What causes fungus to grow on walls and how to tell if you have it

Fungus grows on walls when moisture, warmth, and a food source (usually dust, drywall, or paint) come together. The most common culprits are poor ventilation in bathrooms and kitchens, roof leaks, foundation cracks that let water seep in, and high indoor humidity. If your home stays above 60 percent humidity, fungus will find a foothold.

You'll recognize fungus by its appearance: black, green, or white spots or patches, sometimes fuzzy or slimy to the touch. It often smells musty. Fungus can grow on painted drywall, wood framing, concrete basement walls, and tile grout. The difference between mold (a type of fungus) and mildew (another type) matters less for removal than finding and stopping the moisture that feeds it—if you don't fix the leak or humidity, it will return within weeks.

Key Takeaways

  • Fungus on walls always means moisture is present; cleaning without fixing the moisture source will not stop it from coming back.
  • Small patches (less than 10 square feet) on non-porous surfaces like tile can be cleaned with household products, but larger areas or growth on porous materials like drywall usually require professional removal.
  • The first step is always to find the water source—a leak, condensation, or humidity—and stop it before you clean.
  • Proper ventilation, dehumidifiers, and keeping indoor humidity below 50 percent prevent fungus from returning after removal.

When to clean fungus yourself and when to call a professional

The size and location of the fungus determine whether you can handle it or need to hire someone. If the patch is smaller than 10 square feet (roughly a 3-by-3-foot square), is on a non-porous surface like tile or sealed concrete, and you can see where the moisture is coming from, you can clean it yourself. If the fungus covers more than 10 square feet, has grown into drywall or insulation, or you cannot identify the moisture source, call a mold removal contractor.

Fungus that has penetrated drywall is particularly tricky because the spores live inside the material. Cleaning the surface will not remove them. A professional can determine whether the drywall needs to be replaced or whether surface treatment will work. If you have respiratory problems, asthma, or a weakened immune system, avoid exposure to fungus altogether and hire a professional even for small patches.

How to clean small fungus patches on hard surfaces

Start by stopping the moisture. If the fungus is near a window, improve ventilation or fix condensation. If it's under a sink, find and repair the leak. If it's in a bathroom, run the exhaust fan during and for 30 minutes after showers. Once moisture control is in place, you can clean.

For tile, grout, and sealed concrete, use a solution of one part household bleach to three parts water. Spray it on the fungus, let it sit for 10 minutes, and scrub with a stiff brush. Rinse thoroughly with clean water and dry the area completely. For painted drywall, do not use bleach—it can damage the paint and won't penetrate the porous material. Instead, use a commercial mold-killing cleaner labeled for drywall, or a solution of one part white vinegar to one part water. Spray, wait 10 minutes, and wipe clean. Dry the wall with a fan or dehumidifier.

Wear gloves, eye protection, and an N95 mask during cleaning. Open windows and doors to ventilate the space. Do not mix bleach with ammonia or other cleaners—the fumes are toxic.

How to find and fix the moisture source

Fungus is a symptom, not the problem. The problem is water. Look for the source before you clean, or the fungus will return. Common sources are roof leaks (fungus appears on upper-floor ceilings or walls), plumbing leaks (fungus appears under sinks, around toilets, or along pipes), foundation cracks (fungus appears on basement walls, especially after rain), and condensation (fungus appears on cold surfaces like exterior walls in winter or around bathroom mirrors).

If the fungus is on a ceiling, go into the attic and look for water stains on the underside of the roof. If it's on a basement wall, check for cracks and water seepage after rain. If it's in a bathroom, check the caulk around the tub and shower for gaps, and make sure the exhaust fan vents outside (not into the attic). If it's on an exterior wall, check the outside for damaged siding, clogged gutters, or grading that slopes toward the house.

Small leaks can be sealed with caulk or patching compound. Larger leaks—roof damage, foundation cracks wider than 1/4 inch, or plumbing failures—require a contractor. A roofer, plumber, or foundation specialist can assess the damage and give you a repair estimate.

Preventing fungus from returning after cleaning

Once you've cleaned the fungus and fixed the moisture source, keep humidity low and air moving. Use a dehumidifier to keep indoor humidity between 30 and 50 percent. Run bathroom and kitchen exhaust fans during and for 30 minutes after use. Open windows on dry days to bring in fresh air. In basements, a sump pump and proper grading around the foundation prevent water from entering in the first place.

Check the cleaned area once a month for the first few months. If fungus returns, the moisture source was not fully fixed. Call a professional to investigate further. If the area stays dry and clean, your moisture control is working.

Dealing with fungus in walls and insulation

If fungus has grown inside the wall cavity or into insulation, surface cleaning will not work. The fungus is feeding on the insulation material itself, and spores are embedded where you cannot reach them. This is a job for a mold removal professional. They may need to open the wall, remove and replace the insulation, treat the framing, and seal everything back up.

You'll know fungus is inside the wall if you see it on the surface, smell a strong musty odor coming from the wall, or find water damage in the wall cavity. Do not attempt to remove insulation yourself unless you are certain it does not contain asbestos (common in homes built before 1980). If you suspect asbestos, hire an abatement contractor.

What to do if fungus returns after cleaning

Returning fungus means the moisture source is still active. Go back to the source: Is the roof still leaking? Is the bathroom fan still not running? Is the basement still wet after rain? Is the dehumidifier set correctly? Fix the moisture problem first, then clean again. If you've already fixed the obvious sources and fungus still returns, the problem may be hidden—a slow leak inside a wall, a blocked drain, or a humidity level you cannot control without professional help.

At this point, call a mold removal contractor or a moisture specialist. They can use moisture meters and thermal imaging to find hidden water sources. A contractor can also assess whether the fungus has spread too far to clean and whether materials need to be replaced.

Frequently Asked Questions

Is fungus on walls dangerous to my health?

Some people are sensitive to mold spores and may experience allergic reactions, respiratory irritation, or asthma symptoms. Others have no reaction. People with compromised immune systems, chronic lung disease, or severe allergies are at higher risk. If you have health concerns, avoid prolonged exposure and hire a professional to clean.

Can I paint over fungus to cover it?

No. Paint will not kill fungus or stop it from growing. The fungus will continue to feed and spread under the paint, and the paint will peel or bubble as the fungus damages the surface beneath. Always clean and dry the area, fix the moisture source, and let the wall dry completely before painting.

What's the difference between mold and mildew?

Both are types of fungus. Mildew is usually white or gray and grows on surfaces; mold is often black or green and can penetrate porous materials. For removal purposes, the distinction doesn't matter—both require moisture control and cleaning. The real difference is that mold is more likely to cause health problems and is harder to remove if it's deep in a material.

Do I need to throw away drywall if it has fungus on it?

Not always. If fungus is only on the surface of painted drywall and the moisture source is fixed, cleaning may be enough. If fungus has penetrated the drywall paper or the gypsum core, the drywall should be replaced. A professional can tell you which applies to your situation.

How long does it take for fungus to come back if I don't fix the moisture?

Fungus can return within two to four weeks if the moisture source is still present. Some types grow faster than others, and warm, humid conditions speed growth. This is why fixing the leak or humidity is more important than the cleaning itself.