How to clean moldy basement walls and keep mold from coming back

Scrub mold off a bare concrete or block basement wall with detergent and water, then dry the wall completely.¹ Wear long gloves, goggles without vent holes and an N-95 respirator.¹ In most cases you can do it yourself if the patch is under about 10 square feet.¹ Past that size, hire a pro.² Then fix the water problem, or the mold will most likely come back.¹

TL;DR

• White, chalky crust that crumbles and dissolves in water is usually efflorescence, a mineral salt. Green, black or white spots with a musty smell are mold.³ Both point to a moisture problem, and HomeGuide lists both as signs a basement may need waterproofing.⁴
• Detergent and water is the EPA's method for hard surfaces. Bleach can kill mold, but dead mold can still cause allergic reactions, so it has to come off the wall either way.¹
• The 10 square foot line is a size guide. Sewage water, a suspect HVAC system, mold hidden behind finishes and health concerns call for expert advice at any size.¹
• Moldy ceiling tiles, carpet and other porous materials may have to be thrown away and replaced.¹
• Keep basement humidity below 60%, ideally 30 to 50%, and stop outside water before you finish the space.¹

Is it mold or efflorescence on your basement wall?

Mold usually shows up as green, black or white spots with a musty smell.³ Efflorescence is a white, powdery or crystal crust of mineral salt. Water leaves it behind as it soaks through concrete or block and evaporates. It has no smell and turns to grit when you rub it. Either one means moisture is reaching the wall.

Check before you buy cleaners. The two need different handling, and a quick test settles most cases.

A quick test you can do in a few minutes

1. Put your gear on first. If it turns out to be mold, you don't want to have rubbed it bare-handed.
2. Look at where it sits. Efflorescence forms on bare masonry, often along mortar joints, cracks and the lower part of the wall. Mold grows on anything with a film of dust, paint, paper or wood to feed on. It needs organic material and water to grow.²
3. Mist it and rub it. Efflorescence softens, dissolves or breaks into grit. Mold tends to smear.
4. Check the paint. Mold can hide behind peeling or bubbling paint or wallpaper.²

If you still can't tell, treat it as mold. The gear costs little and the cleanup method works on both.

HomeGuide lists "white or gray chalky residue on walls or floors" as a sign a basement may need waterproofing. It sits on the same list as damp walls, musty odors and mold.⁴ So a positive efflorescence test isn't an all-clear. You can brush salt off dry, but it will keep coming back while water keeps moving through the wall.

Can you remove mold from your basement yourself?

Yes, in most cases, if the moldy area is less than about 10 square feet.¹ That's roughly a 3 foot by 3 foot patch.¹ If mold covers more than 10 square feet or there's been a lot of water damage, the EPA points homeowners to its commercial remediation guidance.¹ A realtor-published guide says to hire a professional past 10 square feet.²

Size is only the first question. Several situations make it a pro job even when the patch is small.

People with asthma, allergies or weakened immune systems face more risk from mold.² The same realtor guide limits its DIY advice to people who aren't immunocompromised.²

Color comes up a lot. The EPA notes that molds come in many colors, both white and black.¹ Its 10 square foot guide is based on size and says nothing about color. The realtor guide is stricter and says to hire a pro for black mold.² If black growth covers more than a small spot, or you'd rather not take the chance, the stricter advice is the safer one to follow.

What gear do you need before you clean basement mold?

Wear an N-95 respirator, goggles without vent holes and long gloves that reach the middle of your forearm.¹ The EPA puts an N-95 at about $12 to $25, and it only works if it fits properly.¹ Ordinary rubber gloves are fine with detergent and water. Bleach calls for natural rubber, neoprene, nitrile, polyurethane or PVC gloves.¹

Set up the room before you start scrubbing:

• Air. If you use bleach or another disinfectant, ventilate the area and exhaust the air to the outdoors.¹ A fan in an open basement window works for a small job.
• HVAC. Don't run the heating and cooling system if you know or suspect it's contaminated with mold.¹
• Waste. Keep garbage bags on hand. Rags, disposable gloves and other moldy material go straight into a sealed bag.³
• Clothes. Wash your work clothes right away in hot water, on their own.³

How do you clean mold off a concrete or block basement wall?

Cleaning a bare concrete or block wall takes five steps. Brush off the loose surface growth, scrub with detergent and water, rinse, dry the wall completely and bag the waste.¹³ Fix any active leak first. The EPA says mold will most likely come back if the water problem isn't fixed.¹

1. Brush off the surface layer

Use a dry brush to knock off the top layer of mold. It makes the rest of the wall easier to treat.³ This step puts spores in the air, so the respirator stays on.

