Whats Causing Damp Salts on Internal Wall in Under Stairs Cupboard?

[kevl2000]

Hi,

Under the stairs there are lots of white mold on the walls and some coming through the carpet on the floor. Looking at the other posts this is salt from damp.

This is only happening under the stairs, which is an enclosed space, apart from the door to access, and all walls are internal.

What is this likely to be causing it and how is it best to treat it.

Thanks

[welsh brickie]

fit an airbrick in the wall and a hit and miss plastic vent on the internal wall.
Also paint a pva solution on the internal wall to seal it against further damp