Clay soil and wet basements: what to fix before you finish

Where the water comes from

Most basement moisture starts as roof water. An inch of rain on an average roof is well over a thousand gallons, and if downspouts release it at the foundation, the backfill zone beside the wall, which is looser than undisturbed soil, turns into a trench full of water. The next cause is grading that has settled and now slopes toward the house. After that come window wells without drains, and only then a high water table or failed drain tile.

Three tests you can run yourself

Walk around the house in steady rain and watch where water goes. Tape a two-foot square of clear plastic to the slab and to a wall, seal the edges and look for condensation beneath it after two days. And look for evidence: white powdery deposits on the wall, rust at the base of steel posts, stains on the lowest course of any existing framing, a musty smell that gets stronger in summer.

Fixes, from cheapest to costliest

Work through these in order. Most basements only need the first three.

• Extend every downspout at least six to ten feet from the house, or into buried drain pipe running to daylight
• Regrade so the soil falls away from the foundation about six inches over the first ten feet
• Clean gutters and add capacity where they overflow
• Add drains to window wells
• Test the sump pump, add a check valve and install a battery or water-powered backup
• Seal non-structural cracks by injection
• Install interior drain tile and a sump where water enters at the floor-wall joint
• Excavate and waterproof from outside, which is rarely necessary

Then build a wall that tolerates vapor

Even a basement that never leaks passes water vapor through the concrete. The wall assembly has to handle that. Rigid foam board against the concrete, with seams taped, keeps warm humid indoor air away from the cold wall so condensation cannot form. A treated bottom plate on a gasket stays dry. Framing stands inside the foam. Paperless or moisture-resistant drywall is held slightly off the slab. The details to avoid are fiberglass batts pressed against concrete and polyethylene sheeting on the room side, which traps moisture inside the wall.

Humidity after finishing

A finished basement should stay below about fifty-five percent relative humidity. Supply and return air from the HVAC system helps, and a dedicated dehumidifier plumbed to a drain deals with the summer months. We include a location and a drain for one in our basement plans.