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A musty smell in the house almost always means moisture and microbial growth somewhere: mold, mildew, or bacteria growing on something that stays damp. The odor comes from gases they release as they grow, which is why you can smell a problem long before you can see it. The fix is not an air freshener. It is finding the water.
This guide covers what a musty smell means, where to look room by room, what else can cause it, how to get rid of it, and who to call when you cannot find the source.
Musty, moldy, or sewer: what the smell tells you
- Musty: stale, damp, earthy. Often mildew or early growth on surfaces, or damp materials such as carpet pad, cardboard, or wood.
- Moldy: stronger and more pungent. Often established growth, sometimes inside a wall or under flooring.
- Sewer or rotten egg: usually not mold. A dry drain trap in a little-used sink, tub, or floor drain lets sewer gas into the room. Running water for a minute refills the trap. If it persists, call a plumber.
- Sour or "dirty socks" when the AC starts: often growth on the air conditioning coil or in the drain pan.
Why the house suddenly smells musty
A smell that appears all of a sudden usually has a recent cause:
- A new leak: a supply line, water heater, dishwasher, toilet seal, or roof leak after a storm
- The wet season or a stretch of humid weather: indoor humidity rises and damp materials start to smell
- A house that was closed up: a seasonal home, or a home left with the air conditioning off during a power outage
- A change in the air conditioning: a clogged condensate line, a new system that cools without running long enough to dehumidify, or a thermostat set higher than usual
- Water that was not fully dried: after a spill, a flood, or carpet cleaning
Where to look, room by room
The EPA lists the places hidden mold most often grows, including the back of drywall and paneling, the underside of carpets and pads, around pipes, above ceiling tiles, and inside air conditioning ducts and drip pans.
| Area | What to check |
|---|---|
| Air conditioning | The air handler closet, the drain pan, the condensate line, the coil if you can see it, and supply vents |
| Basement or crawl space | Wall bases, joists, insulation, standing water, and stored boxes |
| Closets | Exterior walls, the back wall behind clothes, and the floor; closets that share a wall with a bathroom or water heater |
| Bathrooms | Under the vanity, the base of the toilet, the ceiling above the shower, and the exhaust fan |
| Kitchen | Under the sink, behind the dishwasher and refrigerator |
| Laundry | The door gasket and drum of a front-loading washer, behind the washer, the dryer vent |
| Floors | Carpet and pad, especially over a slab or near exterior doors; soft spots in vinyl or wood flooring |
| Attic | Roof sheathing near vents and chimneys, damp insulation, bath fans venting into the attic |
| Furniture and storage | Old upholstered furniture, books, and boxes stored in damp rooms |
Follow your nose. A smell that is strongest in one room, one closet, or near one wall is usually close to the source. Pay attention to when it is worst: right after the air conditioning starts points to the system; after rain points to a leak or ground moisture; in the morning in a closed room points to humidity.
When the house smells musty but you see no mold
Some musty smells have no visible mold behind them:
- High humidity. Fabrics, carpet, and wood absorb moisture and smell stale. An inexpensive hygrometer tells you if indoor humidity is above 60 percent.
- A dirty air conditioning coil or drain pan. The odor spreads through the house when the system runs.
- A dry drain trap. Common in guest bathrooms and basement floor drains.
- Damp laundry or towels. Clothes left in the washer, or towels that never fully dry.
- Hidden growth. Inside a wall, under a vanity, or under flooring. If the smell keeps coming back after you have dealt with humidity and laundry, assume there is a source you have not found yet.
Not sure where it is coming from? A local mold inspector can trace it with a moisture meter, and the call to find one is free.
How to get rid of a musty smell
- Find and fix the moisture source. Repair the leak, clear the condensate line, or dry out the space. Without this step, the smell comes back.
- Lower the humidity. The EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Run the air conditioning or a dehumidifier, and use bathroom and kitchen exhaust fans.
- Improve airflow. Open closet doors, leave a gap between furniture and exterior walls, and do not pack closets tight.
- Clean or remove the source. Wash fabrics, clean hard surfaces with detergent and water, and throw away porous items such as carpet pad, cardboard, or ceiling tiles with mold growth. For small areas, wear an N-95 respirator, gloves, and goggles.
- Have the air conditioning serviced if the smell is strongest when it runs.
Baking soda, activated charcoal, and candles can soften odors, but they only mask the smell. They do nothing about the moisture or the growth causing it.
Is a musty smell dangerous?
The smell itself is a warning sign rather than the main problem, but it does mean that the air you breathe at home all day is carrying something from a damp spot. The CDC says mold can cause a stuffy nose, coughing or wheezing, burning eyes, or skin rash in some people, and people with asthma or mold allergies may react more strongly. If anyone in the home has symptoms that ease when they are away, mention the smell to a doctor. See black mold symptoms.
Who to call about a musty smell
| If the source looks like | Call |
|---|---|
| A leak under a sink, a toilet, or a water heater | A plumber |
| The air conditioning: smell when it runs, water around the air handler | An HVAC technician |
| A roof leak or stains on the ceiling | A roofer |
| You cannot find the source, or the smell keeps coming back | A mold inspector |
| Visible mold larger than about 10 square feet, or mold inside walls | A mold remediation company |
A mold inspector is the right call when the smell has no obvious cause. Inspectors check the likely places with a moisture meter and often a thermal camera, which can find damp areas behind drywall without opening walls, and they tell you whether you need testing at all. See what a mold inspection involves and how to test for mold.
Common causes by area
- Seasonal homes in Florida: a house closed for the summer with the air conditioning off or set high often smells musty on return. See local inspection pages for Naples, Boca Raton, and Sarasota.
- After a power outage: days without air conditioning in summer, as in Houston after Hurricane Beryl, can leave a musty smell with nothing visible.
- Basements: common in Denver and Washington, DC.
- Crawl spaces: under older homes in Austin, San Antonio, and Jacksonville.
See also what causes mold in a house, mold in the closet, and signs of mold in your house.
Next step
A musty smell you can trace to a damp towel or a dry drain is a quick fix. One that keeps returning, or that you cannot place, is worth a professional look. If you own the home, call the number on this page; the free call connects you with an independent local mold inspection company that can find the source with a moisture meter before anything is opened.