How to Get Rid of Mold in Basement Walls, Floors and Joists

Quick answer: To get rid of mold in a basement, first stop the water: fix gutters and grading, check the sump and run a dehumidifier to keep humidity below 50% to 60%. Then scrub mold off concrete and wood with detergent and water and a stiff brush, dry everything within 24 to 48 hours, and throw out moldy drywall, carpet and cardboard. If mold covers more than about 10 square feet, or came from flooding, call a pro.

If you’re figuring out how to get rid of mold in basement spaces, you’re dealing with the room most likely to grow it. Basements sit in damp soil, stay cool, and get little air or sunlight. Mold on the walls, the floor or the wood joists overhead is a sign that moisture is winning.

Cleaning is the easy part. The job only sticks if you find where the water comes from. This guide covers both, based on EPA guidance and advice from university extension experts.

Is it mold or efflorescence?

Before you scrub, make sure it’s mold. Basement walls often show a white, chalky deposit called efflorescence: salts left behind when water moves through concrete or block and evaporates.

Sign	Mold	Efflorescence
Color	Black, green, gray, white or orange, often in blotches	White or off-white
Texture	Fuzzy or slimy, smears when wiped	Crystalline or powdery, crumbles
Water test	Doesn’t dissolve	Dissolves or softens when sprayed with water
Where it grows	Wood, drywall, paper, dust, carpet, dirty concrete	Bare concrete, block and brick
Smell	Musty	None

Efflorescence isn’t harmful, but it means water is moving through your walls, and that same moisture can feed mold nearby. Treat it as a warning sign.

What causes mold in a basement?

• Water from outside. Clogged gutters, short downspouts and soil that slopes toward the house push water against foundation walls.
• Groundwater and seepage. Water rises through the slab or leaks through cracks, especially after heavy rain or snowmelt.
• Summer condensation. Warm, humid outdoor air enters the cool basement and condenses on walls, floors and cold water pipes.
• Plumbing leaks. Water heaters, washing machines, sinks and pipes overhead.
• Indoor sources. Unvented dryers, laundry drying on a line, and unsealed sump pits.
• Mold food. Cardboard boxes, old carpet, paper-faced drywall, wood and dust give mold something to eat.

A simple test tells you where the moisture comes from. Tape a 2 x 2 ft square of plastic sheeting tightly to the wall or floor and leave it for 24 to 48 hours. Moisture under the plastic means water is coming through the concrete. Moisture on top means condensation from humid air.

How to get rid of mold in basement spaces: step by step

1. Stop the water first. Clean gutters, extend downspouts so they discharge several feet from the foundation, and make sure soil slopes away from the house. Check that the sump pump works and fix plumbing leaks.
2. Measure humidity. Put a hygrometer in the basement for a few days. Most basements should stay below about 50% to 60% relative humidity.
3. Protect yourself. Wear an N95 respirator, long gloves and goggles without vents, per EPA guidance. Keep kids, pets and anyone with asthma or allergies out while you work.
4. Remove what can’t be saved. Bag and discard moldy cardboard, paper, carpet, carpet pad, ceiling tiles and moldy upholstered items. Bag them in heavy plastic in the basement before carrying them out.
5. Clean concrete walls and floors. Scrub with detergent and water and a stiff nylon brush, then rinse and wipe up the water with a wet/dry vacuum. Don’t use a wire brush on painted block.
6. Clean wood joists and framing. HEPA-vacuum loose growth, then scrub with detergent and water and a stiff brush. Sand remaining surface stains lightly, while wearing your respirator. Wood that’s soft or crumbling needs a contractor to evaluate it.
7. Replace moldy drywall and insulation. The EPA notes that porous materials with mold growth may need to be thrown away. Cut out drywall well past the stained area, and remove wet fiberglass insulation.
8. Dry everything fast. Run fans and a dehumidifier. The EPA says wet materials should be dried within 24 to 48 hours to prevent new growth.
9. HEPA-vacuum and wipe down. Vacuum floors and surfaces with a HEPA vacuum to pick up settled spores and dust.
10. Seal and finish only when dry. Once walls have been dry for several days, you can use a masonry waterproofing paint on concrete, or mold-resistant drywall and primer on finished walls. Sealants don’t stop water pushed in from outside.

If you use bleach on concrete stains, dilute it (no more than 1 cup per gallon of water), wear gloves and eye protection and ventilate with fans blowing air out of the basement. Never mix bleach with ammonia, vinegar or other acids, which creates toxic gas. The EPA says bleach isn’t usually necessary; scrubbing and drying do the real work. Keep kids and pets away until surfaces are rinsed and dry, and follow each product’s label.

