How do I know if there is hidden water damage in my walls?

Start with what you can see, smell and feel

Water inside a wall usually announces itself in small ways before it becomes a hole in the plaster. The EPA's moisture-control checklist for inspecting a foundation interior tells inspectors to look for signs of seepage or wicking (e.g., water stains, damp materials, efflorescence, peeling paint or mold growth).^[1] Look for swollen, warped or moldy wooden materials.^[1] Baseboards, door casings and the bottom of a wall are the places that swell first, because water wicks upward from the floor.

Then use your nose. Check for musty odors.^[1] The EPA's mold guide describes the situation exactly: You may suspect hidden mold if a building smells moldy, but you cannot see the source, or if you know there has been water damage and residents are reporting health problems.^[2] Walk through the house after it has been closed up for a few hours, not with the windows open.

Finally, use your hand. Run the back of it along the wall and baseboard near tubs, sinks, the water heater, the washer and any exterior wall. A spot that is cooler, softer or slightly clammy compared with the wall beside it deserves a closer look.

Where it hides

Mold and moisture rarely sit where you can see them. Mold may be hidden in places such as the back side of dry wall, wallpaper, or paneling, the top side of ceiling tiles, the underside of carpets and pads, etc.^[2] The EPA also names areas inside walls around pipes (with leaking or condensing pipes), the surface of walls behind furniture (where condensation forms), inside ductwork, and in roof materials above ceiling tiles.^[2]

In practice that means checking the wall behind the tub or shower, the wall under a second-floor bathroom, exterior walls below windows, and the area around the water heater and washer.

Check the humidity

A cheap meter tells you whether the room is holding moisture. The EPA advice is to keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.^[2] Relative humidity can be measured with a moisture or humidity meter, a small instrument available at many hardware stores.^[2] Condensation can be a sign of high humidity.^[2] A room that stays damp in dry weather, with no obvious reason, may have a source behind the surface.

Test for a running leak

A leak inside a wall is often a supply line that is weeping all day. The water meter can tell you.

Chesterfield County's utilities page gives the method: the first step in checking the water meter is to make sure no water is being used in the home.^[3] Then record the reading, and wait 15 minutes and check the sweep hand again.^[3] If the sweep hand has not moved, there probably is not a leak.^[3] Newer meters are equipped with leak detectors. This is a red triangle on the register face that will rotate when water is passing through the meter.^[3] Your own utility may describe its meter differently, so check its page.

If the meter moves, close the main valve inside the house and look again. Typically, if the meter has stopped registering, the leak is somewhere in the house.^[3] If it keeps going, the leak is probably on the pipe outside. Chesterfield also lists the outdoor signs: water standing in a yard, water in or around a meter box, or an unusually high bill may indicate a potential water leak.^[3]

Rule out the toilet before you suspect a wall. Prince William Water says to add a few drops of food coloring or dye tablets to the tank behind the commode and let it sit for 3 hours,^[4] without flushing. Colour in the bowl means a leaking flapper, which wastes water but does not soak a wall.

Do not open the wall to look

It is tempting to cut a hole, but a wet wall is not always safe to disturb. The EPA warns that investigating hidden mold problems may be difficult, and that removal of wallpaper can lead to a massive release of spores if there is mold growing on the underside of the paper.^[2] Its advice: If you believe that you may have a hidden mold problem, consider hiring an experienced professional.^[2]

A restoration company or plumber with a moisture meter and thermal camera can usually map the wet area from the outside and open only a small section. See how to choose a water damage restoration company and what it costs in how much water damage restoration costs in Virginia.

Stop and call a pro if

• The meter keeps moving with everything off, and you cannot find a visible leak. The pipe is likely inside a wall, slab or yard, and a plumber has leak-detection tools.
• A stain is growing, or the paint bubbles and the drywall is soft. Shut off the water at the main valve, then read what to do first after water damage. The EPA says it is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth.^[2]
• There is a moldy smell and no visible source. That is the EPA's description of suspected hidden mold, and it recommends an experienced professional.^[2]
• Water is near outlets, a panel or a light fixture. Stay clear, do not touch them, and call an electrician.
• The leak is in a ceiling below a bathroom or from the roof. Do not climb up to look; call a plumber or roofer.

Next steps in Virginia

Once you know there is damage, the questions are who dries it and who pays. Read what to do first after water damage, then browse restoration answers or restoration companies in Virginia to compare businesses.

When you are ready, describe the damage and local restoration companies will quote it.