Mold vs. mildew — when one is cosmetic and the other is a problem

Walk through any older home in summer and you'll find black or pink spots in bathroom corners and on shower grout. People reach for one of two words: "mold" or "mildew." In casual conversation they mean the same thing. In restoration science, they don't.

Mildew, technically speaking

Mildew is a specific type of surface-only fungal growth that lives on top of organic material without penetrating it. It's almost always cosmetic. The pink stuff in your shower grout, the gray fuzz on a leather jacket left in a closet, the black film on a window seal — that's mildew. It wipes off with vinegar, hydrogen peroxide, or any bathroom cleaner with bleach.

Mold, technically speaking

Mold is a broader category that includes species capable of penetrating organic material — drywall, wood, insulation, carpet pad. It establishes a root system (hyphae) inside whatever it's living on. Surface cleaning doesn't reach the hyphae. Three days later it grows back, often worse.

The species that worry restoration professionals are penetrative molds: Stachybotrys chartarum (black mold), various Aspergillus species, Cladosporium. These produce mycotoxins and microbial volatile organic compounds (MVOCs) that affect indoor air quality.

How to tell which you have

• Surface mildew wipes off cleanly with a bathroom cleaner and doesn't come back if the moisture source is controlled.
• Mold growth that comes back within days, smells musty, or spreads onto adjacent surfaces is not mildew.
• Any visible growth on drywall, wood framing, insulation, or upholstered surfaces — assume mold, not mildew.
• A persistent musty smell in a closed room without visible growth often means mold is established somewhere behind a wall or under flooring.

Why this matters for your insurance

Insurance policies almost universally cover mold remediation up to a sub-limit (typically $5,000–$10,000) when the mold is a direct, immediate result of a covered water loss. They almost universally exclude mold from chronic moisture or gradual exposure. The distinction your adjuster will care about is: was this caused by a sudden event, or was it growing for months?

If you suspect mold (not mildew), document it the moment you find it. Photos with date stamps. The earlier you can prove discovery, the cleaner the claim.