Musty Smell in the Basement: What Causes It and How to Get Rid of It for Good

A musty basement is a damp basement. The EPA explains that growing mold gives off strong-smelling compounds behind the "moldy odor," and that a moldy odor suggests mold is growing and should be investigated. Mold won't grow without moisture, the EPA adds, so the lasting fix is finding the dampness: humid air condensing on cool concrete, water getting in from outside, or stored things that got wet.

A musty smell blowing out of the vents points to the cooling system instead; see clogged condensate drain line. A rotten-egg smell near a floor drain is usually a dry drain trap, covered below. And a musty smell says nothing about radon: the EPA says you can't see or smell it and that testing is the only way to know your level. Our radon test kit guide shows how.

Most Likely Causes

1. Humid air and condensation. The EPA says to keep indoor humidity below 60 percent, ideally 30 to 50; the CDC says no higher than 50. NDSU Extension says musty basement odors can come from water vapor moving through concrete from moist soil, and that a damp wall may just be condensation on the cool surface. University of Minnesota Extension adds that warm, humid summer air from open basement windows condenses on cool walls and floors, and many homeowners mistake it for a leak. The EPA counts condensation on walls or pipes as a sign of high humidity.
2. Water getting in from outside. The EPA notes that water can leak in from outside or seep through basement floors. Minnesota Extension names the usual causes: ground that's level or slopes toward the house, and missing gutters or downspouts without extensions, which drop the roof's water in one spot by the wall. Concrete foundations usually develop some cracks, it adds, and repair may be necessary.
3. Mold on stored things and carpet. The CDC says mold grows on paper, cardboard, wood, carpet, fabric, and upholstery. NDSU Extension warns that boxed articles stay wet once they get damp, and Minnesota Extension says moisture seeping through the foundation can make the bottom of carpets moldy.
4. An open or damp sump pit. Minnesota Extension explains that warm air rising through a house pulls moist air into the basement through foundation openings, open sump pits included, so sumps should have an airtight cover.
5. A dry floor drain trap: a sewer smell, not a musty one. Each drain's trap holds water to keep sewer gas out. The City of Kerrville, Texas, explains that in a rarely used basement floor drain, that water evaporates until the seal is gone. Sewer gas contains hydrogen sulfide, which Wisconsin's Department of Health Services says smells like rotten eggs, and it can enter through a floor drain. A musty smell is the "musty, earthy odor" that UGA Extension ties to mold. A smell that's strongest at the drain and fades once the trap is refilled was the trap.
6. A dryer venting indoors. The U.S. Fire Administration notes that codes require dryers to exhaust directly outdoors, and that installations venting into a crawl space or interior walls can cause mold buildup. On a gas dryer it's a carbon monoxide question too: the CPSC calls any fuel-burning appliance a potential CO source and lists a loose or disconnected vent among the clues to a CO problem.
7. A wet crawl space next door. The EPA says humid crawl spaces are common sites of hidden mold, particularly with a bare earth floor, and their moisture can pass into the rest of the building through cracks in walls, floors, and ceilings. See missing crawl space vapor barrier.

How to Find the Source

• Sniff, and note when. Minnesota Extension says a musty smell is sometimes your only clue to mold, so smell walls, carpet, and boxes. It ties dampness after rain to water getting in from outside, and dampness in humid summer weather to condensation.
• Measure the humidity with a hygrometer, a small meter the EPA says many hardware stores sell. UGA Extension says to consider a dehumidifier once the reading tops 50 percent.
• Tape down a square of plastic. NDSU Extension suggests taping a 3-foot-square plastic sheet to the concrete for a couple of days to see whether moisture vapor is coming through the floor; in University of Nebraska–Lincoln Extension's checklist, moisture collecting under the plastic means it is.
• Check what sits on the floor. UGA Extension suggests lifting a carpet corner to look for mold underneath, and Nebraska's checklist asks whether boxes on the floor are damp on the bottom.
• Test white fuzz. NDSU Extension says white, fluffy deposits on basement walls and floors may be mineral salts. Put a few drops of water on it, then vinegar; if it dissolves in neither, it may be mold.
• Smell it but can't see it? The EPA says to suspect hidden mold behind drywall or paneling, on top of ceiling tiles, or under carpet and pad. Disturbing it, such as by pulling off wallpaper, can release a massive amount of spores, the EPA warns, so consider an experienced professional.

What You Can Fix Yourself

Dry Out the Air

Close up first. Minnesota Extension says not to ventilate a basement with warm, humid air in summer, and ENERGY STAR says to keep doors and windows closed while a dehumidifier runs. ENERGY STAR sizes the unit by floor area and dampness, and its chart puts a space that "feels damp and has musty odor that may be intermittent" at 50 to 75 percent humidity; our basement dehumidifier guide walks through it. Set the humidistat at 50 percent or a little below, inside both the EPA's and the CDC's numbers.

ENERGY STAR says most portable units collect water in a bucket and shut off when it's full, and some take a hose to a floor drain or sump instead. UGA Extension says to empty and clean the drip pan regularly, so the unit doesn't become what the EPA calls a source of biological pollutants. ENERGY STAR also says to use a properly grounded outlet and keep drain hoses away from cords.

