Musty Smell in the House: Causes and Fixes
Quick answer: A musty smell in the house almost always means moisture: a slow leak, a damp basement or crawl space, wet carpet, a dirty AC drain or hidden mold. Find and fix the water, dry or remove damp materials and keep humidity under 50 percent. If mold covers more than about 10 square feet, consider professional mold remediation.
That earthy, stale, "old basement" smell has a source. It comes from gases released by mold and mildew as they grow, and from damp materials breaking down. Colorado's dry climate makes many people think mold can't be a problem here. But a single slow leak or a damp corner below grade is plenty. The smell is often the first clue that water is somewhere it shouldn't be.
9 common causes of a musty smell
| Cause | Where to look | What you'll notice |
|---|---|---|
| Slow plumbing leak | Under sinks, behind toilets, around the dishwasher | Warped cabinet floor, stains, soft spots |
| Damp basement | Walls, floor, corners, behind stored boxes | White powdery deposits, damp concrete |
| Crawl space moisture | Under the house | Wet insulation, standing water, smell in floor vents |
| Wet carpet or pad | Near doors, bathrooms, basements | Smell gets stronger when the carpet is walked on |
| AC condensate problem | Air handler, drain pan, drain line | Smell when the AC runs, water near the unit |
| Bathroom ventilation | Ceiling, grout, behind the toilet | Spots on the ceiling, peeling paint |
| Roof or ice dam leak | Attic, top-floor closets, ceilings near exterior walls | Stains, damp insulation |
| Washing machine | Door gasket, detergent drawer, drain | Smell inside the washer or laundry room |
| Stored items | Cardboard, books, clothes in basements and closets | Smell is strongest when the box is opened |
How to find the source
- Follow your nose. Walk room by room. The smell is usually strongest near the source.
- Check below grade first. Basements and crawl spaces are the most common sources.
- Look under every sink. Use a flashlight and feel the cabinet floor for dampness.
- Check around the water heater, washer and HVAC unit.
- Look up. Check ceilings, especially under bathrooms and near exterior walls.
- Measure humidity. An inexpensive hygrometer tells you if indoor humidity is above 50 to 60 percent.
- Move things. Pull furniture and boxes away from basement walls and look behind them.
If you can smell it but can't find it, the moisture may be inside a wall or under a floor. A restoration company can use moisture meters and infrared cameras to find hidden water without tearing things open.
How to get rid of a musty smell
Fix the water source. Repair leaks, clear AC drains, improve grading and downspouts, and seal foundation gaps. Nothing else lasts until this is done.
Dry everything. Use fans and a dehumidifier to dry damp areas. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent.
Clean hard surfaces. Scrub mold or mildew from hard surfaces with detergent and water, and dry them completely.
Remove what can't be saved. Moldy drywall, carpet pad, ceiling tiles and cardboard usually need to go. The EPA says porous materials that are moldy may have to be thrown away.
Ventilate. Run bathroom and kitchen exhaust fans, and open windows on dry days.
Wash fabrics. Launder curtains, bedding and clothes that picked up the smell.
Musty smell after water damage
If the smell showed up a few days or weeks after a leak or flood, the cleanup probably missed some moisture. Common spots:
- Carpet pad that was dried from the top but stayed wet underneath
- Drywall that was painted over without drying the wall cavity
- Insulation inside a wall or ceiling
- The subfloor under vinyl, laminate or hardwood
The EPA advises drying water-damaged materials within 24 to 48 hours to prevent mold growth. After that, mold may already be growing where you can't see it.
Basement and crawl space tips for Colorado homes
- Run a dehumidifier in summer, when humid outdoor air meets cool basement walls. Our dehumidifier guide covers sizing.
- Keep stored items on shelves, off the floor and away from exterior walls
- Use plastic bins instead of cardboard boxes
- Make sure window wells drain and have covers
- In crawl spaces, a sealed vapor barrier over the soil helps keep ground moisture down
When to call a pro
Call a professional if:
- The smell continues after you've dried the area and fixed obvious leaks
- You see mold on more than about 10 square feet
- The smell is coming from inside walls, ceilings or ductwork
- It started after a flood, sewer backup or other water damage
- Someone in the home has asthma, allergies or a weakened immune system
Musty smell you can't track down? Call and we'll help find the moisture behind it.
Call (303) 879-1370Frequently asked questions
Is a musty smell always mold?
Not always, but it usually means moisture, and moisture is what mold needs. Damp carpet, wet cardboard and a dirty AC drain pan can all smell musty. If the smell lasts, look for a water source.
Is a musty smell harmful?
The CDC notes that damp and moldy environments can cause stuffy nose, wheezing, and eye or skin irritation in some people, and can be more serious for people with asthma, allergies or weak immune systems. The smell itself is a warning sign to find and fix the moisture.
Why does my basement smell musty in summer?
Warm, humid outdoor air enters a cool basement and its relative humidity rises. Concrete walls and floors may also let ground moisture through. A dehumidifier set to keep humidity under 50 percent usually helps.
Will air fresheners or candles get rid of a musty smell?
No. They only cover it up. The smell comes back until the moisture source is fixed and damp or moldy materials are dried or removed.
Can a musty smell come from the HVAC system?
Yes. A clogged condensate drain, standing water in the drain pan or dirty coils can all produce a musty odor that spreads through the vents. See AC leaking water inside the house.
Sources
Related guides
Using a Dehumidifier for Water Damage
What size to buy, where to put it, how long to run it, and when a home dehumidifier isn't enough after a leak or flood.
AC Leaking Water Inside the House: Causes and What to Do
Clogged drain lines, frozen coils, cracked pans and failed pumps: why air conditioners leak indoors and how to stop the damage.
Can't find the source of a musty smell?
Call and we'll help you track down the moisture behind it.