Mold on walls: causes, cleanup and when to call

Surface mold or a wall problem?

Mold on a wall gets its water from one of two places, and the fix depends on which:

• From the room. Moist indoor air condenses on a surface colder than its dew point. Mold grows on the face of the paint, usually in a broad, speckled pattern on outside walls, in corners and behind furniture.
• From inside the wall. A leaking pipe, window, roof or shower soaks the drywall from behind. Mold grows on the back of the board and in the cavity first, and what shows on the room side, often a stain with spots around it, is the last part to appear.

Mold from condensation

Condensation mold follows the cold spots. Exterior walls, especially north-facing ones, the corners where two outside walls meet, the strip where the wall meets the ceiling under thin attic insulation, and the wall behind a dresser or headboard all run cooler than the rest of the room. Closets on outside walls are a classic case: cool walls, little air movement and stored clothing holding moisture. In winter, those surfaces can drop below the dew point of the indoor air, and the dampness that collects there is enough for mold. The fix is drier indoor air, more air movement and, where insulation is missing, better insulation. See high humidity in the house.

Mold from a leak inside the wall

Leak mold tends to have a source you can point to:

• Along the bottom of a wall: plumbing in the wall, a past flood or overflow, or water from outside where the soil is graded toward the house.
• Around or below a window: failed flashing or caulk letting rain into the wall.
• On the other side of a shower or tub: a leaking valve, drain or shower pan.
• Below a bathroom or laundry room on the floor above.
• Behind the refrigerator, dishwasher or washer: supply line leaks.

Peeling or bubbling paint, a soft spot, a stain that grows after rain, or a moisture meter reading on an otherwise dry-looking wall all point to water inside. Bubbling paint and soft drywall covers those signs.

How big is it?

Size drives the response. EPA guidance says homeowners can usually clean up mold covering less than about 10 square feet, roughly a three-by-three-foot patch. Patches from 10 to 100 square feet call for more precautions, and anything larger, or any amount that keeps coming back, is a job for a remediation crew with containment. Size is measured on the wall you can see; with leak mold, the hidden side is often bigger.

Cleaning small spots safely

For a small area of surface mold on a painted, washable wall: wear gloves, goggles and an N95 respirator; scrub with water and a little detergent; wipe clean; and dry the wall completely, with a fan if needed. Bag the rags and paper towels. Do not dry-brush or sand mold, which sends spores into the air, and do not mix cleaning products. The guide to cleaning up mold yourself has the full checklist.

When the drywall has to come out

Drywall is paper wrapped around gypsum, and mold that has grown into the paper or onto the back of the board cannot be cleaned out. When growth goes beyond the surface, when the drywall is soft or stained through, or when the mold came from a leak, the affected drywall is cut out with a margin past the visible growth, inside containment, and the framing behind it is cleaned and dried. Mold remediation explains each step, and black mold removal covers dark growth specifically.

Keeping it from coming back

• Fix leaks promptly, and dry anything that got wet within a day or two.
• Keep indoor humidity below 60 percent, ideally 30 to 50, with the AC and a dehumidifier if needed.
• Leave a few inches between furniture and exterior walls, and keep closet doors open now and then.
• Run bathroom and kitchen exhaust fans, ducted outdoors.
• Add insulation where outside walls or ceiling corners stay cold.