Should I re-plaster with lime plaster?

[molecule]

I have a South Devon house, 500mm thick local slate and mortar walls, externally (I believe) cementacious render, internally has been direct plastered with conventional plaster.

An area of staining has been present in the internal paintwork for a number of years and I am wondering how to treat this. I have a feeling that the staining is caused by dampness but I'm not sure of the actual cause, clearly the external render prevents moisture loss from the walls but is this problem treatable by removing the internal plaster and replacing with 'breathable' lime plaster combined with decent ventilation etc? Would this help pervent internal staining?

Perhaps I should go down the 'Vandex' route - however, I am worried about the damp appearing elsewhere if this is used.

 

[Gervase]

As a rule, anything whichm encourages solid walls to breathe is to be encouraged - so yes, remove the modern plaster and replace with lime.
You may still have probloems with penetrating damp, however. If your exterior render is cementitious and therefore not breathable you could have water being channelled through to the inside from a number of sources, so I would think seriously about removing the external render and either limewashing the stonework or applying a breathable, lime-based render.

Look, too, at your guttering and drainage - solid stone walls have a way of conducting water from all sorts of places if they can't breathe, so your staining could be a result of poor drainage or a leaking gutter.
Or, on the other hand, it could be staining from an old chimney breast or bread oven, where either soot or iron salts are leaching through the plaster.

Try to avoid using a tanking slurry like Vandex at all costs. It works against the natural mechanics of the wall, and you won't be able to get it off again.

 

[molecule]

The wall in question is a gable wall, faces east and is fairly protected by other houses so I think that its unlikely to be penetrating now.

A funny thing happened in early spring this year, a very obvious square stain appeared in the wall, approx 3" x 3", a couple of weeks later and it totally disapeared leaving no trace. The stain had never appeared before and hasn't appeared since. Any thoughts on on the cause of this.

I thought that perhaps it may have been caused by a period of north east winds which were unusual in their duration and coldness, perhaps the 'square' is a piece of timber or metal in the wall which has conducted the heat away from the wall in a localised area causing this localised staining. What do the experts think?

 

[Gervase]

Gable end? That does make me think of chimney flues, access plates and bread ovens - if you do get round to taking the modern plaster off the wall you may find a mechanical answer to your staining rather than just damp.

 

[molecule]

there is a chimney flue in the wall but the staining is remote from it. Can the soot salts travel very far?