When a bathtub flexes under weight or is noticeably cold, spraying expanding foam underneath is a commonly recommended solution in the DIY community. Let’s see if this approach is viable.
Can expanding foam be used under a bathtub? Yes, expanding spray foam can be used under a bathtub. It can reduce coldness and flex in an existing install, but an improper application can result in additional problems.
It’s important to understand that the tried-and-true best practice for preventing flex in a newly installed bathtub is with a mortar slab. But if you are dealing with an existing install and removing the tub isn’t practical, spray foam actually offers appealing benefits.
Many people don’t realize this but a bathtub bottom does not usually rest squarely on the floor. In fact, many tub bottoms actually do not touch the floor at all without bracing or support of some kind being installed underneath them.
The reason for this is that the bottom of bathtubs is not flat and level. They are designed with a slight incline to facilitate the movement of water toward the drain.
But this design feature is also a bit of a design flaw. Because, without proper bracing or support, the bottom of the tub is essentially suspended above the floor.
And so, you end up with this void under the bathtub. This is the reason people often have issues with flexing of the bathtub bottom.
How Does Spray Foam Compare to Other Stabilizing Products?
To fill the gap under a tub, there are several potential approaches. However, they do not offer the same benefits. Some, quite frankly, are promoted as solutions without any sufficient evidence.
These are the common products used for filling a void under a bathtub:
Stabilization Product
Pros
Cons
Joint Compound
None
Won’t hold up to pressure under tub
Sand
None
Does not rebound, deflects
Thin Set Mortar Mix
Readily availableInexpensive
Hard to break if you need to move or reset it
Plaster of Paris
Readily availableInexpensive
Easy to manipulate
Mixed Grout
Expensive
Little to no shrink
Spray Foam
Readily AvailableInexpensive
Easy to manipulate
As you can see, Plaster of Paris is a good alternative to spray foam for stabilizing and firming the bathtub upon installation. However, it may not be the easiest product to use when working with an existing tub. Accessing the space would prove messy and confining compared to spraying expanding foam into the gaps.
How Does Spray Foam Compare to Other Insulation Techniques?
Besides providing stabilization, you may be looking for ways to insulate your existing bathtub to reduce heat loss. Often, the tub was not properly insulated upon installation, especially on the exterior wall, allowing the cold air outside the home to decrease the temperature of the tub (source).
Spray foam is one alternative to mitigate this problem. Let’s look at some other options before discussing spray foam in more detail.
Fiberglass batting insulation
Rigid foam board sheet insulation
Blown-in loose fill
All of these options can provide insulation under and around a bathtub. Here are some considerations:
Fiberglass batting would be the easiest to install yourself, but it does potentially create moisture and mold issues.
Rigid foam board provides great moisture resistance, but it does not form into the awkward shapes under and around a bathtub.
Blown-in loose fill is effective for shaping into the gaps, but it is usually only installed by professionals.
As you can see, the alternatives to spray foam for insulating your bathtub have challenges that may not make them the best solutions.
So spray foam does provide an easy do-it-yourself solution for both the insulation and support issues. It’s not going to provide the level of support that a mortar slab will give and it’s no replacement for proper insulation practices.
That being said, if you are trying to improve the stability and insulation of an existing install, it is an option.
Related Reading:
How Does Spray Foam Work and What Are Its Benefits?
Spray foam is a polyurethane foam that comes in either a closed-cell or open-cell foam, which determines its density. They set differently.
Closed-cell foam forms bubbles with solid walls that trap gases inside them.
Open-cell foam forms bubbles with broken walls that allow air to fill them.
This also affects its moisture resistance and air flow capabilities. Let’s compare:
Closed-Cell Spray Foam
Open-Cell Spray Foam
Higher R-value/Insulation effectiveness
Lower R-value/Insulation effectiveness
Lower moisture permeability
Higher moisture permeability
Good air barrier
Lower air barrier unless thick
Higher strength and rigidity
Lower strength and rigidity
Resists water
Not suitable for direct contact with water
As you can see, closed cell spray foam is better for a bathroom application, as it provides better moisture protection, insulation, and strength.
Polyurethane foam is created by a chemical reaction between 2 liquids. However, most common aerosol spray can forms, like Great Stuff, have already been partially reacted and complete the process upon spraying (source).
Great Stuff has several closed-cell products available for different applications. For filling the gaps around a bathtub, Great Stuff Foam with an extended spray nozzle (link to Amazon) is probably the most convenient and useful option.
