cracking skim over gypsum? in Welsh stone farmhouse

[AWhitten]

I have an old Welsh stone farmhouse that was extended by converting the attached stone barn, this would have been done in the 1970s. The original farmhouse has no damp problems, but the barn conversion part has a few, pretty clearly due to the use of some inappropriate materials. The bit I'm looking at right now is localised to just around the windows on one side of the kitchen, as shown in attached photos. I'm not sure whether this is just an inappropriate skim coat that's cracking off of plaster that's okay, or whether the plaster is wrong and should come off too. It looks like gypsum? It's hard and I can't scratch it with a fingernail. It seems to be structurally sound, no crumbling below the skim coat, but I don't know how to tell whether it's contributing to the damp and should come off to be replaced with lime -- advice on this would be very appreciated.

As the rest of the room is okay, I'm thinking that redoing these sections just at the sides of the recessed windows could be a good size project for me to start and learn on. Any advice would be great. Thanks very much.

 

[vinn]

The base coat has not been correctly prepped plus it appears in the pic as very dark, and then there are all the damp signs on that part of the wall surface - the skim is actually being blown off by expanding moisture.
I would imagine that the whole wall needs hacking off back to bare masonry.
You have a solid floor and presumably a solid wall - whats on the other side of that wall?

The external wall might also be allowing moisture to penetrate and spread to the abutting wall with the damp.

It might help if you post pics of the whole of the damp wall and pics showing the outside wall external surface esp. at ground level.

 

[AWhitten]

Thanks, I'll put up some more pics. That's not the floor in the pic, it's the slate windowsill. The walls are solid stone about 2 feet thick. The outside wall is stone that is mostly correctly rendered but has been inappropriately patched with cement in a few spots, that'll be a separate project to deal with. The kitchen walls don't have any apparent damp issues except for at the sides of the windows where they are recessed into the wall, and only on that side of the house.

 

[AWhitten]

Here's some more pics that show the context a bit more. The exterior pic is a few years old and is of the other side of the house, not where the interior pics show -- I'm including it just to show the construction. The weather's no good for taking pics outside right now.

Could you tell me what you think that surface is? You said it appears dark, I thought that was just due to the mineral mix? Thanks!

 

[vinn]

The exterior has been re-pointed in the last few years but with what kind of mix I cant tell - the best mix would be sand and lime at 3:1. Which would also be the best mix for any internal wall surface render.
Foliage on the wall is bad for the fabric - but great to see. Your choice.

The in-situ mix is irrelevant - you have moisture coming through the wall and the wall coverings are blown: the damp areas must be hacked off and rendered with S&L (or similar) and a remedial finish.

If you buy a cheap moisture meter and put it to the backing and skim you will see an emerging pattern showing which areas are most severely affected.

 

[AWhitten]

Thanks, that's very helpful. I'm reasonably confident the majority of the exterior is sand and lime. There are a few unfortunate concrete patch spots but thankfully they are small and will be dealt with. I will take your advice about the moisture meter.