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DIY Guide On How To Use Expanding Foam Under Bathtub

Creaking tub turning your peaceful soak into an annoying nightmare? It indicates that your tub was not positioned properly. But like any ambitious DIYer, you can fix this yourself by using expanding foam.

How to use expanding foam under the bathtub? 

Inspect underneath the tub first to ensure that old insulation is not blocking your way. Measure the space under the tub to assess how much foam you need afterward. Place ridge foam below the tub now. Fill ⅔ of the tub with water to exert pressure afterward. Find access points and spray in the foam in the end. 

This is just a quick summary. We have explained this in great detail for your better understanding. 

What are you waiting for? Let’s get right to it then!

Why Does Your Bathtub Need Expanding Foam?

Expanding foam is usually used as sealants. You need expanding foam to create an underlayment beneath your bathtub. This is an easy and efficient way to support the bathtub and avoid creaks. 

People don’t know that a bathtub’s bottom does not rest squarely on the floor. Rather, they have a slight incline to help water flow toward the drain. 

To level the bathtub with the floor, you need to create some kind of support. This is where expanding foam comes in.

Both Vikrell and Americast make lightweight and affordable bathtubs. But the downside is they wear over time and cause a variety of issues. For instance, you’ll end up with a creaking bathtub.

Squeaky bottoms are typically caused by insufficient support beneath the bathtub. Using spray foam you can easily close the space between the tub and the floor.

We have listed some of the best low-expanding foams for your convenience-

Product 1
Product 2

These spray foams are extremely easy to use.

Let’s check out why expanding foam is the best option for you!

Why Choose Expanding Foam Over Other Stabilizers?

Spray foam isn’t the only option to create adequate support. But it is more affordable than installing a cement board under the tub. Plus it’s simple and easier.  

Let’s check out an overview of other products that are used in the same field- 

Mortar MixCheap and available Impossible to reset
GroutExpensiveLittle to no shrink
SandReadily availableWeak and insufficient support
Plaster of ParisEasy to use & cheapEasy to manipulate
Expanding FoamCheap and easily availableEasy to manipulate

Ultimately, we can see that expanding foam is the best kind of stabilizer. Other stabilizers are messy and hard to manage in the long run. 

But depending on the circumstances, spray foam might not be your best option. If you have cracks on the bathtub, using spray foam can cause further damage. 

If you do decide to use it, make sure to use a low-expanding foam.

Step-by-Step Process of Using Expanding Foam

We’ve divided the entire process into six basic steps. The entire process might take an hour or two. These are simple and easy to follow-

Step 1: Check the Underlayment

You should start by inspecting the area underneath the tub. Check to see if your rough-in space has gaps or not. It might also be a cause of the squeaky noise. 

Examine the presence of a mortar bed. Sometimes, old insulation can be molded to the tub’s bottom. Check if any previously molded foam is still attached to the tub. If it is partially separated, remove it completely.

Step 2: Measuring the Underneath

You now need to measure out how much space you have under the tub. The gap between the subfloor and the tub bottom usually ranges between ½  to 5 inches.

If the gap is closer to 5 inches, you’d have to use extra ridge insulation foam. 

Step 3: Place the Ridge Foams

In this step, you’ll have to place some ridge foam underneath the tubs. This will keep the spray foam sturdy. An additional benefit is that you won’t have to use an excessive amount of spray.

You can place ridge foam depending on the gap between the tub-bottom and the sub-floor. If the space is near to 4 inches or more, you’d have to use ridge forms.

If you don’t have ridge foams,  you can put a drop cloth or plastic film. Using these will make it easier for you to remodel in the future.

Step 4: Fill the Tub

Spray foams can expand up to 24 hours after you apply them. This is why it’s better to fill the tub to add some weight. But don’t fill it up the whole way. ⅓ of the tub should be kept free.

Sometimes when you apply a lot of foam, it expands and presses the subfloor down. After that, if you remove the water, the tub will spring up and might crack. As a result, you’ll have to be careful with the next step.

Step 5: Find An Access

You should be aware of the type of bathtub you’ve installed. If your tub has an acrylic shower surround, the access point will be different than normal. In that case, the procedure is different. 

With the shower surround, you’d have to make a fixable hole in the tub. Insert the nozzle and get ready to spray after that. But normally, you access through the void under the tub and spray directly. 

Step 6: Apply Expanding Foam

We’ve already mentioned the dangers of using too much spray foam. Don’t worry! We have a remedy for that. It’s quite simple.

You just have to spray in stages. The corners tend to harden quicker than the center. If you fill the whole area up at one go, it will lead to bulging. As a result, you’ll get cracks in your tub.

To avoid this, start spraying at the center and let it harden. Do the outer edges and wait again. 

Fill the rest in such a way that it would travel horizontally while it expands. You’ll have to wait for at least 4 to 5 hours before you can use the tub. 

This was the whole procedure. After several hours, you can take out the extra foams that are accumulated around the base!


Question: What is the lifespan of spray foam?

Answer: With proper maintenance, expanding foam will last up to 80 years. The application of spray foam under the bathtubs could last a lifetime. But because of moisture retention, roofing application only lasts over 50 years. 

Question: How can I stabilize my freestanding bathtub?

Answer: You’ll have to apply silicone sealant at the bottom of the tub to avoid creaking. This kind of bathtub stands on its own. That’s why it doesn’t require much hassle. 

Question: What is normally used under a bathtub for support?

Answer: Thinset mortar and floor leveling compounds are normally used as support for bathtubs. You can also use wedge-shaped wooden shims. It helps to fill out the gaps between surfaces.

Final Words

That’s all from us expanding foam under the bathtub. Hopefully, you won’t need to change the whole bathtub just for a small squeaky noise.

This procedure is simple and effective. But if you notice any cracks, consult an expert.

Good luck fixing your bathtub!