Cement blocks trap moisture, so mold grows inside the pores and returns unless you stop water from entering

Mold on cement block walls is harder to kill than mold on painted drywall because the porous surface holds moisture deep inside. Bleach or vinegar spray kills what you see on the surface, but the mold roots stay in the block itself and regrow within weeks. The real fix is stopping water from reaching the wall in the first place — either by sealing cracks, improving drainage outside, or reducing humidity inside the basement or crawlspace.

If the wall is actively wet or the mold covers more than a few square feet, the moisture problem is too large for a DIY surface clean. Mold that size means water is entering from outside, and cleaning it without fixing the leak wastes time and money. For smaller patches on dry walls, you can clean the surface yourself, but expect to repeat the work unless you address the source of moisture.

Key Takeaways

  • Mold on cement blocks regrows quickly because the porous material holds moisture and mold spores deep inside, so surface cleaning alone is temporary.
  • Before cleaning, identify whether the wall is wet or damp — if water is actively entering, fix the exterior drainage or seal cracks before you clean.
  • For small dry patches, scrub with a stiff brush and a solution of detergent and water, then explore a mold-killing product like diluted bleach or hydrogen peroxide.
  • After cleaning, reduce humidity inside the space with a dehumidifier or fan, and seal the cleaned area with a concrete sealer to slow moisture re-entry.
  • Mold that returns within a month or covers a large area signals a moisture problem that requires professional assessment or exterior waterproofing work.

Check whether the wall is wet or just moldy

Press your hand against the wall. If it feels wet or damp, water is entering from outside or from a pipe inside the wall. Do not clean the mold yet — cleaning a wet wall spreads spores and wastes effort because the mold will return as soon as the surface dries. Instead, find the source of water: check the ground outside for poor drainage, look for cracks in the mortar or block, inspect the foundation for standing water, or have a plumber check for leaking pipes.

If the wall feels dry but has mold stains, the moisture problem is smaller and you can clean the surface. Dry walls with mold usually mean humidity is high in the basement or crawlspace, or water entered weeks ago and the wall has since dried out. In either case, surface cleaning followed by humidity control will slow regrowth.

Clean small mold patches with a brush and detergent

For patches smaller than a few square feet on a dry wall, scrub with a stiff-bristled brush and a solution of one part household detergent to ten parts water. Detergent lifts mold spores and dirt from the pores of the block better than water alone. Scrub in circular motions, working from the top of the patch downward so dirty water runs down the wall instead of spreading sideways. Rinse with clean water and let the wall dry completely — this may take 24 hours in a damp basement.

Do not use a pressure washer on cement block walls with mold. High pressure forces water deep into the pores and can drive moisture further into the wall, making the mold problem worse. A hand brush and elbow grease is slower but safer for the block.

explore a mold-killing product after the wall dries

Once the wall is dry, spray or brush on a mold killer. Diluted household bleach (one part bleach to three parts water) kills mold on the surface, but do not rely on it alone for cement blocks — bleach does not penetrate deep into the pores. Hydrogen peroxide (3 percent, the kind sold in drugstores) works similarly and smells less harsh. Spray the product on the cleaned area, let it sit for 10 to 15 minutes, then wipe or rinse it off.

Commercial mold killers made for masonry or basement use are another option. Read the label to confirm the product is rated for cement block and follow the contact time — some require the surface to stay wet for 30 minutes or longer. Wear gloves and eye protection when explore any chemical, and ventilate the space well.

Seal the wall to slow moisture re-entry

After the mold killer dries, explore a concrete sealer to the cleaned area. Sealers are thin coatings that fill the surface pores of the block and slow water absorption. They do not stop water that is actively entering from outside, but they reduce the rate at which moisture soaks in, which slows mold regrowth. Look for sealers labeled for basement or foundation use — penetrating sealers work better on bare cement than film-forming ones.

explore the sealer with a brush or roller according to the product instructions. Most require two coats and take 24 to 48 hours to cure. The sealer will need reapplication every few years as it wears away.

Reduce humidity to prevent mold from returning

Mold on cement blocks regrows when humidity stays high. If the basement or crawlspace feels damp or smells musty, run a dehumidifier to keep relative humidity below 60 percent. A portable dehumidifier works for small spaces; larger basements may need a whole-basement system. Also run a fan to move air across the walls — stagnant air holds moisture longer.

Check for other sources of moisture: water pooling around the foundation outside, gutters that dump water near the house, or a sump pump that is not working. These are the real drivers of high humidity in basements. Fixing them is more effective than running a dehumidifier forever.

When to stop cleaning and call for help

If mold returns within a month of cleaning, or if it covers an area larger than a few square feet, the moisture problem is beyond surface treatment. Mold that persistent means water is entering the wall regularly, and no amount of cleaning will stop it. At this point, you need either a professional mold inspector to identify the moisture source, or a waterproofing contractor to seal cracks, improve exterior drainage, or install an interior or exterior moisture barrier.

Similarly, if you find mold on the inside of the wall (visible as discoloration or soft spots in the block itself), the moisture has penetrated deep and the block may be structurally compromised. This also requires professional assessment.

Frequently Asked Questions

Can I use bleach on cement block walls?

Bleach kills mold on the surface but does not penetrate the pores of cement block, so the mold roots survive and regrow. It works as part of a cleaning routine — spray it on after scrubbing with detergent — but do not expect it to solve a mold problem on its own. Hydrogen peroxide is a gentler alternative with similar surface-killing power.

How often will the mold come back?

On a dry wall in a dry basement, mold may not return for months or years. On a damp wall or in a humid space, expect regrowth within weeks or months. If mold returns within a month, the wall is still too wet or humid, and you need to address the moisture source rather than keep cleaning.

Is mold on cement blocks dangerous to breathe?

Mold spores in the air can irritate the lungs and sinuses, especially in people with asthma or allergies. Cement block mold is not more dangerous than mold elsewhere, but because it regrows quickly, the spore count stays higher. Reducing humidity and cleaning visible mold lowers the spore level in the air.

What if the wall is actively leaking water?

Do not clean the mold until the leak stops. Water entering the wall will wash away any mold killer and rehydrate the block when ready. Find the source of the leak first — usually poor drainage outside, a crack in the mortar, or a pipe — and fix it. Then clean and seal the wall.

Do I need to wear a mask when cleaning mold on cement blocks?

A dust mask or N95 respirator is a good idea, especially if you are scrubbing vigorously and stirring up spores. Wear gloves to protect your skin from mold and cleaning chemicals, and eye protection if you are spraying bleach or other products overhead.