Tony Bryer wrote:

In article , Stuart Noble
wrote:

Providing a coating adheres to the surface, and is in itself
waterproof, no moisture should get in in the first place. This
is near impossible to achieve with render because invariably
there is a gap between it and the wall


Moisture will get in to the wall because it is being generated
within the building and the internal surfaces are far from being a
perfect water barrier. IMO it is much better to have a wall that
can breathe.

That is why you have impermeable membranes and controlled ventilation in
(some) cavity walls.

As usual th short answer is it depend on what is happening, why the wall
is damp, and what harm its doing.