Mold on finished basement walls

Drywall on basement walls is where mold hides. Water or condensation from the concrete behind it soaks the back of the drywall and the insulation long before you see anything on the front. If you see stains near the floor or notice a musty smell, check behind baseboards. Our guide on how to get rid of mold on walls covers drywall cleanup and replacement in detail.

How long does basement mold cleanup take?

A small area on concrete takes an afternoon. Removing drywall, drying and rebuilding a finished section can take a week or more. Fixing grading or drainage outside depends on the job, but it’s what keeps the mold from coming back.

The equipment that helps

In almost every basement, a properly sized dehumidifier is the tool that keeps mold from returning. For more options and sizing, see our best basement dehumidifier guide.

Want the details? Read our Waykar 80-pint dehumidifier review.

Not sure which drainage setup you need? Our best dehumidifier with pump guide explains lift height and hose routing.

How to keep mold out of your basement

• Run a dehumidifier all year if needed. Set it to about 50%. Most basements need it most during humid summers.
• Keep water away from the foundation. Clean gutters twice a year, keep downspout extensions in place and maintain soil slope away from the house.
• Maintain the sump pump. Test it each spring by pouring water into the pit, and consider a battery backup.
• Insulate cold water pipes. Sweating pipes drip onto floors and boxes all summer.
• Close windows on humid days. Opening basement windows in summer lets in humid air that condenses on cool walls.
• Vent the dryer outside. Check that the vent is connected and not leaking into the basement.
• Store things in plastic bins. Keep them off the floor on shelves and a few inches away from exterior walls.
• Choose basement-friendly finishes. Rugs you can wash instead of wall-to-wall carpet, and mold-resistant drywall kept off the concrete floor.

If your basement still smells after cleanup, our guide on how to get rid of musty smell in a basement covers odor sources, and our best crawl space dehumidifier guide helps if a crawl space connects to your basement.

When to call a professional

Call a mold remediation professional if mold covers more than about 10 square feet (the EPA’s guideline), if it followed flooding, a sewer backup or groundwater, if it’s in your HVAC system or ductwork, if it keeps returning after cleaning, or if anyone in the home has health symptoms. Call a waterproofing or drainage contractor for recurring seepage, cracks that leak, or a sump that can’t keep up, and a structural contractor if wood framing is soft or damaged.

Frequently asked questions

Can I get rid of basement mold myself?

Yes, for small areas. The EPA says homeowners can usually clean mold patches under about 10 square feet themselves with protective gear, detergent and water, and prompt drying. For larger areas, mold after flooding or sewage, or mold in your HVAC system, hire a professional. Either way, fix the water source or the mold comes back.

What humidity should my basement be to prevent mold?

The EPA recommends 30% to 50% indoor humidity, and most basements should stay below about 50% to 60%. Mold grows more easily when humidity stays above 60% for long periods. A hygrometer tells you where your basement stands, and a dehumidifier set to 50% keeps it there through humid months.

Is white stuff on basement walls mold?

Often it’s efflorescence, not mold. Efflorescence is a white, crystalline salt deposit that dissolves or softens when you spray it with water and crumbles when brushed. White mold looks fuzzy, grows on wood, paper or dust, and doesn’t dissolve. Efflorescence isn’t harmful, but it shows water is moving through the wall.

Does bleach kill mold on basement concrete?

Diluted bleach can remove mold stains from concrete, but the EPA says it usually isn’t necessary. Scrubbing with detergent and water, then drying quickly, removes the growth. If you use bleach, dilute it, ventilate well, wear gloves and eye protection, and never mix it with ammonia or acids. Bleach does nothing to stop the moisture.

Should I remove carpet with mold in the basement?

Usually, yes. Carpet and carpet pad are porous and hold water against the slab, so mold can grow deep in them where cleaning can’t reach. The EPA notes that porous materials with mold growth may need to be discarded. Replace it with washable area rugs or a flooring type that tolerates basement moisture.

Will a dehumidifier get rid of mold in the basement?

A dehumidifier won’t remove mold that’s already growing, but it stops new growth by keeping humidity low. Clean or remove existing mold first, fix water entering from outside, then run a properly sized dehumidifier set to about 50%. Together, those steps keep most basements mold-free.

Ready to pick a unit? Compare sizes and drainage options in our best basement dehumidifier guide, or start with the complete guide on how to get rid of mold.