Hidden Moisture Behind Wallsthe clues, the tools and the fix

In Del Mar homes, the common hidden sources are shower pans and tub surrounds, drain lines inside walls, toilet wax rings, supply lines behind fixtures, refrigerator ice-maker lines, windows and sliding doors on the weather side, decks over living space, and roofs and skylights. Slab leaks also show up at the base of walls far from the pipe.

Coastal air adds a quieter source: condensation. Cold water lines and poorly insulated exterior walls can collect moisture from humid indoor air. It is less dramatic than a leak but can still feed mold in a closed cavity.

Bathrooms come first. Look at the wall outside the shower and the ceiling below any upstairs bathroom, press on the floor around the toilet, and smell the vanity cabinet. In the kitchen, open the sink cabinet and look at the cabinet floor, then check the floor and baseboard beside the dishwasher and refrigerator. In the laundry, look behind the washer at the hose connections and the wall behind them.

Then check the weather side of the house after a storm: the floor inside sliding doors, the corners under big windows and the ceilings below decks. Finish in closets and cabinets on exterior walls, where air does not move and smells collect. A simple pinless moisture meter can help you compare a suspicious wall with a dry one, though a contractor's readings are what count for a claim.

In attached homes, the wall you share with a neighbor can be wet from their side. If you smell damp on a party wall and find nothing on your side, tell the neighbor and the HOA. Readings on both sides usually settle where the water is coming from, and the CC&Rs decide who fixes it.

Once the source is repaired, the wall cavity has to be dried. For clean water, that often means removing the baseboard, drilling small holes at the base of the wall behind where the baseboard goes, and pushing dry air into the cavity while a dehumidifier runs in the room. If the insulation inside is wet, if there is mold, or if the water was dirty, a section of drywall is removed so the cavity can be cleaned and dried directly.

Drying is checked with daily readings until the wall matches a dry reference wall. In coastal homes, closed-room drying with a dehumidifier is usually faster than relying on open windows, because outdoor air is often humid.

Re-caulk tubs and showers when caulk cracks, and have a shower that stains the wall outside it checked for a failed pan. Replace supply lines every several years. Clean sliding door tracks and weep holes before winter and check seals on windows that face the ocean. Put leak sensors in places you rarely look, such as under sinks, behind the refrigerator and beside the water heater.

Pay attention to smells. A musty odor in one room that comes and goes with rain or showers is worth investigating while the damage is still small.

If your water bill rises without a reason, check the meter with all water off. A moving meter means water is going somewhere.