Even a basement with no seepage at all can be damp, smell musty and grow mould, purely from humidity. It is a separate problem from water entry and it has separate remedies.
The targets
Under 50% is the working target for a basement.
50 to 60% is tolerable and is where dust mites and some mould begin to be comfortable.
Above 60% sustained is where mould growth becomes likely on any organic material present — cardboard boxes, paper, dust on a cold surface, the paper facing on drywall.
Buy a hygrometer and leave it in the basement. It costs very little and it is the only way to know which conversation you are having.
Why basements are humid
They are cold. Air holds less moisture as it cools, so air arriving from upstairs or from outside reaches its dew point and deposits water on cold surfaces.
They are below grade, surrounded by soil that is at a stable, damp temperature.
Moisture moves through concrete as vapour even where there is no liquid seepage, and a bare slab passes a surprising amount.
Why opening a window makes it worse
On a warm humid day the outdoor air holds far more moisture than the cool basement air. Bringing it in and cooling it deposits that moisture on every cold surface.
This is counter-intuitive and it is the most common self-inflicted basement humidity problem. Ventilation helps only when the outdoor dew point is below the indoor dew point, which in summer it usually is not.
The dehumidifier
Basement-rated, meaning it works at lower temperatures. A standard unit ices up in a cool basement and stops removing water.
With a drain, plumbed to a sump or a floor drain. A bucket that has to be emptied gets emptied for two weeks and then does not.
Sized for the space and for how wet it is. Manufacturers publish coverage figures; a badly undersized unit runs continuously and never reaches target.
Set to about 45 to 50% and left alone.
Sources to remove before dehumidifying
Dryer venting into the basement, which puts several litres of water into the air per load. It must vent outside.
An unvented bathroom or kitchen downstairs.
A bare earth floor or crawl space connected to the basement — this is a large source and the fix is a sealed vapour barrier over the earth, taped and run up the walls.
A bare concrete slab, which passes vapour continuously. A sealed floor covering or a vapour barrier under a finished floor addresses it.
Removing sources is more effective than removing the water afterwards, and cheaper to run.
Insulating cold surfaces
Cold pipes and duct runs condense and drip, which reads as a leak. Insulating them removes the condensing surface.
Insulating the walls themselves is worth doing and worth doing correctly — an assembly appropriate for below-grade concrete, without a vapour barrier trapping moisture in the wrong place. Getting this wrong creates a hidden mould problem inside the wall.
Finishing a basement
The order matters. Fix water entry, get humidity stably under 50%, run it for a season and confirm it holds, and only then frame anything.
Finishing over an unresolved moisture problem hides it behind materials that feed mould, and everything that follows is more expensive to undo than it would have been to do properly.
What good looks like
A hygrometer reading 45 to 50% year round. No musty smell. Cardboard boxes that are dry to the touch. No condensation on pipes in July.
If those are true and the walls are still damp after rain, that is seepage and it is a different article.