How to tell if mold is behind your walls

Section 1.0Five signs worth acting on

1. Smell. Musty and strongest near one wall or cabinet.
2. Stain. Yellow or brown rings, often low on a wall or under a sill.
3. Paint. Bubbling or peeling near a fixture. EPA says paint over mold is likely to peel².
4. Baseboard. Swollen, soft or pulling away.
5. Meter. The flow triangle moves with every tap off³.

Section 2.0What sits behind the drywall

Drywall is gypsum with paper on both faces. CDC says Stachybotrys grows on gypsum board and paper and needs constant moisture⁴⁵. The back face of a wall over a leaking pipe is dark, still and wet. Growth there can be heavy while the room side looks fine.

Section 3.0Map it before you cut

A moisture meter across the wall shows where the wet edge is. Readings that climb toward one fixture point at the source. The same idea works on ceilings under a bathroom or an AC closet. Open the wall where the reading peaks, with a small hole first.

Section 4.0Opening the wall safely

CDC says to wear an N95 respirator at a minimum, plus goggles and gloves⁶. EPA says goggles should have no vent holes⁷. If what you find is larger than about 10 square feet, EPA sends you to its larger job guidance⁸. In the City of Las Vegas, minor drywall repair needs no permit only if no framing is touched and the wall is not fire rated⁹.

Section 5.0Do you need a test

Rarely. EPA says sampling may help find the source when it is unclear¹⁰, but if you can see mold, sampling is usually unnecessary¹¹. Our testing page explains when it helps.

Section 6.0Can mold be removed without removing drywall

Surface growth on the paint side sometimes can be. Growth into the paper usually cannot. EPA says porous materials may have to be thrown away when moldy¹². Cutting a clean section past the stain is faster than fighting it.