Condensation, seepage, or structural? A decision tree

These three problems look similar on a basement wall and cost between $300 and $40,000 to fix. Answering six questions honestly narrows it faster than a free inspection, and without anyone in your house.

The four condensation tests, in full

1. Tape a square of plastic sheet tightly to a damp patch of wall or floor, sealed on all four edges. Wait 24–48 hours. Moisture on the room side of the plastic is condensation from the air. Moisture trapped behind the plastic is water coming through the masonry.
2. Check the timing against the weather. Condensation peaks in warm humid weather, especially the first hot spell of summer, and is worst on the coldest surfaces. Seepage peaks during and just after rain and snowmelt, and follows the water table.
3. Look at where it is. Condensation is even, over broad areas, worst on cold water pipes, uninsulated ductwork and the wall below grade. Seepage has a source: a crack, a joint, a tie-rod hole, the wall-to-floor joint, a window well.
4. Run a dehumidifier for a week at 50% target humidity, with the basement windows shut. If the problem substantially disappears, you had a humidity problem: a $200–$500 portable unit, or $1,300–$2,600 for an installed ducted one, is the whole answer.

The plastic-sheet test is standard building-science practice for distinguishing vapour from bulk water. It is free and it is the first thing an independent inspector will do.

What a structural signal looks like

Water is a nuisance. Movement is a safety issue. Treat these as structural until an engineer says otherwise: a horizontal crack in a block wall; stair-step cracking through mortar joints; a wall that is visibly bowing or leaning inward; any crack where one side has moved out of plane relative to the other; doors and windows that have gone out of square; a crack wider than about 6 mm (a quarter inch). Rate a crack specifically.

Page data updated 2026-08-12