Mold in Basement: Causes, Smell, Removal and Cost

Below-grade rooms sit against soil, stay cooler than the rest of the house, and often hold stored boxes, carpet and laundry. That's a good setup for mold.

• Water through concrete. NDSU Extension says moisture seeping through concrete walls and floors will cause moist conditions.
• Summer humidity. Bob Vila's cost guide notes that basements tend to be cooler and humidity tends to settle in the basement in the summertime. The EPA explains that when warm, humid air contacts a cold surface, condensation may form.
• Leaks. Fixr says basements get damp, have high humidity, and are common locations for leaking pipes.
• Poor drainage outside. FEMA lists a damp basement or crawl space among the causes of water damage, and says the ground around the house needs to slope away to keep basement and crawlspace dry.
• Floods. NOAA reports that Hurricane Harvey was the costliest hurricane in Texas history, with an estimated $160 billion in damages. Any below-grade space that took on floodwater needs a careful look. See mold after a flood in Texas.

Mold isn't fussy about what it eats once it has water. Fixr notes it also grows in basement wood, insulation, and drywall, and the EPA shows a photo of mold growing on a suitcase stored in a humid basement.

Many of the same problems show up in other ground-level spaces:

• Slab floors. A concrete slab can stay damp after a leak or flood. Texas A&M AgriLife Extension notes that if the subflooring is concrete, removal of the floor covering will hasten drying of the slab.
• Garages. The EPA lists clothes dryers vented into a garage as a moisture problem. EPA's house tour says clothes dryers must be vented to the outdoors.
• Storage rooms and closets. The EPA says high room humidity can cause mold to grow on many items indoors, including furnishings or clothing. Cardboard boxes against an exterior wall are a common spot.
• Raised homes. If your home sits on piers instead, see our mold in crawl space guide.

A musty odor is often the first sign. The EPA says you may suspect hidden mold if a building smells moldy, but you cannot see the source. FEMA lists basements, along with air ducts, attics and wall cavities, among hidden sources of mold.

Work through this checklist, adapted from University of Georgia Extension's home inspection list:

• Inspect ceiling, joists, and stairs for mold growth.
• Inspect the floor for foundation cracks.
• Check that the sump pump is functioning properly and the floor drain is intact.
• Measure humidity. UGA says that if over 50%, you should consider using a dehumidifier.
• Look behind stored boxes, under carpet and behind finished walls on exterior sides.
• Outside, check the slope and grade of the house to ensure that the ground slopes away from the house.

Be careful about tearing into finished walls to chase a smell. The EPA lists breaking moldy porous materials such as wallboard as an activity likely to disturb mold. Our mold smell in the house guide covers other ways to narrow it down.

The EPA's rule is simple: the key to mold control is moisture control. Here's the order to work in.

1. Stop the water

Repair leaks, fix gutters and grading, and get the sump pump working. The EPA says to fix plumbing leaks and other water problems as soon as possible. EPA's Indoor airPLUS program, for the new and renovated homes it certifies, requires a floor drain with trap seal OR moisture monitoring system with audible alarm in the lowest area of a basement or crawl space. A water alarm is a cheap version of the same idea.

2. Dry the air

EPA's house tour says that if a room such as a basement is too humid, use a dehumidifier to help prevent mold growth, and to clean the dehumidifier regularly. Aim for the EPA's target: below 60 percent (ideally between 30 and 50 percent). Our dehumidifier for mold guide explains sizing.

3. Clean or remove by material

Table: Basement mold removal by material.

For hard surfaces, the EPA says to scrub mold off hard surfaces with detergent and water, and dry completely. It also warns that absorbent or porous materials, such as ceiling tiles and carpet, may have to be thrown away if they become moldy. TDLR says that merely applying a chemical such as bleach to drywall, without removing the mold source, is not a permanent effective solution. For more on drywall, see mold on walls, and for framing and shelving, mold on wood.

If you use bleach on concrete, the CDC says no more than 1 cup of household laundry bleach in 1 gallon of water, and never mix bleach with ammonia or other cleaners.

4. Protect yourself

For areas under 10 square feet, the EPA's minimum is gloves, eye protection, and an N-95 respirator. The CDC says people with asthma or who are allergic to mold may have severe reactions, so they shouldn't do the cleanup. If you're worried about symptoms, see a doctor; a mold test can't diagnose illness. TDLR recommends that people consult a health care provider about health concerns.

5. Don't paint over it

The EPA says do not paint or caulk moldy surfaces. Clean and dry first.

Table: Basement mold removal cost, national estimates. No source gave Texas-only prices.

Fixr says professional basement work includes scrubbing down affected areas, drying out the space using dehumidifiers and air scrubbers to remove spores. Repairs are extra: Fixr says repairing mold damage costs between $75 and $3,000, in addition to the remediation costs. See our mold remediation cost guide for more.

• More than about 10 square feet. The EPA says that if mold growth covers more than 10 square feet, consult its professional remediation guidance.
• 25 contiguous square feet or more. TDLR says that in most instances areas of visible mold less than 25 contiguous square feet in area may be cleaned or removed by people who are not licensed. Larger jobs generally need a licensed remediation contractor, though owners of residential buildings with fewer than 10 dwelling units are exempt for their own property.
• Floodwater or sewage. The EPA says to call a professional if damage was caused by sewage or other contaminated water.
• A smell with no visible mold. The EPA says if you believe that you may have a hidden mold problem, consider hiring an experienced professional.

Texas law says a license holder may not perform both mold assessment and mold remediation on the same project. Use a mold testing company to find hidden mold, and a separate mold removal company to do the work. Find licensed companies in Houston, Austin, Lubbock or anywhere in Texas, verify any license with our license check, and see how we verify listings. Not sure testing is worth it? Read is mold testing worth it.