What causes mold on basement walls and how to treat it

Mold grows on basement walls because basements are damp. Water seeps through concrete, humidity rises from the ground, or pipes leak—and mold spores that are already floating in the air land on the wet surface and start to multiply. You can see it as black, green, or white patches, often fuzzy or slimy.

Small patches (less than 10 square feet) you can clean yourself with a stiff brush, detergent, and either white vinegar or a diluted bleach solution. Larger areas, mold that keeps coming back, or mold near your HVAC system or insulation should be handled by a professional mold remover, because disturbing it can spread spores through your house.

Before you clean, you need to stop the water from coming in. If you treat the mold but leave the wall wet, it will return within weeks.

Key Takeaways

  • Mold returns quickly if the wall stays damp, so find and fix the water source—seepage, a leak, or condensation—before you clean.
  • For small patches, scrub with a stiff brush and either white vinegar or a 1-to-10 bleach-to-water solution, then dry the wall completely.
  • Wear gloves, eye protection, and an N95 mask when cleaning mold, because spores can irritate your lungs and eyes.
  • Mold larger than 10 square feet, or mold that returns after you clean it, should be removed by a professional.
  • Basement walls stay dry when you control humidity with a dehumidifier, seal cracks in the concrete, and keep gutters and downspouts clear.

Identify where water is entering the wall

Mold will not stay gone unless you stop the moisture. Walk your basement after heavy rain and look for water running down the wall, pooling at the base, or seeping through cracks. If you see nothing during rain, the problem may be humidity—the air itself is too wet—rather than active leaks.

Check your gutters and downspouts from outside. Clogged gutters dump water against the foundation instead of carrying it away. Downspouts should extend at least 4 to 6 feet from the house; if they dump water close to the wall, that water soaks into the ground and pushes back through the concrete.

Look inside for pipe leaks. Run your hand along any visible pipes, especially near the moldy wall. Even a slow drip will keep a wall wet enough for mold. If you find a leak, repair or replace the pipe before you clean the mold.

Clean small mold patches yourself

For patches smaller than 10 square feet, gather a stiff-bristled brush, white vinegar or household bleach, a spray bottle, gloves, safety glasses, and an N95 mask. Open basement windows or run a fan to move air out of the space while you work.

Mix either straight white vinegar in a spray bottle, or a solution of 1 part bleach to 10 parts water. Spray the moldy area until it is wet. Let it sit for 10 to 15 minutes. Scrub hard with the stiff brush to break up the mold and work the cleaner into the concrete. Spray again and scrub a second time. Rinse with clean water and a cloth, then dry the wall as much as you can with towels.

Do not mix bleach and vinegar—the combination creates toxic chlorine gas. Choose one or the other. Vinegar is safer to use in an enclosed space; bleach is stronger but requires better ventilation.

After cleaning, run a dehumidifier in the basement for several days to pull moisture out of the air and the concrete. If the mold returns within a month, the wall is still too wet, and you need to find the water source or call a professional.

When to call a professional mold remover

Call a professional if the moldy area is larger than 10 square feet, if mold is growing on insulation or near your furnace or air handler, or if the mold returns after you clean it. A professional has equipment to contain spores, remove contaminated materials safely, and identify moisture problems you may have missed.

Mold removal companies typically charge by the square foot or by the hour. Costs vary widely by region and by how much of the wall is affected, so get quotes from at least two companies. Ask whether the price includes fixing the water problem or only removing the mold—many charge separately for moisture control.

Before hiring anyone, check that they are licensed in your state. Some states require mold remover licenses; others do not. Ask for references and verify they have liability insurance.

Prevent mold from returning

After you clean the wall, keep it dry. Run a dehumidifier year-round if your basement stays damp—aim for humidity below 50 percent. A hygrometer (a cheap tool sold at hardware stores) tells you the humidity level.

Seal cracks in the concrete with concrete caulk or hydraulic cement. These products are sold at any hardware store and come with instructions. Small cracks are straightforward to fill yourself; large cracks or cracks that leak water should be sealed by a basement waterproofing contractor.

Keep gutters clear of leaves and debris so water flows away from the house. Check downspouts after storms to make sure they are not clogged. If water pools against the foundation after rain, you may need to grade the soil away from the house or install a sump pump.

Insulate cold water pipes to prevent condensation from dripping onto the wall. Pipe insulation is inexpensive and takes minutes to install. If a pipe is actively leaking, repair it rather than just catching the drips.

Safety gear and ventilation while cleaning

Wear nitrile gloves to protect your skin from mold and cleaning chemicals. Wear safety glasses or a face shield to keep spores and spray out of your eyes. Wear an N95 mask—not a cloth mask or surgical mask—to avoid breathing mold spores. N95 masks are sold at hardware stores and online.

Open all basement windows and run a fan that exhausts air to the outside, not back into the house. If your basement has no windows, run a portable air scrubber with a HEPA filter, or open the basement door and run a fan in a nearby window to pull air out.

Do not use a household vacuum to clean up mold debris, because the vacuum will spread spores through the air and into your home. Use a damp cloth or a wet/dry shop vacuum with a HEPA filter.

What to do if mold is hiding inside walls

If you see mold on the surface of a basement wall but suspect it has grown inside the wall cavity (between the concrete and any drywall or paneling), do not straightforward paint over it or cover it. Mold inside a wall will continue to spread and can damage the framing.

Cut out the affected drywall or paneling and inspect the cavity. If you see mold on the concrete or on wood framing, the area is too large or complex for a DIY fix. Stop and call a professional. Removing mold from inside walls requires controlling dust, identifying all affected materials, and ensuring the wall dries completely before you close it back up.

Frequently Asked Questions

Can I paint over mold to cover it?

No. Paint will not kill mold, and mold will continue to grow underneath the paint. The mold will eventually break through the paint or spread to other areas. You must remove the mold first, dry the wall completely, and then paint if you want to.

Is black mold more dangerous than other colors of mold?

Black mold is not inherently more toxic than other colors, though some black molds produce toxins. Any mold can irritate your lungs, eyes, and skin. If you have asthma, allergies, or a weak immune system, avoid cleaning mold yourself and hire a professional instead.

How long does it take for mold to come back after cleaning?

If the wall is still damp, mold can return in two to four weeks. If you have dried the wall and fixed the water problem, mold should not return. If it does, the moisture source is still active, and you need to investigate further or call a professional.

Do I need to replace the drywall if there is mold on it?

Drywall that is wet or moldy should be removed and replaced, because drywall absorbs water and mold grows deep into the material. Concrete can be cleaned and dried, but drywall cannot. If your basement walls are drywall and they are moldy, the drywall needs to come out.

What is the difference between mold and mildew?

Mildew is a type of mold, usually white or gray and powdery, that grows on surfaces. All mildew is mold, but not all mold is mildew. Both are cleaned the same way—with a brush, detergent, and either vinegar or diluted bleach.