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Quick answer
A musty smell in the basement usually means something is damp: humid air on cool concrete, water seeping through walls or the floor, an open sump pit, or mold growing on stored boxes, carpet or the back of finished walls. [4][6] Measure the humidity first and bring it below 60 percent, ideally 30 to 50 percent, with a dehumidifier and closed windows. [3] Then look for the wet spot. If the smell stays after the air is dry, or seems to come from inside finished walls, EPA says to consider an experienced professional for hidden mold. [3]
Do this first: measure the air and rule out gas
Rotten egg is a different smell
A musty, earthy smell points to dampness and mold. Hydrogen sulfide in sewer gas smells like rotten eggs, and Wisconsin's health department says to evacuate and call the fire department if you believe there are high concentrations of sewer gas in an enclosed space. [1] If you smell gas or suspect a gas leak, CDC says to leave right away, contact emergency authorities and not do anything that could cause a spark. [2]
Buy or borrow a humidity meter. EPA says they cost about $10 to $50 at many hardware stores. [3]
Take a reading in the basement and one upstairs. Above 60 percent in the basement is a problem on its own. [3]
Close basement windows in humid summer weather. University of Minnesota Extension says warm, humid outside air condenses on cool basement walls and floors, and homeowners often mistake that for a leak. [4]
Run a dehumidifier with doors and windows to the basement closed. [5]
Look for standing water, wet spots on the floor, damp bands at the base of walls, and water in the sump pit. If water is near outlets or the electrical panel, do not walk into it; turn power off at the panel only from a dry spot, or call your utility or an electrician.
Check stored things on the floor: cardboard boxes, books, rugs and clothing. A 2015 federal guide from EPA, CDC, FEMA, HUD and NIH, written for cleanup after floods and other disasters, says you may see or smell mold on cardboard boxes or books. [6]
If the smell fades within a few days once the humidity drops, damp air was likely a big part of the problem. If it stays, keep going: something is wet or moldy that you have not found yet.
Why do basements smell musty?
A musty smell often comes from mold. EPA says molds release compounds called mVOCs that can be the source of the musty odor linked to mold growth, and that a moldy odor suggests mold is growing and should be investigated. [7] Basements are easy places for it because they sit against damp soil and stay cool.
Condensation. Humid air meets cool concrete and leaves moisture on walls and floors, especially in summer. [4]
Moisture through the concrete. Water can be drawn up through pores in the footing and slab, which UMN says creates the damp ring at the base of many basement walls. [4]
Rain and groundwater. Poor grading, short downspouts and failed drain tile let liquid water in. [4]
Open sump pits. The house draws moist air in through foundation openings, including open sump pits, which is why UMN says a sump should have an airtight cover. [4]
Finished spaces. UMN notes molds and mildew can grow in damp carpets and beneath wall coverings, and that most basements in Minnesota connect to the rest of the house through ductwork or other openings. [4]
ENERGY STAR describes a slightly to moderately damp space, one with a musty odor that may come and go, as typically 50 to 75 percent relative humidity. [5]
Where to look in a basement
Work around the basement with a flashlight, starting where the smell is strongest.
Common sources of a musty basement smell, what to check and the usual fix
Source
What to check
Usual fix
Walls and floor
Damp spots, a damp band at the base of walls, peeling paint, white mineral deposits
Tell seepage from condensation with a foil or plastic test, then fix outside drainage or humidity
Sump pit
An open pit, standing water, a pump that is not cycling
An airtight, childproof cover and a working pump
Floor drain
A drain that is rarely used; a sewer odor
Pour water in so the trap does not dry out
Stored boxes, books and fabric
The bottoms of cardboard boxes and anything stored on the floor
Throw out moldy porous items; store things off the floor in a dry space
Carpet over the slab
Lift a corner and look at the underside and pad
Remove moldy carpet and pad; UMN says slabs must be warm and dry before carpeting
Finished walls
Stains, swelling or a smell strongest at outside walls and baseboards
Find the water first; hidden mold calls for caution
Furnace or air handler
Standing water or debris in the condensate pan, wet insulation near the coil
Fix the drain; do not run a system you suspect has mold
The dehumidifier itself
A bucket or drip pan that sits full
Empty and clean it regularly, or drain it by hose
Laundry area
Leaking washer lines, a dryer that vents indoors
Fix leaks; vent the dryer outdoors
Table sources: University of Minnesota Extension, [4] Natural Resources Canada, [8] Wisconsin Department of Health Services, [1] CDC, [6] NC State Extension, [9] EPA [10][3] and ENERGY STAR. [5] The page on water coming through the basement floor explains the foil and plastic test.
