Mold in the basement: causes, cleanup and prevention

Why do basements get moldy?

Basements sit below grade, surrounded by damp soil, and their walls and floors stay cooler than the rest of the house. That combination brings in water from three directions: liquid water through the walls and floor, leaks from plumbing and appliances, and humid air that condenses on cool surfaces.

EPA explains that mold spores are always in indoor air and begin growing when they land on wet surfaces. “None of them will grow without water or moisture,” EPA says. Basements also tend to be closed off with little air movement. EPA notes that in unaired places such as basements, molds “can produce a strong musty odor.”

The Minnesota Department of Health lists the moisture sources that most often feed basement mold:

• Flooding in the basement.
• Water pooling at the foundation from gutters or poor grading.
• Leaking pipes, water heaters and washing machine hoses.
• Condensation on cold water pipes, walls and floors in humid weather.

Stored items make it worse. Cardboard, paper, fabric and dust are all mold food. EPA’s own mold guide includes a photo of mold growing on a suitcase stored in a humid basement. For a full overview of identification and cleanup, see the mold hub.

How do you find the water source in a basement?

Cleaning is wasted effort until the water stops. EPA says if you clean up the mold but don’t fix the water problem, “most likely, the mold problem will come back.” Use the pattern of damage to narrow down the cause.

EPA’s prevention advice for buildings is direct: “Don’t let foundations stay wet. Provide drainage and slope the ground away from the foundation.” The Minnesota Department of Health adds installing and using a sump pump where groundwater is a problem.

How do you remove mold from basement concrete?

Concrete and block are hard surfaces, but they are porous enough to hold moisture. Start only after the water source is fixed and the area is under about 10 square feet, the size EPA says most people can handle themselves.

1. Put on at least an N95 respirator, gloves and goggles without vent holes (CDC, EPA).
2. Turn off any furnace or air handler that pulls air from the basement. EPA says not to run the HVAC system if it may be contaminated.
3. Remove standing water with a wet vacuum. EPA’s water damage table lists water extraction plus dehumidifiers, fans or heaters for wet concrete and block.
4. Scrub the mold off with a stiff brush, water and household detergent. EPA and the Minnesota Department of Health both recommend detergent and water for hard surfaces.
5. Rinse with clean water and collect the runoff with the wet vacuum or a mop.
6. Dry the surface completely with fans and a dehumidifier.
7. HEPA-vacuum the area once dry. EPA’s remediation table lists HEPA vacuuming for concrete.

EPA does not recommend bleach as a routine practice. If you use it, CDC’s limit is no more than 1 cup per gallon of water, never mixed with ammonia or other cleaners. Do not paint, seal or coat concrete until it is clean, dry and staying dry. EPA warns that paint over moldy surfaces is likely to peel. For the full DIY method, see how to get rid of mold.

What should you throw out after basement mold?

Hard, non-porous items can usually be cleaned. Porous materials that have grown mold usually cannot. EPA says absorbent or porous materials such as ceiling tiles and carpet “may have to be thrown away if they become moldy,” because mold fills their crevices.

Bag moldy items in heavy plastic before you carry them through the house. If the basement is finished and you find mold behind the walls, see mold on drywall for how to decide how much to remove.

What if the basement flooded or the sewer backed up?

Flood and sewage water change the job. EPA’s cleanup guidance says that if the damage was caused by sewage or other contaminated water, “call in a professional who has experience cleaning and fixing buildings damaged by contaminated water.” EPA’s remediation tables also note that OSHA requires protective equipment and containment when water is contaminated.

The Minnesota Department of Health says porous materials touched by sewage should always be removed. That includes carpet, drywall and insulation. EPA says not to use fans before determining that the water is clean or sanitary.

If the basement is still under water, stay out until the power is off. Our guide to sewage backup in the basement covers that situation step by step, and mold after a flood explains the timeline for mold prevention after a storm.

How do you keep a basement from getting moldy again?

Prevention comes down to keeping water out and keeping humidity down.

• Manage outside water. Clean and repair gutters, extend downspouts and slope the ground away from the foundation (EPA, MDH).
• Control humidity. EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent. CDC says no higher than 50 percent. Put a hygrometer in the basement itself.
• Dehumidify in warm, humid weather. EPA advises dehumidifying when outdoor air is warm and humid, and increasing ventilation when outdoor air is cold and dry. Opening basement windows in humid summer weather adds moisture.
• Stop condensation. EPA recommends insulating cold surfaces such as cold water pipes and raising the air temperature.
• Skip carpet. CDC advises against carpet in basements that may have a lot of moisture.
• Store smart. Keep boxes off the floor and away from outside walls. Use plastic bins.
• Dry spills fast. EPA says dry wet areas within 24 to 48 hours.

A dehumidifier with a continuous drain hose or built-in pump keeps working without someone emptying the bucket. Our dehumidifier guide explains sizing, and mold prevention covers the rest of the house.

How much does basement mold removal cost?

Cost guides disagree, partly because they use different data. This table shows the published basement figures with their dates.

The low end usually covers cleaning a small area of concrete. Costs rise when drywall, insulation or framing must be removed, when containment is needed, or when the job includes waterproofing. Waterproofing and drainage work is usually priced separately from mold removal. See the mold remediation cost guide for how to read a quote line by line.