Quick answer
A musty smell usually means something is damp and mold or mildew is growing, even if you can’t see it. The EPA explains that molds release volatile compounds with strong odors and that a moldy odor suggests mold is growing and should be investigated. Follow your nose to the strongest spot, then check the usual suspects: basements and crawl spaces, closets on exterior walls, under sinks, behind furniture, carpets that got wet, air conditioner drip pans and humidifier tanks. Fix the moisture, clean or remove what’s moldy, and the smell goes with it.
What a musty smell is
The EPA’s mold course explains that some compounds produced by molds have strong smells and are volatile, meaning they’re quickly released into the air. These microbial volatile organic compounds (mVOCs) can be the source of the moldy or musty smell associated with mold growth. The EPA’s conclusion is simple: a moldy odor suggests that mold is growing in the building and should be investigated.
The smell is useful: it often shows up before mold is visible. The EPA suggests you may suspect hidden mold if a building smells moldy but you can’t see the source. It lists typical hiding places: the back side of drywall, wallpaper or paneling, the top of ceiling tiles, the underside of carpets and pads, inside walls around leaking or condensing pipes, inside ductwork, and roof materials above ceilings.
Where musty smells usually come from
| Place | Why it gets damp | What to check |
|---|---|---|
| Basement or crawl space | Cool surfaces, ground moisture, seepage; the EPA notes crawl spaces with high humidity are common sites of hidden mold | Damp walls, stored boxes, wood framing, humidity reading |
| Closets on exterior walls | Cold wall, no air movement | Back wall, corners, stored shoes and clothes |
| Under sinks and behind appliances | Slow leaks at drains, supply lines, dishwashers, fridges | Cabinet floor, wall behind, soft or stained wood |
| Behind furniture on exterior walls | The EPA lists walls behind furniture, where condensation forms, as a hidden mold site | Pull furniture out and look at the wall |
| Carpets and pads that got wet | Absorbent materials hold water | Underside of carpet and pad after any spill or leak |
| Air conditioning | Standing water in drip pans or clogged drain lines | Drip pan clean, drain line flowing |
| Humidifiers | Microorganisms can grow in tanks with standing water | Tank and surfaces; clean per EPA guidance |
How to find the source, step by step
- Map the smell. Walk the house after it has been closed up for a few hours, such as first thing in the morning. Note where the smell is strongest, room by room.
- Measure humidity in the rooms that smell. The EPA recommends below 60%, ideally 30–50%; a room that sits higher is a strong lead.
- Open things up. Closets, sink cabinets, the space behind the washer and dishwasher, storage boxes against basement walls.
- Look for water history. Stains, bubbling paint, soft drywall, warped floors or rusty stains point to past or ongoing leaks.
- Check the HVAC and appliances. The EPA advises keeping air conditioning drip pans clean and drain lines unobstructed; also look inside humidifiers and dehumidifier buckets.
- Check the crawl space or basement with a flashlight: wet soil, puddles, damp wood.
Watch out: the EPA cautions that investigating hidden mold can be difficult and that disturbing possible growth sites, such as removing wallpaper with mold behind it, can release a lot of spores. If you believe mold is inside walls or ceilings, the EPA suggests considering an experienced professional.
Should you test the air for mold?
Usually not. The EPA says that in most cases, if visible mold is present, sampling is unnecessary, and that no EPA or other federal limits have been set for mold or mold spores. The CDC does not recommend mold testing and says that whatever the type, mold should be removed. Spend the effort finding and fixing the moisture instead.
Not every bad smell is mold. A rotten-egg or sewer smell points to plumbing, such as a dry drain trap in a rarely used sink or floor drain; running water into it for a minute refills the trap. A gas smell is an emergency: leave the house and call your gas utility or 911 from outside.
Getting rid of the smell for good
Air fresheners hide the smell; they don’t remove the cause. The EPA’s guidance comes down to three steps:
Fix, clean, dry
- Fix the water problem: leaks, seepage, condensation or high humidity.
- Clean or remove moldy materials. Scrub hard surfaces with detergent and water and dry them. Absorbent materials such as ceiling tiles and carpet may need to be thrown away if moldy.
- Dry and ventilate. Dry wet materials within 24–48 hours, run exhaust fans, and use a dehumidifier where humidity stays high.
- Check the result. The EPA says cleanup isn’t finished until the moisture problem is fixed and visible mold and moldy odors are gone.
Worked example: a smelly closet
Assumed situation: a hall closet on an exterior wall smells musty in winter. The hygrometer reads 45% in the hallway and 68% inside the closed closet.
- The 23-point difference means the closet traps moisture against a cold wall.
- Empty it and inspect the back wall: small spots on the paint near the floor.
- Clean the spots, leave the door open for a few days, and store items a few inches off the back wall.
- Recheck after a week. If the closet stays close to the hallway reading and the smell is gone, the fix worked.
Illustrative example with assumed readings.
Health: the EPA notes that exposure to mVOCs has been linked to symptoms such as headaches, nasal irritation, dizziness, fatigue and nausea, while more research is needed on indoor exposures. The CDC notes that people with asthma or mold allergies may react severely. If anyone in the household has symptoms, talk to a doctor; this guide doesn’t give medical advice.
Frequently asked questions
Does a musty smell always mean mold?
Usually it means dampness and mold or mildew growth somewhere. The EPA says a moldy odor suggests mold is growing and should be investigated.
Why does my house smell musty only in summer?
Humid summer air raises indoor humidity and condenses on cool basement and crawl space surfaces. Keep humid air out, run the AC or a dehumidifier, and check the basement.
Will a dehumidifier get rid of a musty smell?
It helps dry the air and slows new growth, but existing mold still needs to be cleaned or removed, and leaks still need fixing.
Should I hire a mold inspector?
Consider a professional if you suspect mold inside walls or ceilings, or after water damage. The EPA advises that any sampling be done by experienced professionals.
Sources
- U.S. EPA. Mold Course, Chapter 1: moldy smell and mVOCs.
- U.S. EPA. A Brief Guide to Mold, Moisture and Your Home: hidden mold, sampling, cleanup, AC drip pans.
- U.S. EPA. Mold Course, Chapter 2: crawl spaces, humidity.
- U.S. EPA. Use and Care of Home Humidifiers.
- CDC. About Mold.
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