Mold in Bathtub: Caulk, Jetted Tubs and How to Remove It

Tubs hold water, soap scum and body oils, and they stay wet after every use. TDLR, the Texas agency that licenses mold professionals, says bathtub or showers corners and joints, including tile crevices, are more susceptible to mold growth. UF/IFAS Extension says mildew grows on soap scum and moisture.

Where mold in bathtubs usually shows up:

Table: where mold grows in a bathtub and what to do.

Black mold in a bath tub, usually along the caulk, is almost always ordinary mold or mildew. Color doesn't tell you the species, and you don't need to know it. The CDC says you do not need to know the type of mold, and the EPA says that in most cases, if visible mold growth is present, sampling is unnecessary.

The CDC notes that for some people, mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash. If you have symptoms, talk to a doctor; a mold test can't diagnose illness. See our black mold guide for more.

Acrylic, fiberglass, porcelain-enameled steel and cast iron tubs are all hard surfaces. That makes mold removal from the bathtub itself fairly simple.

1. Ventilate. Turn on the fan, open a window if you have one.
2. Wear protection. The EPA's minimum for small mold cleanups is gloves, goggles/eye protection and an N-95 respirator. If you use bleach, the EPA recommends gloves made from natural rubber, neoprene, nitrile, polyurethane or PVC.
3. Scrub with detergent and water. The EPA's default is to scrub mold off hard surfaces with detergent and water, and dry completely. Use a soft brush or non-scratch sponge on acrylic and fiberglass.
4. For stubborn spots, Bob Vila suggests a cleaning paste by combining equal parts baking soda and water, or two parts baking soda to one part hydrogen peroxide. If you use bleach, the CDC's limit is no more than 1 cup of household laundry bleach in 1 gallon of water.
5. Rinse and dry. Towel the tub, ledges and corners.

Never mix bleach with vinegar or ammonia. The Washington State Department of Health says when chlorine bleach is mixed with an acid, chlorine gas is given off, and lists vinegar among acid products. See vinegar vs bleach for mold, and our guides to hydrogen peroxide for mold and baking soda for mold.

Mold on bathtub caulk is the most common tub problem. Caulk is soft, it flexes, and water can get behind it. Surface spots sometimes clean off. Mold that sits inside or under the bead won't.

Try cleaning first. Bob Vila suggests scrubbing caulk with a baking soda paste and a toothbrush, and notes that if baking soda doesn't leave the caulk clean enough, it may be necessary to use bleach.

Replace it if the spots stay. The EPA says do not paint or caulk moldy surfaces. Bob Vila is blunt about why: moldy caulk doesn't just go away because you cover the mold with new caulk, and the mold continues to grow and spread inside the walls, as it thrives in dark, moist locations. Its rule: never caulk over damaged or moldy caulk.

How to replace moldy tub caulk

1. Soften the old caulk. Apply a caulk remover. Bob Vila notes that the caulk remover, rather than making the sealant magically disappear, simply softens it, and that you should allow at least four hours.
2. Cut and pull it out. With the caulk now soft, remove it either with a caulk remover tool or a simple utility knife. Be careful on acrylic and fiberglass tubs, which scratch.
3. Clean the joint. Scrub mold out of the gap with detergent and water, wearing gloves and goggles.
4. Let it dry completely. Bob Vila says caulk won't adhere to a wet surface. Run the fan for a day if you can.
5. Pick the right caulk. Check that the label says it's designated for use in kitchens and baths. Bob Vila notes some clear shower caulks contain special inhibitors that protect against mold and mildew.
6. Tape and apply. Painter's tape makes a great guide to ensure a clean bead. Pull the tape while the caulk is still wet.
7. Let it cure. Bob Vila says to allow the caulk to dry and cure for at least 24 hours before returning to regular use, or follow the label.

Plan on checking it yearly. Bob Vila notes a thorough caulking job should last at least a year.

Jetted tubs have a hidden problem: water stays in the plumbing. Bob Vila explains that when you drain the water from a jacuzzi tub, some water stays in the lines, and that over time, oil, bacteria, mold, and mildew can collect in the jets, which will release those unwanted particles back into the tub when it is filled with water. If you see black flakes come out when you run the jets, that's what's happening.

How to flush the jets (summarized from Bob Vila):

1. Fill the tub with hot water a few inches above the jets.
2. Add a cleaner. Bob Vila's first option is to add about 2 cups of white vinegar into the water. Its alternative is 1/2 cup of bleach along with a few teaspoons of powdered or liquid dishwashing detergent. Use one, never both.
3. Run the jets on high for 10 or 15 minutes, or until debris from the internal plumbing stops washing into the water in the tub, then drain.
4. Scrub the tub and jet openings with baking soda and a toothbrush.
5. Refill with plain warm water and run the jets again to flush out any mold or bacteria that remains.

Check the manual before using bleach. Bob Vila notes that some manufacturers do not recommend bleach, which can dry out the gaskets over time. It also says standard bubble bath products should not be used in a jetted tub.

How often: if you use the tub often, Bob Vila says to aim to clean it about once a month.

• Dry the tub after use. TDLR says to use a towel or squeegee to dry off wet surfaces after bathing, paying attention to corners and joints.
• Clean weekly. UF/IFAS says to clean showers/tub regularly so mildew cannot grow on soap scum and moisture.
• Hang bath mats and toys to dry. Anything that traps water against the tub will grow mold underneath.
• Run the fan. The EPA says to run the bathroom fan or open the window when showering. The same goes for baths. See our mold in bathroom guide.
• Fix drips. A leaky faucet keeps the tub wet around the drain.

Some tub mold isn't in the tub at all. Watch for:

• Loose or cracked tiles, or a gap opening between tub and wall
• Soft or stained drywall on the other side of the tub wall
• Stains on the ceiling below an upstairs tub (see mold on ceiling)
• A musty smell that stays after cleaning

The EPA warns mold can hide on the back side of dry wall, wallpaper, or paneling and inside walls around pipes. Fixr says that while mold removal from the tub costs $500 to $1,500, removing it from behind the tub increases the cost to that of treating other walls and porous areas, which it puts at $1,000 to $12,000 for walls. Our mold in walls guide covers what happens next.

Who to call. For the work, hire a licensed mold removal company. If you want to know how far it has spread first, hire a separate mold testing company: Texas law says a license holder may not perform both mold assessment and mold remediation on the same project. TDLR says visible mold less than 25 contiguous square feet can, in most instances, be handled by people who aren't licensed. Verify any license with our license check, or browse companies across Texas.