What are you seeing?
Start with the thing you noticed. Each entry below says what it usually points to and where to read on. None of this replaces someone looking under the house, but it helps you describe the problem when you call.
Under a house on a crawl space
A musty smell indoors, strongest when the air conditioning comes on
Air from the crawl space is being drawn up into the rooms. Down there the soil is damp, the wood is holding moisture, or both. The smell tends to peak in July and August.
Wood floors that cup, or spots that feel soft or springy
Boards cup when their underside is wetter than their top. Soft spots can mean the subfloor or a joist has been damp for a long time. Ask for the wood to be checked with a moisture meter.
Insulation hanging down or lying on the ground
Fibreglass batts soak up moisture, get heavy and drop. Fallen insulation is a sign of long-running humidity, and wet batts hold water against the joists above them.
Puddles or mud under the house after rain
Surface water is reaching the foundation, or groundwater is rising into the low part of the crawl space. This has to be drained before any liner goes down.
Ducts and cold pipes that drip in summer
Humid outside air is condensing on cold metal. It is the plainest sign that the vents are letting in more moisture than they carry away.
In a basement
Water on the floor after a hard rain
It most often comes in where the wall meets the floor, pushed by wet soil outside. Note which wall, and how long after the rain it shows up.
White powder or crust on block or concrete
Water is moving through the wall and leaving its minerals behind as it dries. The powder itself is harmless. The moisture path behind it is the thing to deal with.
A dark, damp band low on the wall
Either seepage from outside or room air condensing on a cool wall. A square of plastic taped to the wall for two days tells you which.
A sump pump that runs all the time, or never
A pump that never rests may have a stuck float, a failed check valve letting water fall back, or more water arriving than it can move. One that never runs in wet weather may have failed.
Outside
Puddles against the foundation, or soil that slopes toward the house
Roof and yard water is being fed to the foundation. Fixing this is often the simplest part of the whole job and sometimes the only part needed.
Soil stains or a wet line on the outside of a basement wall
Water sits against the wall after rain. Where an inside fix is not enough, the wall can be sealed and drained from outside.
Describe it to a local company
Say what you noticed, where in the house, and when it shows up. You will be put through to an independent local waterproofing company.
Calls may be recorded. The company pays this site for the introduction. See the referral disclosure.