found this but does'nt really give a straight answer.
Whilst the insertion of a remedial damp-proof course will control the rising dampness it is unlikely to stop it; nevertheless it will do no harm in that it will at least reduce the flow of moisture into the material (NB. Certain considerations however are necessary for injection damp-proofing cob construction {Trotman , P.M. "Dampness in Cob Walls" BRE, 1995}).
The main problem is where old lime plasters still remain. This may be badly stained, heavily salt contaminated and deteriorated. In this state it clearly shows that there has certainly been a problem, and replacing it with a similar material isn't likely to do much better in the longer term! So to remove these material and apply a new lime plaster is highly likely to lead to similar problems - lime plasters are very permeable (although apparently no more vapour permeable than sand/cement mixes of the order of 1 : 6) and as such are highly likely to let through the dampness/colouration/salts already in the substrate. As such, spoiling may occur again in a relatively short time and lime plaster is a very expensive sacrificial coating! Also note that hygroscopic salt contamination alone can pass into new permeable material from the substrate without any 'free' water being present; this can occur when such salts become 'deliquescent' (ie, they become liquid and therefore mobile) under conditions of high humidities.
There is no problem with the above provided that the owner is prepared to accept that this may occur, but as stated above, this will make lime plastering a very expensive 'sacrificial' material, ie, as it spoils it will have to be periodically replaced if the owner requires a clean decorative surface. There is always an argument that lime plastering should be used but if it allowed the walls to 'breath'(?) then there shouldn't have been a problem in the first place!
There may also be a problem with the use of the stronger sand/cement mixes as described above; to meet their required design functions mixes of the type necessary may be far too strong for the background. It may be possible to use expanded metal lath to aid bonding but perhaps the best practice, should it be acceptable, would be to use a dry lining technique to provide a non-spoiling decorative surface. However, it may be possible to use one of the specialist 'renovating' plasters which are usually less 'strong' than the traditional dense sand/cement mixes (see below).
The answer to damp-roofing and replastering such properties is clearly with the owner - what are they expecting and what are they willing to accept. If they expect clean non-spoiling decorative surface then some kind of action with reference to replastering/finishing will need to be taken, but if they are quite happy with some degree of staining/spoiling then consideration can be given to leaving the old material but being aware what has actually caused it to deteriorate in the first place. BUT REMEMBER - make sure no wood or other biodegradable material is left in contact with any dampness - it will be at a high risk to rot developing!!
