Musty basement smell: what's causing it and how to get rid of it
Last reviewed · Not medical advice
Basements smell musty because they're below ground: concrete wicks groundwater in, and in summer, humid air condenses on the cool walls and floor. Cardboard, carpet, and anything stored on the slab soaks that up and grows mildew. Get the humidity down with a dehumidifier, get stored stuff off the floor, and do the plastic-sheet test. It tells you whether water is coming through the concrete (a drainage problem) or condensing on it (a humidity problem). They have different fixes.
Most likely causes
Ranked for this situation. Open any cause for checks and fixes.
- 1
Damp basement or foundation seepage
most commonWithin daysSeepage and condensation on below-grade walls is the default explanation for a musty basement.
Points to this:- Musty smell starts in the basement and drifts upstairs
- White chalky deposits (efflorescence), damp spots, or water lines on walls
- Worse after rain or in humid months
Check it, fix it, who to call
Safe checks- Tape a 1-foot square of plastic sheet to the wall or floor for 48 hours. Moisture under it means water is coming through the concrete; moisture on top means humid air is condensing on it.
- Check that gutters and downspouts discharge well away from the foundation.
Fix- Fix grading and gutters first — it's cheap and often enough.
- Run a basement dehumidifier.
- Get stored items off the floor and out of cardboard.
DIY stops here: Active seepage or standing water needs a waterproofing contractor.
Who fixes it: Basement waterproofing contractor
More on this - 2
Indoor humidity too high
most commonBasements often sit well above 60% humidity in summer even with no leaks at all.
Points to this:- Smell everywhere, worse in humid weather or when the house is closed up
- Condensation on windows; clammy air; towels that never dry
Check it, fix it, who to call
Safe checks- Put a hygrometer (an inexpensive humidity gauge) in the smelly area for 24 hours.
- EPA guidance: keep indoor humidity below 60%, ideally 30–50%.
Fix- Run a dehumidifier, and AC in summer.
- Use bathroom and kitchen exhaust fans; vent the dryer outdoors.
- Wash or dry-clean fabrics that picked up the smell.
DIY stops here: If you can't get below 60% with a dehumidifier, find the moisture source (basement, crawlspace, leaks).
Who fixes it: You (DIY)
More on this - 3
Stored items that absorbed dampness
commonCardboard boxes, old books, and upholstered furniture on the slab are mildew sponges.
Points to this:- Smell is strongest near storage
- Old furniture or books smell musty up close
Check it, fix it, who to call
Safe checks- Move suspect items out of the room for a few days and see if the room improves.
Fix- Move storage into plastic bins, off the floor.
- Air out, clean, or discard items that keep smelling.
DIY stops here: None — this is a sorting job.
Who fixes it: You (DIY)
More on this - 4
Sewage ejector or sump pit
commonWithin daysA dry or stagnant sump pit, or a poorly sealed ejector lid, smells musty to sewer-ish.
Points to this:- Smell centered on a basement pit with a lid
- Pump runs constantly or not at all
Check it, fix it, who to call
Safe checks- Check that the ejector lid is bolted down with its gasket in place.
- Pour water into a dry sump pit to see if the smell eases.
Fix- Reseal the lid; make sure the pit vent runs outdoors.
- Have a plumber service or replace a failing ejector pump.
DIY stops here: Opening a sewage ejector pit or servicing its pump is a plumber's job.
Who fixes it: Plumber
More on this - 5
Carpet or pad that got wet
commonWithin daysCarpet laid directly on a slab stays damp from below — finished basements are prone to it.
Points to this:- Smell strongest at floor level in one area
- Worse on humid days or after carpet cleaning
Check it, fix it, who to call
Safe checks- Press a dry paper towel hard into the carpet — dampness shows up.
- Lift a corner at the tack strip and check the pad and subfloor.
Fix- Dry the area fully with fans and a dehumidifier.
- Replace pad that stayed wet more than a day or two, or that is stained.
DIY stops here: Large areas or any sewage-contaminated carpet go to a restoration company.
Who fixes it: Water damage / restoration company
More on this - 6
Mold or mildew growing on surfaces
less commonWithin daysOn drywall, wood framing, and the backs of stored items, rather than on bare concrete.
Points to this:- Black, green, white, or gray speckled growth
- Growth near a moisture source
Check it, fix it, who to call
Safe checks- Measure the affected area.
- Under about 10 square feet (roughly 3 ft × 3 ft), EPA guidance says most homeowners can clean it themselves.
Fix- Fix the moisture first.
- Scrub hard surfaces with detergent and water, and dry completely (EPA).
- Porous materials like ceiling tiles and carpet that have gone moldy may have to be thrown away.
DIY stops here: More than about 10 sq ft, mold after significant water damage, or mold you suspect is in the HVAC system: bring in a professional.
Who fixes it: Mold remediation company
More on this - 7
Dried-out drain trap
less commonA basement floor drain that has dried out — more sewer than musty.
Points to this:- Smell is near a guest bath, basement floor drain, unused shower, or utility sink
- Started after a trip, or in a room nobody uses
- Goes away after running water
Check it, fix it, who to call
Safe checks- Run water in every sink, tub, and shower for 30 seconds, and pour a bucket of water into floor drains.
- Flush toilets that rarely get used.
- Wait a few hours. If the smell is gone, it was a dry trap.
Fix- Run water in rarely used fixtures every couple of weeks.
- For floor drains that dry out constantly, a plumber can fit a trap primer or a trap-seal insert.
DIY stops here: If refilling the traps doesn't stop it within a day, the problem is past the trap — call a plumber.
Who fixes it: Plumber
More on this - 8
Front-load washer gasket or drum
less commonIf the laundry lives down there.
