Mold vs. mildew: what is the difference?

Both are fungi, and both need moisture to grow. The difference that matters in a house is where the growth is and how deep it goes. Mildew sits on top of a surface: shower grout, a window sill, a bathroom ceiling above the tub. It usually wipes off a hard surface with a household cleaner and a dry cloth.

Mold on porous materials is different. On drywall paper, wood framing, ceiling tiles or carpet pad, growth goes into the material, not only onto it. Wiping the surface removes the stain you can see and leaves the growth inside. That is why spots come back in the same place after cleaning.

Three signs point past mildew. The spots return after cleaning. The growth is raised, fuzzy or spreading into a larger patch. Or the area is near a leak, a damp basement wall or a spot that got wet and did not dry. A musty smell with no visible growth is another sign, because it often comes from a wall cavity or under flooring.

In all cases the cause is the same: moisture that stays. Fix the moisture and dry the material, and mildew and mold both stop growing. When the area is larger than a small patch, or the material is drywall, wood or carpet, a moisture meter reading shows how far the water went before anything is opened.