Can you use expanding foam under a bathtub? Yes, this DIY solution is a reliable way to take care of specific issues. 

People might use expanding foam when they notice that their bathtub is too cold or has begun to flex underweight.

However, you should be aware that misusing foam can lead to further issues. You might even damage your tub or your bathroom floor. 

If you’re installing a new bathtub, the best way to fix it in place is by using mortar. Expanding foam works better for existing installs when you can’t remove the tub.

Benefits of Using Spray Foam Underneath Bathtubs

using spray foam under a bathtub for insulation and leveling

Spray foam is typically used to help with insulation issues and warping issues in bathtubs.

The bottom of your bathtub doesn’t rest perfectly on the floor. Instead, it is raised slightly off the base, which allows it to drain at a slight angle. However, this leads to empty space below the tub.

As time goes on, your weight can cause the bottom of the tub to buckle. As a result, it sinks down and becomes uneven. If there’s a lack of insulation, that can cause the tub to feel very cold even if it is filled with hot water.

Spray foam can be sprayed underneath your bathtub to provide bracing. This prevents the bathtub floor from flexing while also providing essential insulation.

Open or Closed Cell

There are two types of spray foam: open and closed cell. Each type sets differently, so the density is different between the two.

With closed-cell designs, the foam creates solid-walled bubbles and causes gases to become trapped inside.

With open-cell designs, the walls of the bubbles are broken allowing air to go in and expand within them.

So, which is better to use in your bathroom?

Closed-cell designs typically fare better underneath bathtubs. Because the bathtub walls are solid, less moisture gets inside and the less likely mold can grow. Closed designs tend to be more rigid and strong, and they’re also water-resistant.

Open-cell designs, on the other hand, can be easily permeated by moisture in the air. They also aren’t designed to come in close contact with water because water can get trapped in the foam. Additionally, these designs are typically less rigid.

Low or High Expansion

Different foams have different expansion levels. Some expand significantly, while others only expand a little.

When it comes to your bathtub, it’s best to get mildly expanding foam. Avoid high-expansion models, as these are likely to damage your bathtub or walls.

Make sure you follow the manufacturer guidelines for how to use the foam of your choice. Filling gaps under your tub can be done using an extended spray nozzle and a low-expansion foam.

Expanding Foam vs. Other Stabilizers

Expanding foam is one of several stabilizers that you might consider. Some competing stabilizers, like sand and joint compounds, have absolutely no scientific backing. They don’t provide any support or stability.

For new tub installations, thin-set mortar is the most common stabilizer. This mixture levels your tub and prevents it from moving. However, keep in mind that mortar is extremely difficult to break if you need to reset or move it.

To lay down a mortar solution, you need to de-install the entire tub. For those who can’t pull up their bathroom tile and plumbing, it’s simply not practical. However, the foam expands into gaps in the tub without requiring you to de-install it.

Plaster of Paris is another option similar to spray foam in its maneuverability. However, packing it into small spaces under your tub might be a challenge for some DIY home renovations.

Mixed grout is sometimes used for stabilizing a tub, but this material is expensive and won’t expand to fill a space like expanding foam.

Expanding Foam vs Other Insulation Materials

Expanding foam will not replace the need for proper insulation, but it can help with a cold bathtub bottom.

Fiberglass insulation is easy to DIY but functions poorly in bathrooms due to mold and moisture problems.

Rigid foam boards are similar to spray foam in texture, but you have to cut and shape them to the exact size of the gap.

The blown-in loose filling is a good insulation for filling the gaps in a tub, but it’s considered a professional solution rather than DIY.

What Are the Risks of Using Expanding Foam Under the Bathtub?

There are some risks to this endeavor, but most can be alleviated as long as you pay attention.

Make sure that you can still access the controls and plumbing during your installation. That way, you can easily make potential repairs in the future. While spray foam is much more malleable than mortar, it’s still inconvenient to remove it.

With that said, it’s a good idea to put foam around the pipes that lead into your walls and floor to prevent air from leaking. You want to make sure you can access the joints and connections of your piping.

Another issue is that expansion foam does exactly what it sounds like: expands. If you spray too much underneath your tub, you could lift it from the ground and destabilize the base. That’s a headache that might require you to reinstall the entire tub from scratch.

One solution is to fill your tub with water prior to spraying. That way, the water’s weight presses the tub down while you’re working. The foam will fill the space surrounding the tub instead of simply lifting it up.

Be sure that you have enough open area for your foam to expand into. If there isn’t enough room, the foam might warp your walls or doors.

Expanding Foam Can Be a DIY Bathtub Fix

If you need to stabilize your tub without removing it entirely from its fixture, expanding foam might be the best solution for you. It has marked advantages over expensive solutions like grout, and it can also be easily manipulated into new shapes as time goes on.

Expanding foam works best when your bathtub has warped or can’t retain heat. Keep in mind that it’s not the best solution for new installs. With a brand new tub, you’ll want to use mortar to set the base in place instead.