Is it Mold or Mildew?

Many people commonly refer to various building molds as “mildew", and numerous mold cleanup products use the words “mildew” or “mildewcide” in their name and instructions. With most people, interchanging the two words isn’t a big deal. But if you regard yourself as a mold remediation expert, you might consider being more conscious of your phrasing.

Mold is a distinct species and genus within the fungi family. (Did you flashback to biology class?) It’s one of the main decomposition methods that nature utilizes. Molds or fungi feed upon dead and decaying organic matter, so they enhance the decaying processes of nature on fallen trees, plants, dead animals, leaves, etc. Without life forms like mold, we’d be buried under piles of dead stuff. So that’s great. But problems arise because mold doesn’t know the difference between dead trees in the forest and a wet wall stud in a home. And since mold spores are always floating around, they easily find what they need to grow and reproduce; food, moisture, and darkness.

Mold can also grow on many nonporous materials such as concrete and brick. And with the right temperature, moisture, and food, mold will grow most anywhere. A rolling stone may gather no moss, but one sitting still sure can. Although it's more prevalent and grows more quickly on porous surfaces like drywall and wood (because there's more food), over time even a large area of concrete or brick can be covered by it. This is especially true in high humidity when the temperature between the surface and the air is slightly different.

Mildew is likewise a segment of the fungi family, but a different genus. Mycologists regard it as a parasite because it feeds only on living plants. They’re divided into two sub-groups:

1. Oidium-Erysiphe, familiarly named Powdery Mildew?

2. Peronosporaceae, familiarly named Downy Mildew

These take the form of a white or gray powdery or splotchy deposit on plant leaves and stems, and are often caused by poor air circulation within or around the plant, and dampness or high humidity. And that’s an important distinction because most buildings will never have a mildew infestation unless they're growing a ton of plants inside. But since the majority don’t, mildew is rarely an issue with indoor contamination.

Often, the term mildew will be used generically to refer to mold growth with a flat growth tendency. In other words, it doesn't grow roots and thus it's non-destructive and easily scrubbed away. It, too, will grow anywhere there's enough moisture, such as bathroom sinks and showers, basement walls, or fabrics.

For these reasons, a mold expert or a home inspector should never lump mold and mildew together or say they’ve found mildew growing in or on a building. Although the average customer wouldn't know any difference, some might, and you don’t want to end up saying anything that could damage your reputation.