Why Does My House Smell Musty or Like Mildew?

A persistent musty, earthy smell — especially in a damp basement, crawl space, or under a sink, even with no visible mold — is most often mold or mildew growing wherever moisture has accumulated; the EPA and CDC both note that a musty smell alone is reason enough to suspect hidden mold and address the moisture source, without needing to test first. The same general 'musty' description has real look-alikes worth ruling out: a heavy cockroach infestation produces an oily, wet-cardboard-like odor concentrated in kitchens and bathrooms; an early-stage dead animal can smell musty for a day or two before it turns clearly sickly-sweet and rotting; and a musty smell that only appears when your air conditioner runs points to moisture on the coil or in the ducts, not the room itself.

An earthy, mushroom-like odor that seems to live in the basement or a closed-up room is most often mold or moisture — the compounds mold produces as it grows have exactly that musty character. But 'musty' isn't a single diagnosis. A large, established cockroach population produces a genuinely oily-musty smell of its own, an early-stage dead animal can smell musty before it turns clearly rotten, and a musty smell that only shows up when your AC runs points somewhere else entirely: your ductwork.