Do not tear into finished walls to chase the smell
EPA says investigating hidden mold requires caution when it means disturbing places where mold may be growing. For example, removing wallpaper can lead to a massive release of spores if mold is growing underneath. If you believe you have a hidden mold problem, EPA says to consider hiring an experienced professional. [3]
What humidity should a basement be, and how to get it there
EPA says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent. [3] ENERGY STAR calls 30 to 50 percent the optimum and says anything above that range may promote mold growth. [5] CDC sets a tighter line: keep humidity as low as you can, no higher than 50 percent. [11]
ENERGY STAR's dehumidifier sizing guide by basement condition
Condition without a dehumidifier
Typical humidity
Pints per day, space under 2,000 sq ft
Pints per day, over 2,000 sq ft
Slightly to moderately damp: feels damp, musty odor that may come and go
50 to 75 percent
20 to 30
30 or more
Very damp: consistently feels and smells damp, damp spots on walls and floors
75 to 90 percent
25 to 40
40 or more
Wet: walls or floor sweat, or seepage is present
90 to 100 percent
30 to 50
50 or more
Set the humidistat. ENERGY STAR dehumidifiers have a built-in humidistat that cycles the unit to hold the level you choose. If yours does not show humidity, hang a separate hygrometer nearby. [5]
Drain it automatically if you can, with a hose to a floor drain or the sump. [5]
Keep basement doors and windows closed while it runs. [5]
Watch the temperature. If the basement is usually below 65 degrees F, ENERGY STAR suggests a model rated for lower temperatures, because frost can form on the coils. [5]
Cut moisture at the source: extend downspouts, keep gutters clear, slope soil away from the foundation, and vent the dryer outdoors. [5]
A dehumidifier treats the air, not the water. UMN Extension says it can reduce humidity and odor symptoms but is not a permanent solution if water is getting into the basement, and that drying the air can pull moisture through the concrete faster. [4]
Cleaning up the mold you find
CDC says that if you see or smell mold, you should remove it, and that you do not need to know the type, since all molds are treated the same for removal. [11][12] Fix the moisture first, or EPA says the mold will most likely come back. [3]
Safe to do yourself
EPA says that if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, in most cases you can handle the job yourself. [3] Wear an N-95 respirator, gloves and goggles without vent holes, scrub hard surfaces such as concrete with detergent and water, and dry them completely. [13][14] Throw out moldy cardboard and other porous items. EPA says absorbent materials may have to be thrown away if they become moldy. [3] Do not paint over mold. [3]
People with asthma, allergies, a weakened immune system or chronic lung disease should not take part in mold cleanup, according to CDC. [14] If anyone in the home has symptoms or health concerns, talk to a doctor.
When to call a mold remediation company
The smell stays after the air is dry. EPA says you may suspect hidden mold when a building smells moldy but you cannot see the source, and to consider an experienced professional if you believe you have hidden mold. [3]
Finished walls, ceilings or carpet are involved. EPA lists the back side of drywall and the underside of carpets and pads among hidden mold spots. [3]
Mold covers more than about 10 square feet. EPA points larger jobs to its remediation guide for schools and commercial buildings, which it says applies to other buildings too. [3]
The basement flooded or had a sewer backup. EPA says to call in a professional with experience cleaning buildings damaged by contaminated water. [3] Treat it as Category 3. [15]
The HVAC system may be involved. EPA says not to run a system you know or suspect is contaminated with mold, and that wet or moldy duct insulation should be removed and replaced. [3][10]
For budgeting, HomeAdvisor's June 2026 cost guide puts mold remediation at an average of $2,368, with a typical range of $1,223 to $3,757. [16] Insurance usually does not cover mold unless it comes from water damage the policy covers, says Washington's insurance commissioner, and some insurers offer limited mold coverage. [17] Check your own policy. Renters: EPA says to report moisture problems to the building owner or manager right away. [3]
Common questions
Why does my basement smell musty but I cannot see mold?
The air may be damp enough to smell musty, or mold may be growing where you cannot see it, such as under carpet, inside boxes or behind finished walls. EPA says a moldy odor suggests mold is growing and should be investigated. [7] Check the humidity first, then the hidden spots. The page on musty smells with no visible mold covers the rest of the house.
Will a dehumidifier get rid of a musty basement smell?
Often it helps a lot, especially when condensation is the cause. UMN Extension says a dehumidifier can reduce humidity and odor symptoms, but it is not a permanent fix if water is getting in through walls or the floor. [4] Moldy items still need to be cleaned or thrown out.
What humidity should a basement be?
EPA says below 60 percent, ideally 30 to 50 percent. [3] ENERGY STAR also calls 30 to 50 percent the optimum and says higher levels may promote mold growth. [5]
Why does my basement smell worse in summer?
Warm, humid summer air condenses on cool basement walls and floors, which UMN Extension says homeowners often mistake for a leak. It advises not ventilating the basement directly with warm, humid outside air and recommends ventilating through an air conditioning system instead. [4] EPA also lists air conditioners and dehumidifiers among ways to reduce humidity. [3]
Can a sump pump or floor drain make a basement smell?
Yes. UMN Extension says the house can draw moist air in through an open sump pit, so sumps should have an airtight cover. [4] Wisconsin's health department says sewer gas can enter a home through a floor drain and suggests flushing floor and sink drains with water so the traps do not dry out. [1]
Is a moldy basement smell harmful?
CDC says damp and moldy environments may cause a variety of health effects, or none at all, and that people with asthma or mold allergies may have severe reactions. [11] If you have symptoms or health concerns, talk to a doctor, and fix the moisture either way.
