20m x 6m is quite a large space for a single dehumidifier so it will take a while to bring the humidity levels down. Also, if there's any carboard or wood stored in the place, that will act as a buffer and it will take a while to decrease it's moisture content. Has the floor been laid with a damp proof membrane underneath?
I try and keep my area below 70% relative humidity, which seems to do the trick.
I try and keep my area below 70% relative humidity, which seems to do the trick.