Musty smell in your house

What causes a musty smell in a house?

Usually mold or mildew growing on something that stays damp. The CDC describes mold as something that can look like spots of many colors and can smell musty.³ The EPA notes that in unaired places, molds can produce a strong musty odor.²

The growth itself is often small. What matters is that something is feeding it water: a slow drip, condensation, or air that stays humid.

Can you smell mold you can’t see?

Yes, and that’s common. The EPA says you may suspect hidden mold when a building smells moldy but you can’t see the source.¹ It lists places hidden mold tends to grow:¹

• The back side of drywall, wallpaper or paneling
• The underside of carpets and pads
• Inside walls around leaking or condensing pipes
• Wall surfaces behind furniture, where condensation forms
• Inside ductwork

Peeling back wallpaper or carpet to check can release a lot of spores if there is growth underneath, so the EPA suggests considering an experienced professional for suspected hidden mold.¹

Where should you look first?

Follow your nose to the room where the smell is strongest, then check where water collects:

1. Under the kitchen and bathroom sinks, including the back wall of the cabinet
2. Around the base of toilets and behind the washing machine
3. Closets on exterior walls, especially in homes near the coast
4. The AC air handler and its drip pan. The EPA advises keeping drip pans clean and drain lines flowing.¹
5. Behind furniture pushed against exterior walls
6. Under the house, if it sits on a raised foundation

If the smell is strongest when the AC or heat first comes on, start at the air handler and drip pan, then read our guide to mold in AC vents. If it’s strongest near the floor in a house on a raised foundation, the source may be underneath; see mold in the crawl space.

Pay attention to timing too. A smell that disappears when windows are open and returns when the house is shut up often points to humidity rather than a leak. A smell that stays put in one cabinet points to something local.

Is it mold or something else?

Plenty of musty smells have small, ordinary causes. Wet towels left in a hamper, a load of laundry that sat in the washer, a bath mat that never dries, or a rug that got wet and stayed wet. Dry or wash those and the smell usually fades with them.

A smell that survives cleaning, drying and airing out is the one to take seriously. If you find spots you aren’t sure about, our guide to mold vs mildew helps, or you can send us a photo.

Does a musty smell matter if nothing is visible?

It does. California’s Department of Public Health lists mold odor alongside visible water damage, damp materials and visible mold as signs that indicate an increased risk of respiratory disease for occupants.⁴ The smell is a symptom of moisture, and moisture is what lets growth spread.

Covering it with air fresheners or candles removes the one clue you have. Find the source first, then clean it and fix the water problem. The EPA is direct about it: if you clean up mold but don’t fix the water problem, the mold will most likely come back.¹

For symptoms or health concerns, talk to your doctor.

When should you call an inspector?

Try the simple steps first. Dry out obvious damp items, run bathroom and kitchen fans, and check the spots listed above. A hygrometer will tell you whether a room is staying above the EPA’s recommended range of below 60%, ideally 30 to 50%.¹

Book a visit when the smell persists for more than a week or two after that, when it’s strongest near a wall or floor with nothing visible, or after a leak you aren’t sure dried out. A moisture and humidity assessment is built for exactly this: room-by-room readings that trace where the water is coming from. It starts at $495 for one dwelling up to 2,000 sq ft. The mold check can help you decide whether you need one.