Basement musty smell
easy 1–3 h
Tools: hygrometer, flashlight, plastic sheet and tape, dehumidifier, fan · Materials: mould cleaner or detergent, waterproof storage bins, hydraulic cement for small wall cracks
Steps
- Measure the humidity: a basement above 60 % relative humidity smells musty on its own, and a dehumidifier set to 50 % often solves it.
- Tape a square of plastic to the floor and to a wall for two days: moisture under the plastic means water is coming through the concrete; on top means condensation from the air.
- Look for the sources — a downspout at the wall, a sump that runs constantly or never, efflorescence lines showing where water reached, and cardboard boxes on the floor.
- Move stored items into sealed bins on shelves, off the floor and away from the walls, and throw out anything already mouldy.
- Clean surface mould with detergent and dry the area with a fan; fix the outside grading and downspouts that feed the wall.
- Run the dehumidifier and recheck in two weeks; a smell that returns after rain is water intrusion, not humidity.
Stop and call a pro if
- water shows on the floor or walls after rain, or the plastic test shows moisture coming through the concrete
- mould covers more than about a square metre or is behind finished walls
- the smell is sewage-like — a floor drain trap has dried out or the sewer line is involved
What a pro does that you can't
A waterproofing contractor seals cracks, adds an interior drain and sump or exterior drainage, and a mould remediator removes growth behind finishes when the source has been running for years.
DIY cost: $20–40 for a hygrometer and cleaner; $150–300 for a dehumidifier