Musty smell in a home is one of the most common odor complaints — and one with the most variation in underlying cause. Before you can eliminate it, you need to identify the source.

What Causes Musty Smell in a House

Musty odors are almost always moisture-related. Common sources in OC homes: unconditioned crawl spaces that accumulate moisture and mildew, HVAC systems where the evaporator coil creates condensation that can grow mold, attic moisture from poor ventilation in coastal OC homes, slow plumbing leaks under sinks or behind walls that create sustained moisture for months, window condensation on single-pane windows creating mildew conditions, and homes closed up for weeks developing musty odor from moisture trapped in the space.

How to Find the Source

• Walk through room by room — is the odor stronger in a specific area or uniform throughout?
• Check under sinks and around plumbing fixtures for moisture
• Inspect HVAC vents — is the odor stronger when the system runs?
• Check accessible crawl space or basement if present
• Look for water staining on ceilings or walls indicating a roof or plumbing leak

Treatment by Source

Crawl space: Crawl space encapsulation sealing the vapor barrier to the walls is the long-term fix for crawl space moisture.

HVAC: Filter replacement, HVAC coil cleaning, ensuring drain lines are clear, plus professional ozone treatment with the system running to treat ductwork.

Plumbing leak: Repair the leak first. Then address any mold remediation needed in affected materials. Odor treatment alone does not work with active moisture present.

Vacant home: Ozone or hydroxyl treatment followed by ventilation typically resolves musty odor in homes that were simply closed up without active mold growth.