What the smell is
The typical musty or earthy basement odor usually comes from mold and mildew. As mold grows it releases gases known as microbial volatile organic compounds (mVOCs), which have a strong smell even in small amounts, according to the U.S. EPA. The EPA also cautions that odor is not a reliable test of indoor air quality: a smell tells you there is a problem, not how big it is.
Mold needs moisture. So a musty smell almost always means something in the basement stays damp – the air, the walls, the floor or stored materials.
Common sources
- High humidity. Basements are cool, so summer air reaching them gets damp. The EPA recommends keeping indoor humidity below 60%, ideally 30–50%.
- Seepage through walls or at the floor edge, even if you never see a puddle. White crust on walls (efflorescence) is a clue.
- An open sump pit. Water and wet soil in an uncovered pit add humidity and smell. Newer editions of the International Residential Code require a gas-tight, removable sump cover, according to the DOE’s Building America Solution Center.
- Wet or old materials: carpet on concrete, cardboard boxes, drywall or paneling against damp walls, old insulation.
- A crawl space connected to the basement.
- Dry floor drain traps. If a floor drain is rarely used, the water in its trap can evaporate and let sewer gas in – a different, sewer-like smell. Pouring water into the drain refills the trap.
How to find the source
- Measure humidity with a hygrometer over a few days.
- Follow your nose: the smell is usually strongest near the source.
- Check the back of stored boxes, carpet pads, the bottom of drywall and paneling, and behind furniture against outside walls.
- Tape plastic sheeting to the wall for a day or two: moisture behind it means seepage; moisture in front means humid air.
- Look at the sump pit: is it covered and sealed?
How to get rid of the smell
- Dry the air: run a properly sized dehumidifier set around 45–50% – see the dehumidifier guide.
- Stop the water: fix gutters and grading, seal cracks, and install drainage where water seeps in – see basement waterproofing.
- Seal the sump pit with a gasketed lid.
- Remove moldy materials that can’t be cleaned, such as wet cardboard, carpet pad and drywall; follow EPA guidance and consider professionals for larger areas.
- Store smarter: plastic bins on shelves, not cardboard on the floor.
- Don’t just mask it: air fresheners and paint over damp walls hide the symptom while the moisture keeps feeding mold.
When to call a professional
- The smell persists even with humidity under control.
- You see visible mold on wood, drywall or belongings, or the affected area is large.
- There is water on the floor, damp walls or efflorescence.
- You plan to finish the basement.
See basement mold and humidity for how contractors handle it.