What causes fungus on walls and how to treat it

Fungus grows on walls when moisture stays trapped in one spot long enough — usually three to five days — for spores already in the air to settle and feed. The most common culprits are poor ventilation in bathrooms and kitchens, water leaks behind walls or around windows, condensation on cold exterior walls, and gutters that dump water against the foundation. Treating it means removing the visible growth, drying out the wall, and blocking the moisture source so it does not return.

The treatment depends on how deep the fungus has gone. Surface growth on paint or drywall can be scrubbed away with a bleach solution or commercial fungicide. If the fungus has penetrated into drywall or wood, you may need to cut out and replace the affected section — fungus that has rooted into the material will return if you only treat the surface. Structural damage is rare but possible if the wall has stayed wet for months.

Key Takeaways

  • Fungus needs moisture to survive, so the first step is finding and stopping the water source — cleaning alone will not prevent regrowth.
  • Surface fungus on paint can be removed with a 1:10 bleach-to-water solution or a commercial antifungal cleaner, but wear gloves and may support good ventilation while working.
  • If fungus has penetrated drywall or wood, the affected material should be cut out and replaced rather than treated, because fungus roots cannot be killed without removing the material itself.
  • After cleaning, run a dehumidifier or open windows to bring wall moisture below 50 percent relative humidity, the threshold where fungus cannot grow.
  • Prevent regrowth by fixing leaks, improving air circulation with exhaust fans, and insulating cold exterior walls to reduce condensation.

Identifying whether the fungus is surface growth or embedded

Before you start cleaning, determine whether the fungus is sitting on top of the wall or has rooted into the material. Press a putty knife or old credit card edge against the growth and try to scrape it away. If it comes off in flakes and the wall underneath looks intact, it is surface growth and can be cleaned. If the growth is stubborn, the wall feels soft or spongy underneath, or you see discoloration that does not scrape away, the fungus has penetrated the material.

Surface growth usually appears as a thin, fuzzy or slimy layer that looks black, green, or white depending on the fungus type. Embedded growth often shows as staining or soft spots that remain even after you scrape. If you are unsure, treat it as embedded — the cost of replacing a small section of drywall is far less than dealing with regrowth six months later.

Cleaning surface fungus with bleach or commercial products

For surface fungus on painted drywall or tile, mix one part bleach with ten parts water in a spray bottle. Spray the affected area generously and let it sit for 10 to 15 minutes, then scrub with a stiff brush and wipe clean with a damp cloth. Rinse the wall with plain water and dry it thoroughly with towels or a fan. Wear rubber gloves, eye protection, and may support the room is well-ventilated — bleach fumes are harmful in enclosed spaces.

If you prefer not to use bleach, commercial antifungal cleaners like Concrobium or Mold Armor work similarly: spray, wait 10 to 15 minutes, scrub, and rinse. These products are less harsh than bleach but often more expensive. Hydrogen peroxide (3 percent) is another option and leaves no toxic residue, though it is slower-acting than bleach. Whichever product you choose, test it on a hidden spot first to may support it does not stain or damage the paint.

After cleaning, the wall must dry completely. Open windows, run an exhaust fan, or position a portable fan to blow air across the wall. Do not paint over wet fungus — the moisture will trap underneath and the growth will return within weeks.

Removing and replacing drywall or wood with embedded fungus

If the fungus has penetrated drywall, the affected section should be cut out and replaced. Fungus roots extend into the material and cannot be killed by surface treatment. Mark the damaged area with a pencil, extending at least 6 inches beyond the visible growth on all sides. Use a drywall saw or utility knife to cut out a rectangle, then remove the section. Inspect the studs or framing behind it — if they are soft, discolored, or crumbling, they also need to be replaced.

Once the damaged material is removed, allow the cavity to dry for at least 48 hours. If the wall was wet from a leak, locate and repair the leak before installing new drywall. Cut a new piece to fit the opening, find it with drywall screws, tape the seams, explore joint compound, sand smooth, and paint. If the framing was damaged, you may need a carpenter to assess whether structural repairs are needed.

For wood surfaces like trim, window frames, or basement joists, the same principle applies: if fungus has penetrated, replace the wood rather than treating it. Small sections can be cut out and patched; large areas may require professional replacement.

