Where Hidden Mold Hides

Evidence: Well supported (EPA: indoor spores cannot be eliminated, but mold growth indoors should be prevented and removed).

Much of that growth is out of sight. A veteran inspector who examined homes for three decades often found that the patch everyone worried about was a minor player, while the mold that mattered was hidden somewhere nobody had looked.

Mold needs food, usually wood, paper or other plant-based material, and it needs moisture. So trace every path water has taken, including leaks and overflows from before you owned the house. Inside a wall, leaking water trickles downward, pools at the base, seeps beneath the drywall and trim, and sits where it cannot dry. Some common molds keep growing in a closed wall afterward on nothing more than the dampness of that hidden space, even once the leak is fixed.

Evidence: Well supported for moisture as the root cause of indoor mold (EPA, CDC). The point about continued growth in closed walls is Inspector's practice (precautionary).

Bathrooms and laundry:

• Beneath the trim at the foot of the wall behind and next to the toilet, where an old overflow may have soaked in.
• Along the base of the wall on the far side of a shower or tub, checked from the bedroom or closet next door.
• Under the wooden trim or posts at each side of a shower, where water slips past the curtain.
• Under the base molding next to the washer, the dishwasher and the refrigerator.

Sink cabinets:

• The back crack where the cabinet floor meets the rear wall, around the pipes, and the edges of the pipe holes. If neighboring cabinets are moldy too, general dampness is more likely than one leak.

Windows and exterior doors:

• Under the base molding below a window that has leaked, and under the sill and side posts of doors where rain can seep in.
• Any wall where the ground, a patio or a downspout pushes water toward the house.

Basement:

• The underside of the bottom step. One long-time inspector used this spot as a gauge: heavy growth there suggested the basement could support a lot of mold, while little or none was a good sign.
• The ceiling joists and the subfloor above them. Check discolored and clean-looking areas separately, so you learn whether you face one spot or a whole ceiling.

Air conditioning:

• Deep inside a supply duct with the vent cover removed. Spots on a vent cover usually come from local condensation and say little about the system; growth farther inside is the bigger concern.

Furniture and beds:

• The undersides and lower backs of upholstered and older wooden pieces, and the slats and undersides beneath a mattress or box spring. These matter most in a damp house, for second-hand or vintage furniture, and for anything that was once stored in a basement.

Evidence: Inspector's practice (precautionary) for this list of places and for the bottom-step gauge.

Black specks on windowsills, bathroom ceilings, caulk, shower curtains and refrigerator gaskets are nearly always a common mold that lives on condensation. Wipe them off, dry the area and move on. They seldom explain why a house makes someone ill.

Turn the usual question around: is there any reason mold would not grow at this spot? If there was food and water and the area could not dry quickly, finding growth should not surprise you. Finding it is progress, because a known source can be removed. Learning where mold is not growing matters just as much, since it keeps any cleanup as small as it can be.

This page is for education only. It is not medical, legal or engineering advice. The inspector whose work this builds on was an environmental consultant, not a physician. Talk with your doctor about health concerns, and use licensed professionals for electrical work and large mold jobs. Products, prices and labs mentioned reflect the years they were recorded and may have changed.

Source: the field work of environmental inspector May Dooley, MS, MA, CMC

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