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The house is late Victorian with 18" thick solid stone walls (sandstone/lime mortar with no cavity). We've knocked all the plaster off the walls as it was poor condition. In two areas there was evidence of damp which coincides with damp areas that we had believed was due to condensation as they were behind cupboards.
At the weekend we battened the wall and put 25mm Kingspan insulation in prior to fitting a vapour barrier and plaster boarding it. After 3 days, I've just removed the insulation panels to check the damp areas and there is evidence of water on one stone in each position - not just a damp feeling but actual water glistening!
In both cases, there is no obvious area on the outside of the wall that points to water ingress, so how has it got in? Is it possible that there are porous stones that run right through the walls? How is that different from the porous lime mortar that the house is built with?
If so, what is the best solution? My thoughts are to tank the inside area with cement/PVA slurry (it stopped the inspection pit in my garage from flooding), and possibly using a waterproofer very locally on the outside.
Any suggestions are welcome!
Mike

there is a possibility the property is suffering from hydroscopic salts ask your local rentokill man to investigate the survey should be free.first indications are damp patches at any height on any wall and are not localised these hydroscopic salts are water bearing,as the name implies,and they love plaster,the second stage is a white fluffy fungus appearing on or around the surface of penetration followed by blistering of the plasterwork.although you say the external stonework is not defective it would not be a waste of time to check the guttering for blockages.

It is perfectly normal for wind blown rain to run through a wall.
Hence the developement of cavity walls.
The best solution is to paint the outside of the wall with two or three coats of silicone paint (see B&Q etc:) this will cause the water to run off.
Poorly mixed motar is usually the problem and a wall exposed to strong winds.

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