People use "mold" and "mildew" interchangeably, but they are different things with different implications for your home.
What Mildew Is
Mildew is a specific type of fungal growth that stays on the surface of materials — it does not penetrate into the material itself. It is typically flat, powdery, and grows in gray or white patches. It is most commonly found in bathrooms on tile grout, shower curtains, and window sills.
Mildew is common, relatively easy to clean, and generally does not cause structural damage. It can often be addressed with standard bathroom cleaning products and improved ventilation.
What Mold Is
Mold is a broader category of fungal growth that can be many colors — green, black, white, orange, or purple. Unlike mildew, mold penetrates into the material it grows on. Mold on drywall is inside the drywall. Mold on wood framing is inside the wood.
Because mold penetrates materials, surface cleaning does not remove it. Scrubbing mold off drywall leaves the mold that has grown into the material. Professional remediation — physical removal of contaminated materials — is usually required for true mold growth.
How to Tell the Difference
| Mildew | Mold | |
|---|---|---|
| Appearance | Flat, powdery | Fuzzy, raised, slimy |
| Color | Gray, white | Green, black, white, varied |
| Smell | Musty but mild | Strong, earthy, musty |
| Location | Surface only | Penetrates materials |
| Damage | Surface staining | Structural damage |
| Treatment | Surface cleaning | Professional remediation |
The smell test is useful: mold typically has a stronger, more pervasive musty odor than mildew. If you can smell something but the visible growth looks minor, there may be more mold growing inside walls or under flooring than is visible.
When to Call a Professional
Mildew: Generally does not require professional remediation. Clean with an appropriate bathroom cleaner, improve ventilation, and address any moisture source causing recurrence. A bathroom exhaust fan that actually vents to the outside (not just to the attic) solves most recurring bathroom mildew problems.
Mold: Requires professional assessment if it covers more than 10 square feet, if it has penetrated building materials, if it is inside walls or under flooring, if it returns after cleaning, or if anyone in the home is experiencing symptoms.
The key question: is this a surface problem or has it penetrated into the material? Surface growth that wipes off cleanly and does not return is likely mildew. Growth that comes back, spreads, or leaves staining after cleaning is likely mold.
When in doubt, have an independent inspector take a look. A professional assessment costs $200–$600 and tells you definitively what you are dealing with and what needs to happen next.