Musty Smell After Water Damage: What It Means and What to Do

A musty smell that lingers after water damage is a sign, not just a nuisance. EPA says a moldy odor suggests mold is growing and should be investigated, even when you can’t see it. In most cases something behind a wall, under the flooring or in the carpet pad never finished drying. The smell goes away when that source is found, dried and cleaned, not when it is covered up.

What causes the musty smell

EPA explains that some compounds produced by mold are volatile and quickly released into the air. These microbial volatile organic compounds, or mVOCs, are what you smell. People often notice the odor before they see any growth. EPA notes the health effects of breathing mVOCs are largely unknown, but exposure has been linked to headaches, nasal irritation, dizziness, fatigue and nausea.

Mildew is not a separate problem. EPA’s mold course describes molds as also called fungi or mildew, so a mildew smell means the same thing: moisture and growth.

What different smells can mean

• Musty or earthy: mold or mildew growing on damp materials.
• Sour or wet-carpet smell: carpet pad that stayed wet. Pad holds water under carpet that feels dry on top.
• Rotten eggs: hydrogen sulfide, which Wisconsin’s health department notes smells like rotten eggs and forms in sewer gas. A dry drain trap can let sewer gas in, and a sewage leak needs Category 3 cleanup.

Where the smell is coming from

If a room smells moldy but you can’t see the source, EPA says mold may be hidden. Common places include:

• The back side of drywall, wallpaper or paneling
• The underside of carpet and carpet pad
• Inside walls around leaking or condensing pipes
• The top side of ceiling tiles and ceiling drywall
• Inside HVAC ductwork

EPA advises not running the HVAC system if you know or suspect it is contaminated with mold, since it can spread spores through the house.

Will the smell go away on its own?

Not reliably. Drying stops mold from growing further, but EPA’s standard for a finished cleanup is that visible mold and moldy odors are gone. If the smell remains after an area seems dry, either something is still wet or mold that already grew hasn’t been removed.

Timing matters. UC Riverside’s mold plan notes mold hyphae may begin growing 24 hours after moisture reaches growth potential, and Nebraska Extension says mold grows on materials that stay damp for more than two to three days.

What doesn’t fix it

• Ozone generators. EPA says ozone is not effective at removing many odor-causing chemicals and, at safe levels, does not effectively remove viruses, bacteria or mold. It can cause chest pain, coughing and shortness of breath. California limits indoor air cleaners sold in the state to ozone emissions below 0.050 ppm.
• Painting or sealing over it. EPA’s guidance is not to paint or caulk moldy surfaces. The growth stays behind the coating.
• Air fresheners and fans. They mask the odor or move it around without removing the source.

How to find and fix the source

1. Find the wet material. A moisture meter or thermal camera shows wet areas behind surfaces.
2. Fix the leak if it is still active.
3. Dry the structure with air movers and dehumidifiers until readings match unaffected materials.
4. Remove what can’t be cleaned, such as moldy pad, insulation or drywall.
5. Clean hard surfaces with detergent and water and dry them completely.
6. Control humidity. EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent.

Musty smells in Murrieta homes

Musty smell FAQs

Will a musty smell go away on its own?

Rarely. A lingering musty smell usually means something is still wet or mold is growing, and EPA says it should be investigated.

How long does it take mold to settle in after water damage?

Mold can begin growing about a day after materials stay wet, and Nebraska Extension says it grows on materials that stay damp for more than two to three days.

Do ozone machines get rid of water damage smell?

EPA says ozone at safe levels doesn’t effectively remove mold or many odor-causing chemicals, and it irritates the lungs. Finding and drying the source works.

Is it a mildew smell or mold?

Mildew is a common name for some molds. Either way, the smell means moisture and growth, and the fix is the same.

What does a rotten egg smell after water damage mean?

It usually points to sewer gas, which contains hydrogen sulfide. A dry drain trap or a sewage leak are common causes.

Sources

• U.S. EPA: What does mold smell like?
• California Air Resources Board: Limiting ozone from indoor air cleaning devices
• Wisconsin Department of Health Services: Sewer gas