2. Scrub with detergent and water

Dip a sponge or cloth in the cleaning solution and wring it out so it isn't soaking wet. Then scrub the area until the mold is gone.³ On block walls, work a stiff brush into the mortar joints. That's where growth tends to hang on.

3. Rinse with clean water

Rinse the wall with a second cloth or sponge and clean water.³

4. Dry the wall completely

Run a fan or dehumidifier until the wall is completely dry.³ Speed matters. The EPA's rule for water-damaged areas is to dry them within 24 to 48 hours to prevent mold growth.¹

5. Bag the waste and clean your tools

Seal the rags and disposable gloves in a garbage bag. Clean reusable tools with hot soapy water or rubbing alcohol, then dry everything before you store it.³

A shadow may stay on the concrete after the growth is gone. The EPA notes that mold can cause staining and cosmetic damage that cleaning won't always reverse.¹ A faint stain on a dry wall is cosmetic. Fuzz or a musty smell coming back means the wall is still getting wet.

Hold off on paint. The EPA says not to paint or caulk moldy surfaces, and paint over mold is likely to peel.¹ Waterproof paint doesn't solve the water either. HomeGuide notes that sealers "won't stop water under pressure."⁴ Building scientist Joseph Lstiburek warns that impermeable wall finishes such as oil or alkyd paints block inward drying and typically lead to mold growth.⁵

One homeowner's 2015 blog account shows how the surface decides the job. The couple's concrete walls and plywood paneling cleaned up in a few hours with 8 parts water to 1 part bleach. Their mineral fiber ceiling tiles had soaked up water and grown black mold, so the tiles came down.⁶

How do you treat mold on drywall, paneling, wood and ceiling tiles?

Hard surfaces can usually be scrubbed clean, but porous materials often can't.¹ Concrete, block and paneling get detergent and water. Painted drywall gets a gentle cloth. Wood may need sanding first.³ Ceiling tiles, carpet and other absorbent materials may have to be thrown away, because mold fills their gaps and crevices.¹

Finished basements hide mold. The EPA lists the back side of drywall and paneling, the underside of carpet and pad, and wall surfaces behind furniture as common hiding spots.¹ A musty smell with nothing visible is the usual clue. Pulling finishes off can release a large burst of spores, so the EPA suggests an experienced professional for suspected hidden mold.¹

What kills mold on a basement wall?

Bleach kills mold, but the EPA's method for hard surfaces is detergent and water, which lifts the mold off.¹ The EPA doesn't recommend chlorine bleach or other biocides as routine cleanup.¹ Killing mold also isn't enough. Dead mold can still cause allergic reactions, so it has to be removed.¹

Here's how the common cleaners compare.

For most basement walls, detergent and water plus fast drying does the job. Bleach earns its place in narrower cases. The EPA gives one example: when immune-compromised people are present.¹ If you do use it, never mix it with anything else. Bleach mixed with ammonia produces toxic fumes.¹ Mixing it with vinegar or other acids is also dangerous, because the combination releases chlorine gas.

Will vinegar kill mold on basement walls?

Home Depot lists undiluted white vinegar as a mold cleaner for hard surfaces in kitchens and baths.³ It says nothing about porous basement concrete or block. On a basement wall, treat vinegar as a cleaning step. You still scrub the mold off, rinse and dry.¹

What are the downsides of using vinegar for mold?

Vinegar has three practical downsides on a basement wall:

• Dead mold still causes trouble. The EPA warns that dead mold can still cause allergic reactions, so spraying it without scrubbing it off leaves the allergen on the wall.¹
• It does nothing about the water. If you clean up the mold but don't fix the water problem, the EPA says the mold will most likely come back.¹
• It can't go near bleach. Vinegar mixed with bleach releases chlorine gas. That makes it risky to switch cleaners partway through a job.

How do you know the basement mold cleanup is finished?

Cleanup is finished when the water or moisture problem is fixed and no visible mold or moldy smell remains.¹ The area should still show no mold or water damage when you check it again shortly after. People should be able to use the space without health complaints.¹ The EPA admits it is ultimately a judgment call.¹

You usually don't need a mold test. The EPA says sampling is unnecessary in most cases when you can see the mold. There are no EPA or federal limits for mold or mold spores to test against.¹ Surface sampling can confirm whether an area was cleaned well. The EPA says it should be done by professionals with specific sampling experience.¹

Put a reminder in your calendar to check the wall again a few weeks after cleaning, and again after the next heavy rain. A clean wall that stays dry through wet weather is the real test.

