The short answer
Set it to 45 to 50 percent. Below 45 percent the unit runs much longer for little gain; above 55 percent mold and dust mites have room to thrive.
The target
Mold needs moisture; keeping a basement below about 50 percent relative humidity takes it away. Dust mites also struggle below 50 percent. Going much lower mostly wastes electricity.
Modes
- Auto or humidistat: the unit cycles on and off to hold your setting. The normal choice.
- Continuous: runs regardless of the reading, for drying out after a leak.
- Fan speed: low is quieter; high moves more air through the coil in large spaces.
Check the reading
A dehumidifier's built-in sensor reads the air right at the unit. Put a separate hygrometer across the room to see whether the whole space is reaching the target.
Products mentioned
Questions readers ask
Is 40 percent too low for a basement?
It's fine but costs more to maintain; 45 to 50 percent is usually enough.
Should a dehumidifier run constantly?
In a damp summer basement it may run for long stretches on its humidistat; that's normal.