Hygroscopic salts on lime plaster

[dllive]

Hi
We have cob/stone walls. Gypsum plaster was put on and within weeks there was a tide mark on all the walls where the salts were attracting damp. We've hacked off all the gypsum and original lime plaster back to the bare walls.
Now we're going to put lime plaster on there (NHA 3.5). Will we get the same problem with the salts on the lime plaster? (dark patches, efflorescence etc...). Is there some treatment we can put on - either before the plaster goes on or as an additive to the plaster? Someone mentioned a plastic membrane which will prevent the salt penetrating the finished surface.
Thanks

 

[StephenStephen]

I'm only an amateur, but in my limited experience you won't get the same problem with lime plaster. I'm curious as to why you're using NHA 3.5, rather than something less hydraulic, or lime putty plaster. Using any kind of plastic membrane in a lime and cob wall sounds like madness.

 

[dllive]

Thats what the plasterer recommenced. What would a less hydraulic type of plaster be? Lime putty sounds like it will be a lot more expensive?

 

[StephenStephen]

Thats what the plasterer recommenced. What would a less hydraulic type of plaster be? Lime putty sounds like it will be a lot more expensive?

non-hydraulic lime plaster is the same as lime putty plaster, which yes, I personally would use indoors. It sets in air (carbonates), so will store if you keep the air out. Pure lime putty seems to be regarded as better (and more expensive) the longer it is stored.

Often it's the delivery which shoots the price up if you're using a smaller amount.

Hydraulic lime is also confusingly called NHL (natural hydraulic lime) and comes with charming descriptions of how hydraulic it is, from feebly hydraulic - NHL 2, through moderately hydraulic NHL 3.5, up to eminently hydraulic lime NHL 5. It sets by hydration, and stores ok if kept dry, though I don't think it keeps forever. personally I'd use NHL 3.5 with sand for outside.

If I remember rightly, lime putty plaster is more expensive if you're paying someone to do it, because it involves a series of layers with time gaps in between, where NHL goes off quicker, so is cheaper in labour. There are also a lot fewer plasterers familiar with putty - i don't think it is more difficult, just different.

...and then there's hydrated lime, which is something else again, and not what you want at all.

But (and it's a big but), almost all of what I've learnt is from books and videos, and I've only been using it for the last year, on one area in my home, and almost all internally, so take everything I say with a pinch of salt and listen to the guys on here who actually know what they're talking about.

 

[dllive]

Because this is to go on interior walls, maybe NHL 2 will do the job. Im paying someone to do it - so as you say - the labour will be more.