Short answer: A musty smell almost always means moisture, and often mold or mildew somewhere you cannot see, such as a basement, crawl space, under a sink or inside a wall. Find and fix the damp, and the smell goes with it.
Musty smells are one of the most common signs of hidden moisture. Here is how to track down the source and when to call for help.
The most common causes
- Mold or mildew in a basement, crawl space or closet
- A slow leak under a sink, behind a toilet or inside a wall
- High humidity with poor ventilation
- Damp carpet, rugs or furniture after a spill or leak
- A washing machine door seal or drain holding water
- Air conditioner drain pans and ducts
How to find the source
- Follow your nose: the smell is usually strongest near the source
- Check under sinks and around toilets, tubs and the water heater
- Look in the basement, crawl space and closets on outside walls
- Check the washing machine seal and the air conditioner drain pan
- Use a moisture meter or hygrometer if you have one
Musty or sewer smell?
A rotten egg or sewer smell is different from a musty one. It often comes from a floor drain or sink trap that has dried out, which lets sewer gas in. Pouring water into unused drains usually fixes it. If the smell stays, call a plumber.
How to get rid of it for good
- Fix any leak you find
- Dry wet areas within a day
- Run a dehumidifier to keep humidity below about 50 percent
- Use bathroom and kitchen fans that vent outside
- Clean small moldy areas on hard surfaces with detergent and water
When to call a professional
Call a mold removal provider if the smell does not go away, you cannot find the source, you see mold on large areas or there has been a leak inside a wall.
Frequently asked questions about musty smell in house
Is a musty smell dangerous?
The smell itself is not the problem, but it usually means mold or damp, which can affect people with allergies or asthma.
Will air fresheners fix a musty smell?
No. They only cover it. The moisture source must be found and fixed.
Why does my house smell musty after rain?
Water may be getting into the basement, crawl space or walls. Check gutters, grading and for leaks.
Can a musty smell come from the air conditioner?
Yes. Clogged drain pans and damp ducts can grow mold. Have the system cleaned and checked.
Related
Read our guide on mold behind walls and our basement mold removal page.
This guide covered musty smell in house. For more background, see the EPA’s mold guide, and if you need help, Sifixit can connect you with a local provider.
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Facts last checked on October 7, 2026 by Ishaq Khokhar.
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