Does a musty smell always mean mold?

Musty smells that aren’t always mold

Several things produce the same earthy, stale odor people associate with mold. Some are harmless, some are moisture problems that haven’t grown mold yet, and some are mold hiding behind another explanation.

Possible source	How to tell
Bacteria in damp soil or materials	Some soil bacteria produce geosmin, the earthy smell of rain on dirt. Common in wet crawlspaces and basements.
AC drain pan or condensate line	Smell is strongest when the air conditioner runs, and the pan holds standing water or slime.
Washing machine or dishwasher	Smell is concentrated at the appliance door, gasket or drain.
Sump pit or floor drain	Smell is strongest near the pit or drain, especially when water sits in it.
Old books, papers and fabrics	Smell follows the items. Take them out of the room and it fades.
A house closed up for a while	Smell clears within a day or two of airing out and running the heating or cooling.

Notice how many of these involve water. Even when the source isn’t mold, it’s often a moisture problem that mold can take advantage of later.

When the smell usually is mold

• It’s stronger in one room, closet or corner than the rest of the house.
• It comes back after you clean and air out the area.
• It gets worse after rain or during humid weeks.
• It’s strongest when the furnace or air conditioner first starts.
• It sits near a wall with a bathroom, kitchen or laundry room on the other side.
• There’s a known leak or flood in the area’s history.

Narrowing it down at home

1. Put a hygrometer in the room. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent.
2. Check the easy suspects: drain pans, appliance gaskets, sump pits, stored boxes and damp towels.
3. Clean or remove any of those, then dry the room with a dehumidifier or air conditioning for several days.
4. Air the space out, then close it up again and check the smell over the next week.
5. If the odor returns in the same spot, stop looking for easy answers. The source is likely hidden.

Resist covering the smell with candles or sprays while you work through this. The smell is your best clue, and masking it makes the source harder to find.

When to bring in testing

A musty smell that survives drying and cleaning is one of the most common reasons people have a home inspected. Mold inside walls, under floors or in the HVAC system often gives itself away by odor long before anything shows. An inspection follows the smell and the moisture readings to the source, and air or wall cavity samples confirm what’s there. Our answer on whether you can have mold without seeing it covers the other signs to look for, and indoor air quality testing explains how air samples are compared with the outdoors.

If the smell is only in one spot and you find the source quickly, a test may not be needed at all. It’s worth it when the source stays hidden, when someone in the home is sensitive, or when you need documentation.

We test and remediate buildings. We don’t diagnose or treat medical conditions, and a home test result isn’t a medical diagnosis. If you have symptoms, please work with a doctor. With your permission, we’re glad to share test results with them.