Points to this:- Clothes smell musty right out of the washer
- Dark residue in the folds of the door gasket
Check it, fix it, who to call
Safe checks- Pull back the gasket folds and look; smell the detergent drawer.
Fix- Clean the gasket and drawer; run the washer's cleaning cycle.
- Leave the door and drawer open between loads.
DIY stops here: Persistent smell after cleaning can be a slow drain or the drain pump — appliance repair.
Who fixes it: Appliance repair
More on this
How we rank causes: by how often each one explains this smell in ordinary homes, according to plumbing, HVAC, electrical, and building-science guidance, adjusted for the context on this page. The labels are judgments, not measured statistics. Anything dangerous that can't be ruled out (gas, fire, electrical) is covered in the safety box at the top of the page, whatever its rank.
Which one is it?
| What you see | What it means | Fix direction |
|---|---|---|
| Damp under the plastic-sheet test | Water moving through the concrete | Gutters, grading, drainage, waterproofing |
| Damp on top of the plastic sheet | Humid air condensing on cold concrete | Dehumidify, insulate, air-seal |
| White chalky crust on the walls | Minerals left by water moving through | Same as seepage — fix the water |
| Water lines or puddles after rain | Active leaking | Downspouts first, then a waterproofing contractor |
| Smell centered on a pit with a lid | Sump or ejector pit | Reseal or refill it |
Narrow it down
Answer in order. Stop at the first yes.
- 1Is basement humidity over 60% on a hygrometer?Yes → Run a dehumidifier sized for the space, set to 50%, draining to a floor drain or pump. Most of the smell often goes with it.No → Humidity alone isn't it. Keep going.
- 2After 48 hours, is there moisture under the taped plastic sheet?Yes → Water is coming through the concrete. Start outside: downspouts, gutters, grading.No → If moisture is on top of the sheet, it's condensation — dehumidify and stop warm, humid air reaching cold surfaces.
- 3Is the smell sewer-like, or centered on a drain or pit?Yes → Check the floor drain trap and the pit lids. Go thereNo → Keep going.
- 4Is the basement finished with carpet or drywall over the concrete?Yes → Check the carpet pad and the bottom of the drywall. Finished surfaces hide the moisture.No → Clear stored items off the floor and re-check in a week.
Safe checks you can do yourself
In the order that finds the source fastest.
- 1
Plastic-sheet test (48 hours)
Tape a 1-foot square of plastic sheet tightly to the wall and another to the floor. Moisture underneath means water is wicking through; moisture on top means condensation.
- 2
Walk the outside after rain
Check that downspouts discharge several feet from the foundation, gutters aren't overflowing, and soil slopes away from the house.
- 3
Hygrometer for a week
Leave a hygrometer in the basement and note the readings in the morning and evening. Above 60% means humidity control is part of the fix (EPA guidance).
- 4
Inventory the floor
Look at what's sitting directly on the slab: cardboard, rugs, furniture, paper. Smell the bottoms.
- 5
Check the pits and drains
Pour a bucket of water into the floor drain. Check that the sump pit isn't bone dry and the ejector lid is sealed and bolted.
How to get rid of the musty smell in a basement
Work from the outside in. Outside: downspout extensions and grading are the cheapest fix and often remove most of the water. Inside: a dehumidifier held around 50%, with storage moved onto shelves or into plastic bins. Then clean what grew: scrub hard surfaces with detergent and water and dry them (EPA); discard moldy cardboard and carpet pad.
If the plastic-sheet test still shows water coming through after the outside fixes, or water shows up after every storm, call a waterproofing contractor. Interior drainage, sump systems, and wall treatments are their work. Painting a sealer over damp walls on its own rarely stops seepage.
When DIY stops
- Standing water or water that appears after every rain.
- Cracks that leak, or foundation walls that bow or shift.
- Mold over more than about 10 square feet, especially on finished walls (EPA's DIY threshold).
- A sewage ejector pit that needs opening or pump work.
Who to call
- Basement waterproofing contractor
Water comes through the walls or floor, or the sump system can't keep up.
Foundation seepage, drainage, sump systems, and crawlspace encapsulation.
- Plumber
A floor drain, sump, or ejector pit is the source.
Drains, traps, toilets, vents, water heaters, and leaking supply lines.
- Mold remediation company
Large growth on finished walls or framing.
Contains and removes mold growth too large or too hidden to handle yourself.
- Home inspector
You want an independent read before paying for waterproofing.
A whole-house look when you can't localize a problem.
Answer a few questions about where and when you smell it. You'll get the causes ranked for your situation.
Frequently asked questions
Is a musty basement normal?
It's common, but it's not something to live with. It means moisture, and moisture is what lets mold grow on anything stored or finished down there. Most basements can be kept smell-free with drainage fixes and a dehumidifier.
What's the fastest way to get rid of a musty basement smell?
Run a dehumidifier, get everything off the floor, throw out moldy cardboard, and open it up to air out on a dry day. That clears a lot within days, but it comes back unless the water source is fixed.
Does a musty basement smell mean there's mold?
Often, yes — on stored items, wood, or drywall. Bare concrete itself doesn't feed mold much, but dust and organic material on it does. If you can see growth, EPA says testing is usually unnecessary.
Why does my basement smell musty only in summer?
Warm, humid outdoor air reaches the cool basement walls and floor and condenses. That's a humidity problem more than a leak. Dehumidify and keep basement windows shut on muggy days.
Can the basement make the whole house smell musty?
Yes. Air rises from the basement into living spaces, and leaky ductwork in the basement can pull basement air into the HVAC system.
Related smells
Sources
- A Brief Guide to Mold, Moisture and Your Home — U.S. EPA
- Mold Cleanup in Your Home — U.S. EPA