The three sources
- Condensation
- Warm, humid summer air meets a cool wall or a cold water pipe and leaves droplets. The dampness is spread evenly, appears in muggy weather whether or not it rains, and is worst with the windows open.
- Roof and surface water
- Rain that lands near the foundation and finds a way in. It appears within hours of a storm, usually at one wall or corner, and stops soon after.
- Ground water
- After days of rain the water in the soil rises to the level of the floor. It seeps in along the joint between wall and floor on more than one side and lingers after the weather clears.
Three checks you can do yourself
The foil test
Tape a square of kitchen foil tightly to a damp patch of wall and leave it a couple of days. Moisture on the room side of the foil is condensation. Moisture underneath it came through the wall.
The storm walk
Go outside in heavy rain with a coat and a flashlight. See whether the gutters overflow, where each downspout empties and whether water runs toward the house or pools against it.
The calendar
Note the date each time the floor is wet and what the weather did in the days before. A pattern of one storm, one wet corner points to the surface. A wet floor after a rainy week points to ground water.
What each answer leads to
Condensation is handled by drying the air: keep the windows shut in humid weather, insulate cold pipes and run a dehumidifier. No drain or pump is involved.
Roof and surface water is handled outside, with gutters, downspout extensions and soil that slopes away. The yard drainage page covers it.
Ground water needs a drain and a pump. That is basement waterproofing, with a sump pump to move the water out.
When it is time to call
Call once you have done the outdoor fixes and the floor still gets wet, or when you can see the water is coming up at the wall-floor joint. Tell the company which of the three checks you did and what you found. It shortens the visit and makes it harder for anyone to sell you more than the house needs.