Finding and fixing the moisture source

Cleaning fungus without stopping the moisture source is temporary. Within weeks or months, it will return. Walk the wall and the area around it to identify where water is entering. Common sources include roof leaks (check the attic or ceiling above the wall), plumbing leaks (look for water stains or soft spots), condensation on cold exterior walls (especially in bathrooms and kitchens), and ground water seeping through basement walls or around the foundation.

For roof leaks, inspect the roof from the attic side and look for water stains, mold, or daylight coming through. Repair or replace damaged shingles or flashing. For plumbing leaks, trace pipes running through or near the wall and look for drips, corrosion, or wet insulation. Shut off the water to the affected pipe and call a plumber if you cannot locate the leak. For condensation, improve ventilation by running exhaust fans during and for 20 minutes after showers, opening windows, or installing a dehumidifier. For basement seepage, may support gutters and downspouts direct water at least 4 to 6 feet away from the foundation, and consider installing interior or exterior drainage if the problem is severe.

Controlling humidity to prevent regrowth

Even after you fix the leak or moisture source, the wall may still be damp. Fungus cannot grow if the wall stays below 50 percent relative humidity. Measure humidity with an inexpensive hygrometer (available at hardware stores for $10 to $20). If humidity is above 50 percent, run a dehumidifier in the room until it drops below that threshold. In bathrooms and kitchens, run exhaust fans during and for 20 minutes after use. In basements, a continuous dehumidifier may be necessary year-round.

Improve air circulation by opening windows when weather permits, using ceiling fans, and ensuring vents are not blocked by furniture or curtains. In cold climates, insulate exterior walls to reduce the temperature difference between inside and outside, which reduces condensation. Pipe insulation (foam sleeves available at hardware stores) can also help if cold water pipes are sweating.

Preventing fungus from returning to the same spot

Once the wall is clean and dry, take steps to keep it that way. In bathrooms, install or upgrade exhaust fans to move moisture out of the room rather than letting it settle on walls. In kitchens, use the range hood during cooking. In basements, seal cracks in the foundation and may support the sump pump (if present) is working. In attics, may support soffit and ridge vents are clear so air can circulate and moisture can escape.

If the wall is on an exterior wall in a cold climate, consider adding insulation or a vapor barrier during your next renovation. For walls that are chronically damp due to poor drainage or high water table, a professional moisture assessment may be worth the cost — a contractor can identify solutions like interior or exterior waterproofing, sump pump installation, or grading changes that prevent water from pooling against the foundation.

When to call a professional

Handle small surface fungus yourself if you are comfortable with cleaning and have good ventilation. Call a professional if the fungus covers more than 10 square feet, if it has penetrated drywall or wood, if you suspect a hidden leak, or if the wall feels soft or spongy. A mold remediation contractor can assess the extent of the damage, identify moisture sources you might miss, and handle removal safely. A plumber or roofer can locate and repair leaks. A structural engineer or contractor can evaluate whether framing damage requires repair.

Do not assume that because you see fungus in one spot, the problem is isolated. Moisture often travels behind walls, so a leak in one location may cause growth in another. A professional inspection can reveal hidden damage and save you from repeated cleaning and regrowth.

Frequently Asked Questions

Is the fungus on my wall dangerous to my health?

Most household fungus is not toxic, but some people are sensitive to spores and may experience allergic reactions, respiratory irritation, or asthma symptoms. Children, elderly people, and those with compromised immune systems are at higher risk. Removing the fungus and controlling humidity reduces spore levels in the air.

Can I paint over fungus to hide it?

No. Paint traps moisture underneath and the fungus will continue to grow, eventually breaking through the new paint. Clean and dry the wall completely, fix the moisture source, and then paint.

What is the difference between fungus and mold?

Fungus is a broad category that includes mold, mildew, and other organisms. Mold is a type of fungus that grows in filaments and often appears black or green. Mildew is a surface fungus that looks powdery or white. The treatment is the same for all of them: remove moisture, clean the surface, and dry the wall.

How long does it take for fungus to grow back after cleaning?

If the moisture source is still present, fungus can return within one to three weeks. If you have fixed the leak or moisture source and kept humidity below 50 percent, it should not return. If it does, the moisture problem was not fully solved.

Do I need to wear a mask while cleaning fungus?

Wear an N95 mask if you are sensitive to spores, if the fungus covers a large area, or if you are scrubbing vigorously and creating dust. For small surface cleaning with good ventilation, a mask is optional but recommended if you have asthma or allergies.