What are you seeing?

Air from the crawl space rises into the rooms above and carries the smell of damp soil and damp wood with it. The odor is often stronger in closets, in rooms over the low end of the crawl space, after rain and in humid weather.

It tells you the space is staying moist. It does not tell you why, so someone needs to look.

Puddles after rain usually trace back to the surface: downspouts that stop at the wall, soil that slopes toward the foundation, or a clogged ditch. Water that lingers for days, or appears with no rain, suggests groundwater or a leaking pipe.

Note when it appears and how long it stays. If you suspect plumbing, turn off every tap and watch the water meter. If the dial still moves, call a plumber.

Fiberglass batts between the joists soak up moisture from humid air. They get heavy, sag and drop. The fallen pieces then sit on the soil and hold water like a sponge.

Pushing them back up does not last. The humidity has to be dealt with first.

Wood flooring that cups at the edges, a floor that gives slightly underfoot, or a chill at ankle level can all come from a damp crawl space. Moist subfloor swells, and wet insulation stops insulating.

Ask for moisture readings on the joists and subfloor. If the framing has sagged or a pier has shifted, that is structural work and a separate job from drying the space.

Cold air-conditioning ducts and water pipes sit in warm, humid crawl space air. Moisture condenses on them and drips onto the insulation and soil. You may see dark spots where the drops land.

This is a humidity problem, not a leak. Closing the space off from outdoor air and drying it is the usual cure.

The chalky crust is mineral salt. Water moving through masonry dissolves it and leaves it on the surface as it evaporates. It brushes off, and it comes back for as long as the wall stays damp.

On a crawl space wall it points to wet soil outside. On a basement wall it often comes with peeling paint and dark patches.

Look for where it starts. A trail from a crack, a window or a pipe is a leak with one source. A damp strip all along the base of the wall means groundwater is rising at the joint between wall and floor.

One source can often be repaired on its own. Water along a whole wall usually calls for a drain system.

A pump that never rests may have a stuck float, a failed check valve or a discharge that returns water to the foundation. A pump that stays silent while the basin fills has lost power or failed.

Lift the float by hand to see whether the pump starts. If there is no battery backup, a storm with a power cut leaves the basin unprotected.

Ponding beside the foundation after rain means the ground is not carrying water away. Settled backfill, short downspouts and flower beds with solid edging are frequent causes.

It is often a simple thing to put right, and a very common reason a crawl space is wet.