Hygroscopic salts and lime plaster

[dllive]

Hi

We have cob/stone walls. Earlier this year they were plastered with gypsum plaster. Shortly after there was damp patches all along the bottom of the external and partition walls. An independant damp specialist said this was due to hygroscopic salts and the dampness was trapped behind the cementitious plaster.

We have removed the plaster and sprayed a salt neutralising product (acetic acid) on the walls. The walls are currently exposed and I am wondering what to do next.

Its obviously sensible to use lime plaster so the walls can breathe, but my concern is the salts will come back.

Ive spoken to a few lime plasterers, none of which have heard of hygroscopic salts.

Is there a salt resisting lime plaster mix that could be used?

If the salts do come back, will the lime plaster handle it better? (the moisture will evaporate)

Thanks

 

[bobasd]

your lime plasterers - whatever that means - are ignorant and sound to be short course semi-skilled chancers,, have they heard of google?
all skilled and NVQ or City & Guilds qualified plasterers are trained in the effects of damp and condensation.
all skilled plasterers can do lime plastering or rendering.

you can use a sand and lime 3:1 mix and Limelite skim finish for your work or pay far more for Limelite remedial plaster. both options work for a number of years in keeping back damp.

 

[blup]

Lime plaster is supposed to help the walls "breathe" but won't remove the salts, and therefore the source of the moisture. It must surely be an improvement on gypsum plaster though.

Isn't using acetic acid to neutralise salts a bit like giving someone caustic soda who has accidentally swallowed acid?

I would leave the wall to dry out for as long as is possible/practicable.

Blup

 

[dllive]

Im fairly sure Ive eliminated all sources of damp (blocked drain, raised ground level at front of elevation etc...) so Im *hoping* the walls wont get damp again. The damp specialist said it was historic damp that had caused the salts to form.

Heres what some of the walls currently look like...

As you can see, the walls are a hodgepodge of cob, stone, brick (all of which seems bone dry):

Is this white dusting along the bottom of the wall the salts?:

When its sunny outside and we havnt had rain for a few days the stone looks perfectly dry. But when it rains some of the stone looks a lot darker as if its wet, although doesnt feel wet to the touch. Could this be moisture condensing on the stone; water wicking through the stone; or my paranoia?!

The gypsum plaster was chipped off about 2 months ago. I thought I would leave it for most of the winter and then take a view on what to do next. Some people have suggested I Newton Lathe the walls (plastic membrane which prevents salts migrating through to finished plaster surface); others have said I should lime plaster them.

I just want to mitigate against going to the expense of putting lime plaster on and having the damp patches and tide lines come back! Im confused what cause of action to do next...

 

[blup]

Yes, the white powder seems to be the salt.

You appear to have removed the sources of ground damp, but not the damp from rain penetrating the walls at the bottom, that will continue for ever. I believe the salts remain in the walls etc and attract the moisture which gathers from the penetration of rain (or whatever). There are different schools of thought on whether to tank such walls (your plastic membrane/gypsum plaster) or allow the wall to breathe (e.g. using lime plaster).

The second course of action seems to me (a DIYER) to make more sense because damp retained in walls will find other ways to escape. In other cases like a cellar, tanking may be the only viable option.

Blup