What Are the Causes of Mold?

Mold spores are everywhere — floating through the air indoors and out, all the time. They're harmless until they land somewhere with the two things they need to grow: moisture and an organic food source. Take away the moisture, and the spores simply can't establish a colony.

Because spores are always present, controlling mold is really about controlling water. Almost every mold problem traces back to a source of moisture that went unaddressed.

Leaks are the usual suspects — a slow drip under a sink, a failing roof, or a pinhole in a supply line can keep materials damp for weeks. Past water damage that wasn't fully dried is another frequent cause, since moisture trapped in walls and subfloor feeds growth long after the surface looks fine.

Humidity does just as much damage without any dramatic leak. Poorly ventilated bathrooms and kitchens, damp crawl spaces, and condensation on windows and pipes all create the steady dampness mold thrives on.

Homes with poor ventilation trap humid air that never gets a chance to dry out. Crawl spaces with bare soil let ground moisture rise into the structure, and tightly sealed homes without good air exchange can hold humidity in.

Climate plays a role too. In the Bay Area, coastal humidity and damp winters keep indoor moisture levels elevated, which is why crawl space and bathroom mold are so common here.

Since mold can't grow without water, prevention is about keeping things dry: fix leaks promptly, ventilate bathrooms and kitchens, keep indoor humidity in a healthy range, and address damp crawl spaces with vapor barriers and dehumidification.

If you've already had water damage, thorough drying is the best insurance against mold. When growth has taken hold, professional remediation removes it and corrects the moisture source so it doesn't simply come back.