For sweating pipes, the EPA says to cover cold-water pipes with insulation. Know the limit, though: Minnesota Extension says a dehumidifier reduces the symptoms of humidity and odor but isn't a permanent fix when water is getting in.

Stop Water at the Source

ENERGY STAR's first steps are outside: extend downspouts away from the foundation, keep gutters and downspouts clear, make sure the soil slopes away, and don't overwater foundation plantings. The Department of Energy's Building America Solution Center says downspouts should end at least 5 feet from the foundation. Our pages on downspouts draining near the foundation and negative grading cover the fixes, and foundation cracks explains which cracks need an engineer. Reconnect a loose dryer duct too; the EPA, the CDC, and ENERGY STAR all say to vent clothes dryers outdoors.

Refill a Dry Floor Drain Trap

Kerrville's fix: pour about a quart of water into the floor drain, and into any sink, tub, or shower you rarely use. For a trap that keeps drying out, it suggests adding ¼ cup of mineral oil to slow evaporation. Wisconsin's health department recommends flushing floor drains with water so the traps don't dry out.

Cover the Sump Pit

Fit the airtight cover Minnesota Extension calls for, and confirm the pump still runs with it on; our guide to testing a sump pump covers the bucket test.

Clean Small Mold the EPA Way

The EPA's line is about 10 square feet, roughly a 3-by-3-foot patch: under that, in most cases, you can handle the job yourself. Fix the moisture too; the EPA says that if you don't, the mold will most likely come back.

• Gear: the EPA suggests an N-95 respirator, gloves that reach the middle of the forearm, and goggles without ventilation holes.
• Hard surfaces: scrub with detergent and water, then dry completely (EPA). The EPA doesn't recommend bleach as a routine practice; if you use it, the CDC says no more than 1 cup of household laundry bleach per gallon of water, with windows and doors open.
• Never mix bleach with ammonia or other cleaners. The CDC says that produces a poisonous gas.
• Don't paint or caulk over mold. The EPA says paint over moldy surfaces is likely to peel.
• Throw out porous things that stay moldy. The EPA says porous materials such as ceiling tiles and carpet may have to go, because the mold may be difficult or impossible to remove completely, and NDSU Extension says to discard carpet that grew mold.
• Know when you're done: the EPA says visible mold and moldy odors should be gone, with no new water damage or mold when you check back.

When to Call a Pro

• Mold over about 10 square feet or a lot of water damage. The EPA says to hire a contractor with experience cleaning up mold and to check references.
• Suspected hidden mold behind finished walls or under flooring (EPA).
• Sewage or other contaminated water: the EPA says to call a professional experienced with contaminated-water damage.
• A sewer smell that returns with the traps full. Wisconsin's health department says a leak behind a wall may need a plumber, and Kerrville says a plumber can smoke-test the drains. Call a licensed plumber.
• Water still getting in after gutters, downspouts, and grading are fixed. Minnesota Extension's next step is an interior or exterior drainage system, a waterproofing contractor's job.
• A musty smell from the vents. The EPA says not to run an HVAC system you suspect is contaminated with mold; call an HVAC technician.

Prevent It

• Watch the number: a hygrometer in the basement, and humidity below 60 percent, ideally 30 to 50 (EPA).
• Dry spills fast. The EPA says areas dried within 24 to 48 hours of a leak or spill, in most cases, won't grow mold.
• Store in plastic. University of Missouri Extension says solid plastic containers, rather than cardboard boxes, protect stored things from moisture.
• Skip wall-to-wall carpet. The CDC says not to use carpet in basements that may have a lot of moisture; Minnesota Extension suggests a painted floor with area rugs you can take outside to dry.
• Don't finish a damp basement. Minnesota Extension says not to finish the walls with insulation and wallboard unless the basement is very dry.

Frequently Asked Questions

Does a musty basement mean I have radon?

No, and a basement that smells fine can still have it. The EPA says radon is invisible, that you can't see or smell it, and that all homes should be tested, musty or not.

Should I have the basement tested for mold?

Usually not. The EPA says sampling is unnecessary in most cases when mold is visible, and the CDC doesn't recommend mold testing: whatever the type, you remove it and fix the moisture.

Will a dehumidifier get rid of the smell for good?

Only if humid air was the whole problem. If water is getting in, Minnesota Extension says, a dehumidifier treats the symptoms. And drying the air doesn't remove mold already growing; the EPA says dead mold may still cause allergic reactions, so it must be removed, not just killed.

Why does the basement smell worse in summer?

NDSU Extension says ventilating with warm summer air typically raises a basement's humidity, so keep the windows closed in humid weather; the CDC says an air conditioner or dehumidifier will help keep humidity low.

The Bottom Line

A musty smell means moisture, and usually mold. Measure the humidity, then check boxes, carpet, the sump pit, the dryer vent, and the crawl space. Dry the air to 50 percent or a little below, fix the drainage outside, and clean patches under 10 square feet with detergent and water. Bigger growth, hidden mold, sewage, or water you can't stop belong to a pro.

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