Can I Use Expanding Foam Under Bathtub? The Ultimate Guide

Installing or replacing a bathtub often requires leveling, sealing gaps, and providing support. Traditionally, this was accomplished using mortar, shims, and caulk. However, some homeowners and contractors now use expanding spray foam for some or all of these purposes.

When used properly, expanding foam can be an effective alternative to traditional bathtub installation methods. But improper use can lead to problems down the road. This comprehensive guide provides expert insight on using expanding foam under bathtubs.

Pros and Cons of Using Expanding Foam Under Bathtubs

Pros:

• Quick and easy application
• Expands to fill gaps, providing insulation and draft stopping
• Bonds strongly to most surfaces
• Remains lightweight despite expansion
• Can reduce cleanup compared to mortar methods
• Often less expensive than mortar bed or underlayment panels

Cons:

• Can be messy if overapplied
• Some fumes during application require ventilation
• Improper use may lead to hollow spots without support
• Post-expansion trimming required for clean finish
• May break down faster than mortar when exposed to moisture
• Not recommended as primary structural support

Types of Expanding Foam for Bathtub Installation

The two main types of expanding foam that can be used under bathtubs are:

1. Polyurethane Foam

Polyurethane foam comes in spray cans, allowing for crack and crevice filling. It cures quickly and bonds well to most surfaces. Polyurethane works well for sealing gaps and providing insulation. However, it does not provide the same level of robust structural support as mortar.

2. Two-Part Pour Foam

Two-part pour foam comes as two liquid components that must be combined during application, expanding 30-40 times in volume as it cures. It is often used for larger cavities that require more foam. Two-part foam provides structural stability, but still should not replace mortar for primary support.

Be sure to select a foam rated for exposure to moisture, such as closed-cell or high-density formulas. Avoid one-part and open-cell foams.

How to Prepare the Area

Before applying expanding foam:

• Clean and dry the area thoroughly
• Fill any large gaps or holes with fiberglass, mortar, or wood
• Cover nearby surfaces with painter‘s tape and plastic
• Ensure adequate ventilation
• Carefully follow all manufacturer’s instructions

Also check that tub drain holes are sealed to prevent foam entry.

Step-by-Step Application Instructions

For spray foam:

1. Shake can well and attach nozzle
2. Test in an inconspicuous area to gauge foam amount
3. Fill gaps and cavities about 1/3 full, allowing room for expansion
4. Apply in multiple layers 30 minutes apart, if needed
5. Smooth and shape foam with a damp tool before full cure
6. Let foam fully cure based on product instructions before trimming

For two-part pour foam:

1. Prepare foam per manufacturer instructions
2. Pour foam slowly into cavities to control amount
3. Allow foam to fully rise and cure before disturbing
4. Consider temporary supports until fully cured
5. Trim excess foam once hardened

Apply in multiple stages if applying large amounts of foam. Follow all safety and PPE recommendations.

Curing and Drying Time

Curing and drying times vary based on temperature, humidity, and type of foam used. Most foams become tack-free within 20-60 minutes. Full cure times range from 4-48 hours. Do not disturb, trim, or expose foam to water until fully cured.

Two-part pour foam may take longer to cure, sometimes up to 24 hours. Support tub temporarily until pour foam has fully risen and hardened. Never rely on foam that is not fully cured for structural support.

Safety Precautions When Using Expanding Foams

• Work in a well-ventilated area
• Wear gloves, goggles, and protective clothing
• Keep foam away from skin and eyes
• Have a fire extinguisher nearby in case of ignition
• Follow all manufacturer safety recommendations

Never leave expanding foam unattended since it can rapidly over-expand. Fully extinguish foam if ignited before it spreads. Consider hiring a professional if unsure about proper use.

Alternatives to Expanding Foam Under Bathtubs

While expanding foam can be useful, it is often best paired with traditional installation materials:

• Mortar bed – Provides primary structural support
• Cement board – Prevents damage to subfloor
• Shims – For leveling tub before final secure
• Underlayment panels – Designed for moisture resistance

Only use foam for sealing, insulating, and gap filling. For freestanding tubs, a mortar base is almost always recommended over foam alone. Consider foam supplemental rather than the sole support material.

Common Mistakes to Avoid

Improper use of expanding foam under tubs can cause both short and long term problems:

• Applying too much foam leading to expansion damage
• Not allowing proper cure time before finishing
• Using foam as primary support rather than shims/mortar
• Failing to trim excess foam resulting in uneven surfaces
• Inadequate ventilation during application
• Applying foam unevenly leaving unfilled gaps
• Using foam near ignition sources before fully cured

Take care to apply foam in recommended amounts. Follow all manufacturer instructions to avoid safety and performance issues.

Signs of Improper Installation

If you suspect the expanding foam under your bathtub was not installed correctly, watch for these signs:

• Creaking, shifting, or sagging tub
• Hollow spots or uneven tub surface
• Cracked or deteriorated foam residue
• Evidence of moisture damage or mold
• Lingering chemical odors from insufficient curing

You may need to remove tub, improve support with mortar or other materials, and reinstall properly. In some cases, complete foam removal and replacement may be required.

Long Term Durability and Maintenance

When properly installed, expanding foam can last for years under a bathtub with proper maintenance:

• Inspect foam once per year for moisture issues or breakdown
• Repair small cracks or gaps with additional foam
• Remove tub and replace deteriorated foam as needed
• Avoid exposing foam to standing water or humidity
• Reapply protective finishes to foam as needed

However, foam does not have the same lifespan as traditional mortar materials. Plan to recheck foam condition during any remodeling or bathtub refinishing.

Cost Comparison

Expanding foam – $3 to $20 per can or kit

Mortar bed materials – $15 to $50 for full tub install

Cement board – $.50 to $2.00 per square foot

Underlayment panels – $100 to $400 depending on tub size

Foam is generally cheaper upfront but may require more frequent replacement than mortar long term. Consider combo approaches for ideal results at reasonable cost.

Building Codes and Restrictions

Most building codes require tubs to be anchored and supported properly without specific materials specified. However, codes generally default to traditional mortar bed and underlayment methods as the expected standard.

Using expanding foam as the sole support under tubs is not recommended and may not meet building inspector approval in all areas. Check local regulations before assuming foam alone will suffice.

Conclusion

When used properly, expanding foam can provide excellent supplemental insulation and waterproofing under a bathtub. But take care to follow usage directions closely. Combining foam with traditional tub supports like mortar or underlayment panels provides the best results. With some planning and proper application, expanding foam can improve the install and lifespan of your bathtub.