On occasions we are asked about the possibility of dry lining walls where rising damp has been encountered and a subsequent damp proof course installed. When considering dry lining the following must be borne in mind;
Rising damp is the upward capillary migration of ground water/moisture in masonry. One effect of rising damp is to carry salts from the ground into the wall. As a result hygroscopic salts (particularly nitrates and chlorides) will normally be present in walls and wall plaster suffering from/affected by rising damp. Hygroscopic salts have the ability to absorb moisture from the atmosphere/surrounding environment therefore potentially leaving a wall damp due to hygroscopic moisture alone.
Where rising dampness is encountered then in addition to carrying out damp proofing works by installing a damp proof course (dpc), part of the remedial treatment process also involves removal of the contaminated plaster back to the substrate.
Once a dpc has been installed it is imperative that subsequent replastering/making good is carried out correctly and in accordance with specification for the overall efficacy of the damp proofing system to be maintained. The function of the new plaster is to prevent hygroscopic salts that are present in the wall from migrating through to the surface, whilst at the same time allowing the wall to breathe and dry out. Generally sand/cement render incorporating Sovereign Rendermix or Sovereign Renderlite Renovating Plaster is used, although please note the use of Sovereign Renderlite Renovating Plasterwill also provide a thermally efficient backing plaster in addition to fulfilling these other necessary functions.
With regard to dry lining following the insertion of a dpc then two considerations must be taken into account. Firstly, gypsum based dab adhesive will readily absorb residual salts and moisture from the underlying wall and as such will allow their movement through, therefore failing in one of the required functions of new plastering. In order to overcome this, a barrier must first be applied to the wall prior to dot and dabbing. This could be achieved in several ways, such as using Sovereign SBR mixed with cement and water as a bonding coat or using the Sovereign Hey’di K11 system (both of which must be allowed to dry/cure), or using Sovereign PT Lath membrane, for example.
Secondly, the potential of condensation forming on the relatively cold wall surface in the void between wall and board must also be considered. Should this occur then pools of water and/or damp spots on the dab and board may be seen. Attempts can be made to overcome this by using an insulation backed board (thermal board) with vapour check, however, with this comes the issue of excessive thickness/loss of space.