How to Find Hidden Moisture

Walk the room with the lights off and a flashlight held flat against the wall. Waves or shadows show swelling. Press baseboards, and look for lifting paint, dark corners and rust on fasteners. Then check the room below and the room behind, since water moves sideways along framing and down through floors.

Hidden moisture is usually lower and farther from the leak than you would guess. Water follows gravity, framing and pipes, then wicks upward through porous materials. So start at the source, then check downward and outward: the base of nearby walls, the room below, the back of cabinets, and under the flooring.

EPA lists many of the places mold may hide, and they double as a list of where water lingers. They include the back side of drywall, wallpaper or paneling, the top of ceiling tiles, the underside of carpet and pad, inside walls around leaking or condensing pipes, behind furniture against walls, and inside ductwork.

You can narrow things down with your eyes, hands and nose, but you generally cannot prove a material is dry without a meter. Moisture can be well above normal in a surface that feels completely dry to the touch.

Moisture meters give numbers that are easy to misread. Many report wood moisture content directly and other materials only on a relative scale, so a number on drywall does not mean the same thing as the same number on a stud. Read the manual, and always compare like with like. A reading only has meaning when you compare it with a known-dry piece of the same material.

Pin meters read between two points and are good for pinpointing, though they leave small holes. Pinless meters scan through a surface and are quick for mapping, but they respond to what is within their sensing depth and can be fooled. Metal such as foil-backed insulation, nail plates or a metal corner bead can produce a high reading on a dry wall, and so can some tile and dense materials.

False lows happen as well. A pinless meter on a thick floor may not sense a wet layer underneath. That is why contractors often use more than one tool and sometimes open a small inspection hole where the readings and the evidence disagree.

A thermal camera shows surface temperature, not water. Evaporation cools a wet surface, so damp spots often show up as cooler patches, and that pattern can reveal the path water took. It is a useful way to see where to test next.

But cool spots have many other causes. Missing insulation, air leaks, a cold pipe, and a nearby vent can all look much like moisture. On the other hand, a wet area that has reached room temperature and stopped evaporating may hardly show at all. Cameras work best with a temperature difference between inside and outside, so results can vary with the weather.

Treat any thermal image as a question, not an answer. A contractor who shows you a colored image should also put a meter on that spot. If someone uses an image to recommend removal without taking a moisture reading, ask them to take one.

Sometimes a small hole answers a question no meter can. If readings are high at the base of a wall and there is insulation behind it, the only way to know how much is wet may be to look. A small inspection cut near the floor, hidden later by a baseboard, can be a fair first step. Pulling a baseboard first can give access with very little damage.

Do not start cutting in an older home without checking first. EPA notes that cutting into pre-1978 paint can put lead dust in the air, and asbestos can be present in older building materials. Turn off power to that wall, and find out where plumbing and wiring run. Disturbing hidden mold can also release spores, so have a plan before you open a suspect area; EPA's cleanup guide covers when professional help makes sense.

For flooring, an access point at the edge of a room or in a closet can show what is beneath. Lifting a corner of carpet is easy; lifting hardwood, tile or laminate may not be, so a pinless scan and readings from the room below may be the better early step.

Usually, finding the moisture matters more. CDC does not recommend routine mold testing, because health effects vary from person to person and there are no set standards for acceptable mold levels. CDC's advice is to remove what you can see or smell and to fix the moisture problem behind it.

EPA similarly notes that sampling is not always necessary, although professional testing is an option if you are in doubt. If you do pay for testing, ask what question it will answer and how the results would change the plan. A positive or negative result rarely tells you where the water is coming from. See mold prevention after water damage for more on the moisture side.

Knowing how water moves makes the search much shorter. It falls until something stops it, then spreads sideways along that surface until it finds an opening. A ceiling board, a subfloor, or the top of a foundation can act like a shelf, carrying water well away from the leak. Pipes and wires passing through framing give it a route from one floor to another.

Porous materials also pull water upward by wicking. Drywall, plaster and masonry can draw moisture above the visible water line, and that is why a stain on a wall may sit higher than the flood itself ever reached. Slopes matter too: most floors are not perfectly level, so water may pool on the far side of a room from where it began.

That's why a good search goes beyond the wet spot. Sketch the room, mark the source and draw arrows showing where water could go. Check the lowest points, then work back. In a multi-story home, go to the room directly below, and then to the rooms alongside it.

A musty smell usually means something is damp, though not necessarily where you notice it. Air moves through a house. It can carry an odor from a crawlspace up into a bedroom, or from a wall cavity into a closet. Odors may also become stronger when the HVAC runs, when doors are closed or when the weather turns warm and humid.

Sewer-like smells are a different problem. A dry floor drain trap, a failing wax ring or a broken vent can let sewer gas in without any flooding. Pouring water into an unused floor drain restores the trap, and that's an easy test. If the smell stays, call a plumber. Don't confuse this with hidden moisture just because the two can smell somewhat alike.

Try an odor walk. Leave for a few hours so your nose can reset, then return and move slowly from room to room. Open closets and cabinet doors, and sniff near the baseboards and vents. Note where the smell is strongest. This low-tech method can give a contractor a useful starting point.