Why Is My Basement Damp? A 48-Hour Test to Find the Cause

Damp basement walls, a musty smell or a fogged window down there all point to too much moisture. The fix depends on where it's coming from, and there are really only two places: water seeping in from outside, or humid air condensing on cold surfaces.

The 48-hour foil test

  1. Cut a square of aluminum foil about 12 by 12 inches.
  2. Tape all four edges tightly to the damp part of the wall or floor.
  3. Wait 24 to 48 hours, then check it.
  • Moisture on the outside of the foil means condensation. The air in the basement is too humid.
  • Moisture under the foil, against the wall, means water is coming through the foundation.

If it's seepage (water coming in)

  • Check the downspouts first. Roof water dumped at the wall is one of the most common causes. Aim to discharge it at least 4 to 6 feet away. More on downspouts.
  • Clean the gutters so they don't overflow down the wall.
  • Check the grading. Soil should slope away from the house, about 6 inches over the first 10 feet.
  • Persistent water, cracks or a wet floor after every storm? Call a waterproofing or foundation professional.

If it's condensation (humid air)

  • Measure it. Put a humidity sensor in the basement for a week. Most basements do best at or below about 50%.
  • Run a dehumidifier sized for the space. A 32-pint unit suits many basements up to about 2,000 sq ft; larger or very damp spaces may need a 54-pint unit. Set it to empty into a drain if you can, so it doesn't stop when the bucket fills.
  • Vent the dryer outside, and don't hang laundry to dry in the basement.
  • Insulate cold water pipes that sweat.
  • Keep air moving. Don't push furniture and boxes tight against exterior walls.

Why it matters

Damp air and wet surfaces are what mold needs to grow, and basements combine both with little airflow. Getting the humidity number down and keeping roof water away from the foundation solves most damp basements.

If you already see mold over a large area, or smell it strongly and can't find it, talk to a qualified remediation professional.

Start with the free 10-minute home humidity checklist.