be found inside your walls.

Fiberglass has an R-value of up to 4.3 per inch, which is not as good as foam’s highest R-value of 7. And another problem with fiberglass is installing it under your tub.

It could be challenging to bring fiberglass rolls or batts into your tight tiny crawlspace and then try to install them where needed.

If you’re in a desperate situation, you can even use standard mail packaging bubble wrap as insulation. Standard clear bubble wrap has an R-value of about two. Just make sure you steer clear of the foil-faced variety.

Alternative Flex Fixes

Expanding spray foam does a good job at fixing flex issues, but there is a better option out there.

The mortar, a mix of sand, lime, and cement, is the best option to fix the bathtub flex. Every bathtub manufacturer recommends mortar.

The issue is that some contractors do not apply mortar when the bathroom is built. That lack of mortar underneath your tub is what leads to flex issues in the first place.

Suppose your building or renovating your bathroom, then applying mortar will be your best option.

If you are dealing with flex after a bathroom has been fully built, then expanding foam will be much easier to apply.

In some cases, a tub or shower is not meant to rest on mortar but on ribs with plastic shims. If these ribs and shims give out, the shower or tub will begin to flex.

Replacing the ribs and shims is an option when building or renovating but as with mortar expanding foam will be easier to accomplish if your bathroom is fully built.


Expanding spray foam is a fantastic product that has multiple uses for your bathroom and beyond.

I would recommend a closed-cell high-density foam as an excellent all-purpose product to keep around your home for leaks, flexes, and other emergencies.

As long as you know how much the spray foam expands and are using the correct foam for your specific application, you can’t go wrong.

James B. Parker

At a young age, James' father taught him carpentry, and he was pretty much destined for a career in the construction industry. He worked as a professional remodeling contractor for some of the most prominent construction companies for several years, before deciding to pursue his passion for writing.

A vast amount of construction experience allows James to provide practical advice for those considering a bathroom remodel.

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