On Friday, 31 January 2020 11:08:51 UTC, wrote:

A cavity wall has a "tide mark" about 200mm above the floor. The wall
was originally an external wall but, as a result of building an
extension, is now an internal wall. The wall has a 1930s DPC and the
floors have modern DPMs.
I assume that the old DPC has failed so am wondering what the best
options are. At the moment I'm thinking of knocking-off the plaster to
about 300mm, coating the wall with something and then
rendering/plastering with an additive.
The question is what to put on the wall: liquid DPM? bitumen? cement
board (hardiebacker)? foil-backed plasterboard?
What does the team think?

Rising damp does exist but is rare. It is very unlikely that this is what's going on. DPCs are therefore not often useful. You propose DPC failure - how exactly does a hard material such as slate entombed in lime or cement fail?

Bottom of interior wall damp is usually from condensation. A wet slab from a plumbing leak can also cause it.

Coating with impermeables normally results in the coated area not drying out, and the matter therefore worsening. I'd look at the slab if practical, and look to see if ambient RH in the area can be reduced. Other options are to scrape & paint the area with lime, which permits it to dry better than emulsion, and use of a dehumidifier. The last 2 are often sufficient to keep things in check.


NT