This will allow you to get further underneath the tub. I would not recommend a highly expanding foam for this but rather a low expanding option. This will help to prevent damage to the tub.
Concerns for Expanding Foam Under-Bathtub Installation
When applying polyurethane spray foam under a bathtub, there are a couple of potential issues to keep in mind.
First is maintaining access to plumbing and controls, should you need to make repairs to them in the future. Spray foam can be removed, but it is easier to leave space to work around these items.I would, however, make sure that any piping leading into the wall or floor is foamed to prevent air leaks. The connections and joints are the areas you want to make sure you have access.
The other concern is providing room for the expansion of the product. You could potentially lift the tub off its base or off the ground by over-spraying, and end up with a problem as this guy did.
OGT 40 - How not to install an acrylic bath with spray foam
This would of course create extra work removing the foam, realigning the tub, and starting over.
You can mitigate this problem by filling the tub with water before spraying the polyurethane foam. The weight of the water would help to keep the tub in place while the foam reacts and fills the spaces around it.
It’s important to understand that the foam is going to push somewhere when it expands. There would need to be plenty of open space around the sides of the tub where the foam can expand to.
Expanding foam can exert tremendous force when swelling in tight spaces. I’ve seen large gap foam warp window and door frames when used inappropriately. It’s even used to raise concrete sidewalks and foundations (source).
The last thing you want is to make a bad situation worse.
Conclusion
Expanding foam can be used in under-bathtub applications. It is not a replacement for a mortar slab in new installs but is a practical solution for improving stability and insulation of an existing tub.
In fact, when working with an existing tub, spray foam may provide the only option for preventing the fiberglass from flexing under weight by filling the spaces underneath.
Likewise, spray foam is a viable and easy option for increasing the warmth in and around an uninsulated bathtub.
Just be sure to fill the tub before applying the product if you don’t want to create more work redoing an over-insulated, floating tub.And as always, if you are not comfortable and familiar with what you doing, seek the advice of a pro.
Recommended Reading:
Can I Install a Bathtub Myself? Considerations And Steps
Does a Bathtub Need a P-Trap?
Can You Install a Bathtub Over Tile? Read This Before You Do
Fiberglass insulation is the least expensive, but any of these types of insulation that are unfaced are suitable for this type of application. Not only is it inexpensive, but blanket insulation is also easy to cut into pieces that will conform to the contours of the tub—an easy project for the DIYer.
Fiberglass insulation is the least expensive, but any of these types of insulation that are unfaced are suitable for this type of application. Not only is it inexpensive, but blanket insulation is also easy to cut into pieces that will conform to the contours of the tub—an easy project for the DIYer.
Avoid using the foam near any ceiling lights or heating appliances. If you spray the foam near a ceiling light, for example, there is a chance that it could catch fire due to the flammable nature of the spray. Additionally, it could also cause damage to the light itself if it cures around any wiring.
Homes with open cell spray foam don't have to replace the foam when it gets wet, or even after it has been submerged. This might come as a surprise because there is a common misconception that all open cell spray foams absorb water like a sponge and retain it, but that's not the case.
The most common way to reinforce a bathroom floor is to double the joists. This means that where you had one joist before, you would now have two. You need to think about plumbing at the same time, because it is likely that they will need to occupy some of the same space.
Whether the walls around your tub are tile, laminate, paneling, or wallpapered drywall, there should be a waterproof seam between the top edge of the tub and the wall. This nice finish is usually accomplished by applying a line of waterproof caulk at the juncture between the tub and wall.
Expanding foam is lightweight – The real reason your concrete slab is sinking is because the soil underneath isn't strong enough to hold its weight. Expanding foam only weighs about 2.5 to 4 pounds per cubic foot. That means that the foam will support your concrete slab and avoid adding extra weight to the soil.
Though it hasn't always been common practice, insulating bathtubs is now a popular way to save energy and money. The water in your bath will lose a great deal of its heat through the sides and base of your bath, particularly if the bath is set against one of the house's outer walls.
Even if there is no huge water leak such as from a hole in the roof, the steam and moisture of showers, bathtubs, and boiling pots can potentially affect the structure and safety of a house or building. This is why vapor barriers are needed in areas that are more moisture-prone.
However, expanding foam would effectively 'double insulate' the wires, preventing the heat from escaping. This could result in the wires getting too hot and melting. That in turn could cause a power failure, or even worse, a fire.
As a product, spray foam insulation does not attract mold. And unlike wood or metal, spray foam does not rot, rust or deteriorate. This means that an area properly sprayed with foam insulation will never become a food source or a hospitable environment for a mold colony.
