Damp trapped?

[inwood]

I've now replastered the dining room. However I appear to have a damp problem where there 'appeared' to be none before.

Original plaster is a daub base, with well haired lime skim onto stonework (600-900mm stone walls). It had been emulsioned/wallpapered/emulsioned a few times so this was removed as carefully as possible. This left about 60% intact original plaster but not a sufficiently good surface (after all the scraping) to be left as the finished surface.

I then went about repairing with lime scratch and devil coats up to the level of the existing plaster before skimming the whole surface to give a good finish.

I now have what appear to be damp areas where the old daub/lime skim is behind the new finish coat. These just don't appear to be drying out - all the areas where the plaster was fully replaced have dried out very well. If you stand back from the wall you can clearly see where the original plaster remains.

My question is therefore 'is daub less breathable than lime'? and have I effectively trapped the moisture now in the daub by skimming over the top? (although I can't see how). I must admit this is the only external wall that is left to repoint (and badly in need of it!) but I can't see why the two backgrounds are so different in their breathability. My only thought was that the daub probably has a lot of clay in it which may hold the moisture content higher.

Any thoughts/suggestions as to best approach to resolving?. Since it's only two walls the worst case scenario would be to start again but I'd rather not have to.

 

[Andrew Bodenham]

An interesting scenario indeed. I'm presuming that the overall skim coat was lime based, yes? In which case I can't really see how moisture could be trapped beneath it. You'll not have used any PVA or similar bonding solutions?

How long has the work been done? Have you got any heating in the room? My only thought is that if it's an external wall, and there was any difference in the insulating properties of the old and new plaster then you may be getting condensation forming on the areas which have the old plaster underneath - possibly made worse if the new full thickness areas are still drying out a bit and adding to the humidity. Far fetched really - I'm clutching at straws here!

Come on, experts....

 

[inwood]

Double posted, sorry, correct version below.

 

[inwood]

Thanks Andrew, yes skim was lime and silver sand (1:1) and no bonding agent - just scratched up the previous finish coat.

Thought this might help to visualise issue (photo taken begining of July):

You can see the damp area to the bottom right of the beam. No internal heating..... yet!!! but very little condensation on the walls normally (except today when it was warmer outside than in - opened doors and condensation formed on most walls but never got close to looking as damp as this). If the daub is holding the moisture it might explain why the original beam (which was replaced by the one in the photo) was in such a poor state (it had lost about 70% of its depth to insect attack).

My concern is that this is the first area where the original plaster has been skimmed - upstairs I've managed to retain between 60% and 90% of the original but these rooms need to be skimmed - I don't want the same thing to happen.

The odd thing is that the original plaster didn't look or seem damp before it was skimmed, although I never tested how damp it was.

Damp does seem worse when it rains and seems to disappear (although not fully) when dry, still that doesn't explain why it's not drying out completely.

The mind boggles.

 

[Lime]

It looks like condensation to me.

 

[inwood]

It looks like condensation to me.

Mmmm, pretty sure it isn't - it's been there four months now and isn't getting any better. That also wouldn't explain why the damp areas are only limited to where the original daub with lime skim sits behind my skim.

 

[Lime]

It looks like condensation to me.

Mmmm, pretty sure it isn't - it's been there four months now and isn't getting any better. That also wouldn't explain why the damp areas are only limited to where the original daub with lime skim sits behind my skim.

Well it's your wall and you are in a position to judge better than I.
It looks like condensation onto a cool wall from a door opening.
Differing wall surfaces will have differing evaporation rates and as a consequence the surface temperature of this area may be lower too.
Obviously pointing up the outside must be a priority.

 

[Gervase]

It looks like penetrating damp from defective pointing to me (just to add yet another opinion to the mix!). You sometimes find a stubborn patch in rubble-stone walls where cracks in inappropriate pointing, the lie of the stones and the fill combine together to channel moisture to the inside, and what you have looks very similar to something I encountered a year or so back. That was cured by repointing and several coats of limewash. Be warned, though - it takes a long time for the damp patch to go, so you may be looking at a dark area for some months after you sort out the exterior. Once you've got some heating on, of course, that will speed up the drying.
The fact that the damp is where the original daub remains is intriguing - it could be that the earth/clay from which the daub was made is naturally hygroscopic. That said, there's no reason why it shouldn't eventually surrender its moisture content once the source is stopped. The fact that the end of the beam was knackered is a classic symptom of penetrating damp - moisture gets in through cracks in defective cement pointing, can't escape and ends up saturating the stonework and anything in it - including beams and joist ends, which rot. Thus I'd say that your damp daub/render is a symptom rather than a problem in itself.
In addition to repointing, be sure to check your guttering, hoppers and downpipes on that outside wall. If they overflow in heavy rain, there's a risk of even more water being blown against the wall, which will exacerbate the situation. I'd hazard a guess that the wall faces the prevailing wind, but that's not always the case, given that buildings can cause strange eddies and vortexes around them. Next time it's blowing a hooligan and cats and dogs are coming down (in the next day or so, given our current weather), take a look outside and watch how the water hits the wall.