How do you stop mold from coming back on basement walls?

Control the moisture. The EPA calls moisture control the key to mold control, because mold spores won't grow where moisture isn't present.¹ Dry any wet area within 24 to 48 hours. Keep indoor humidity below 60%, ideally between 30 and 50%.¹ Then stop outside water from reaching the wall.

Measure the humidity

A humidity meter costs about $10 to $50 at a hardware store.¹ Hang it in the basement and read it through a humid summer week. ENERGY STAR notes that all of its certified dehumidifiers include a built-in humidistat. If yours doesn't show relative humidity, add a separate hygrometer.⁷

Size a dehumidifier to the room

ENERGY STAR sizes portable dehumidifiers by how damp the space is and how big it is. Capacity is measured in pints of water removed per day.⁷ Our basement dehumidifier sizing guide walks through the choice in more detail.

Source: ENERGY STAR dehumidifier buying guidance.⁷

ENERGY STAR says it's better to oversize than undersize.⁷ Cool basements need a second look. Frost can form on the coils below 65 degrees F, so a model rated for lower temperatures may work better.⁷ Run a drain hose to a floor drain or sump so the unit keeps working when you're away.⁷

Stop summer condensation on cold walls

Concrete below grade stays cold because it touches the ground. In summer, warm, moist air hits those cold walls and condenses on them.⁵ So a basement with no leak can still grow mold in summer. Dehumidifying helps. The EPA also suggests covering cold surfaces such as cold water pipes with insulation.¹

Move roof and ground water away from the wall

Keep gutters clean and extend downspouts away from the foundation.⁷ Make sure the ground slopes away from the house so water doesn't collect against the foundation.¹ Lstiburek names these same steps first for keeping groundwater out of basements.⁵

When water comes through the wall

If you see seepage, damp patches after rain or salt crust that keeps returning, the wall needs a water fix, and a dehumidifier alone won't be enough. HomeGuide's national cost guide puts basement waterproofing at $6,000 to $18,000 on average, depending on the methods used and how bad the problem is.⁴ Smaller pieces cost less:

• Sump pump installation: $600 to $2,500+⁴⁸
• Epoxy injection for a crack: $250 to $800+ per crack⁴
• Waterproof paint or concrete sealer: $2 to $10 per square foot⁴

Our guide to basement waterproofing methods explains what each fix does and when it's the right one. When you're ready to act, get a written quote to find and fix the water in your basement. The price for your house depends on where the water enters, and the contractor prices it after the free on-site estimate.

Should you finish a basement that has had mold?

Yes, once the water source is fixed and the walls have dried. The one vetted, licensed basement contractor you're matched with checks moisture at the free on-site estimate, along with ceiling height and layout. If the walls are still damp, fix the water before anyone talks about drywall.

The wall build matters as much as the timing. Lstiburek's basement digest warns that frame walls filled with insulation and covered with an interior plastic vapor barrier often lead to odor, mold, decay and corrosion problems.⁵ He recommends rigid foam or spray foam against the concrete. Those allow drying, aren't sensitive to moisture damage and don't support mold growth.⁵ He also says to skip impermeable finishes such as vinyl wall coverings and oil or alkyd paint.⁵

If you're starting from bare concrete, see what finishing a bare basement involves. If mold showed up behind an existing finished wall, the work becomes a tear-out and rebuild. That falls under remodeling an already finished basement. Homeowners in Georgia and Colorado can see local detail on finishing a basement in metro Atlanta and finishing a basement in Denver.

What should a mold or waterproofing quote show?

A good quote names where the water comes from, what material comes out and what gets cleaned in place. The EPA says to hire a contractor with experience cleaning up mold and to check references.¹ Ask them to follow EPA's Mold Remediation in Schools and Commercial Buildings or the ACGIH guidelines.¹

Questions worth asking before you sign:

• How is water getting into my basement?⁴
• Which methods do you recommend for my basement, and why?⁴
• Which materials are you removing, and which are you cleaning?
• How will you keep spores out of the rest of the house while you work?
• Will you check the HVAC system for mold before it runs again?¹
• How will we confirm the job is finished?¹
• Are you licensed and insured, and will the estimate be itemized and in writing?⁴

Published price ranges for mold removal disagree. HomeGuide puts mold remediation at $1,500 to $15,000, depending on how severe the problem is and how big the basement is.⁴ A realtor-published guide says basement mold removal can cost $1,500 to $4,000.² Budget from HomeGuide's range. It's wider and names what moves the price, while the realtor figure gives no scope. Your own number comes from the on-site visit.

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