GREAT STUFF™ Outdoor Insulating Foam Sealant is the easy-to-use solution that brings long-lasting, airtight and water-resistant seals to the home beyond the household.
Since you are joining two different materials together, you should use caulk in the space where the floor tile and tub meet. Grout is not flexible, so it's not the best material for the job, while caulk allows for movement and also provides a better sealant.
Flooring repairs can be intimidating, but the subfloor under the tub generally consists of plywood secured over floor joists and is fairly simple to replace – once you get access to it. The entire tub unit requires removal before subfloor replacement can begin.
So you are installing a new bathroom and suddenly have hit a snag-which should you install first, the flooring or bathtub? The short answer is that standard bathtubs such as alcove and corner bathtubs should always be installed first, followed by the wallboard, the subfloor and the vinyl.
Concrete provides a strong support for these products. Expanded foam has nowhere near the density or rigidity. It is plenty strong enough to hold the post up under normal circ*mstances, but has little structural strength.
Low-expansion foam expands up to 30 times its liquid size, while high-expansion foam can expand as much as 300 times its liquid size, quickly putting pressure on a window frame to the point where it's difficult to open and close the window.
Like the space under the kitchen cabinets, this is not a large area to insulate, but it is quick, easy and cheap to do. It also has the added benefit that your bath water will stay warmer for longer, so you can soak away those aches for longer after a hard day of insulating the rest of the house!
Fill gaps up to 1/4 inch in width with caulk. If you have tile on the wall adjoining the tub, select a caulk that matches the color of the grout. If your gap is smaller than 1/8 inch in width, use regular caulk; fill gaps larger than 1/8 inch with a sanded caulk.
Some tub materials retain heat better than others. The best bathtub materials for warmer baths are cast iron, copper, solid surface, stone, and acrylic. Some models and manufacturers, such as the Aquatic HotSoak series, feature in-line heating systems that keep the water consistently warm for your entire bath time.
You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good. This circ*mstance may prevent water vapour from drying, which in turn can cause rot and mold.
Vapor barriers can often double as added insulation, protecting buildings against excess cold and heat. Failure to use a vapor barrier can lead to higher energy costs and an ideal environment for mold and mildew growth.
What is barrier-free bathing? Originally, barrier-free baths and showers were made as a more accessible bathing option. Barrier-free bathtubs include a door to get into them instead of having to step over the ledge and showers are virtually flat to the ground so they are easy to step into.
The gas is emitted from the foam as it expands and dries out. It is heavier than air, sinks and forms an invisible cloud. If an ignition source is present, the gas can catch fire, potentially with explosive force.
Yes, expanding foam can be used as insulation. Expanding foam can be defined as a two-component mixture composed of two materials that react when mixed. This causes the foam to expand and harden, providing an insulating material.
But in many spray foamed attics, it can be hard to manage humidity. During the spring, summer, and fall, relative humidity (RH) in a spray-foamed attic often exceeds the comfort threshold of 60%. When it exceeds that threshold by too much – and for too long – you could experience problems, like mold growth.
Spray foam insulation can cut your energy bill by up to 50 percent thanks to its energy efficiency and temperature regulation. This will increase the value of your home while appealing to future home buyers.
Foam will be tack free in about 20 minutes. It can be cut, sanded, plastered, or painted after twelve hours and will be fully cured in 12 -24 hours. If exposed to sunlight, the foam should be painted or coated.
Spray foam (expanding foam in the UK) is a chemical product created by two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place.
Over time, the catalyst become less active until it eventually loses its ability to catalyze the reaction. As a result, you can get a poor application including foam shrinkage. As the catalyst dissipates, the density of the foam increases which means there is a decrease in yield.
Both regular thinset mortar and floor leveling compound are suitable. A series of strategically placed piles is easier to control than a full bed, and it provides comparable support for the tub.
Cement backer board is used as a sub-surface for tiling, usually for showers or bathtubs. Backer boars are generally placed against plywood or OSB board. This treatment is unnecessary if there is already cement backing.
Flooring repairs can be intimidating, but the subfloor under the tub generally consists of plywood secured over floor joists and is fairly simple to replace – once you get access to it. The entire tub unit requires removal before subfloor replacement can begin.
The short answer is that standard bathtubs such as alcove and corner bathtubs should always be installed first, followed by the wallboard, the subfloor and the vinyl.
Introduction: My name is Mr. See Jast, I am a open, jolly, gorgeous, courageous, inexpensive, friendly, homely person who loves writing and wants to share my knowledge and understanding with you.
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