Waterproofing problems

Condensation versus seepage is the first thing to establish, because people spend thousands solving the wrong one. Tape a square of plastic sheeting tightly to the wall and leave it a couple of days. Moisture on the room side of the plastic is humid air condensing — a dehumidifier problem. Moisture behind the plastic is water coming through the wall. EPA recommends keeping indoor humidity below 60 percent, ideally 30-50 percent, measurable with an inexpensive meter.

Water at the floor-wall joint after heavy rain is the classic hydrostatic pressure pattern. Water rises in the soil, finds the seam where the wall meets the footing, and comes in. Sealers will not stop it. This is what interior drainage systems are designed for.

Efflorescence — the white, chalky, powdery deposit on block or concrete — is mineral salt left behind when water evaporates out of masonry. It is not mold and it is not itself damaging, but it is proof that water is moving through the wall. Scrubbing it off without addressing the water just means it comes back.

Musty smell with no visible water usually means mold somewhere out of sight, often behind finished walls or under carpet. EPA's guidance: fix the water source first, because cleaning mold without fixing the moisture means it returns. A patch under roughly 10 square feet can usually be handled yourself; bigger than that, hire a remediator. EPA also notes that if you can see mold, testing is generally unnecessary — you already know.

Bowing, leaning or horizontally cracked walls are a different category. That is soil pressure moving your foundation, and it is a structural engineer's question, not a waterproofing salesperson's. Pay a licensed structural engineer for an independent opinion before you accept a repair proposal from the company that would perform the repair.

How to choose

Log when it happens

Write down the date, the weather and where water appeared. 'Only during multi-day rain' points to surface water. 'Every spring regardless' points to a water table. This log is the most useful thing you can hand a contractor.

Walk the outside in the rain

Genuinely — stand outside during a downpour and watch where the water goes. Overflowing gutters and downspouts dumping at the wall are visible in about two minutes and are frequently the whole problem.

Get an engineer for structural symptoms

Bowing walls, stair-step cracks widening over time, or doors that stopped closing need an independent licensed structural engineer, paid by you, with no stake in the repair.

Treat radon as a separate question

Basement work and soil-gas entry are related but not the same issue. EPA's position is that testing is the only way to know your level.

Photograph everything before work starts

Dated photos of walls, floor and belongings protect you in a warranty or insurance dispute later.