Short answer
A dehumidifier controls moisture contained in the air. Wall treatment targets a cause within the building, for example rising damp. You therefore need to distinguish air humidity from wall moisture.
When to think about air treatment
Air treatment may concern a house, a buried basement, an industrial space, a museum, collections, or certain agricultural uses.
When to think about wall treatment
A damp band at the bottom of the wall, saltpeter, or recurring deterioration of the substrate may indicate that the problem is not only coming from the ambient air.
Do the marks keep coming back despite cleaning?
That is often a sign that the cause needs analysing, not just the surface.
Why ask for advice
Buying a device without understanding the cause may improve the feeling of humidity without treating the visible issue in the wall. Diagnosis helps point you toward the right approach.
Frequently asked questions
Can a dehumidifier remove saltpeter?
No, not if the saltpeter is linked to rising damp. It may act on the air, but not on the cause inside the wall.