The walls in our downstairs bathroom had been damp for some years before we moved in, due, we think, to damaged render and brickwork externally. We had the external wall repaired and rendered last summer, and took the panelling off the internal wall before Christmas, and knocked off the old plaster during the bathroom re-fit.
We then had all walls skimmed, and the new plaster has mostly dried out, apart from a patch in the middle of the external wall, close to where the render and brickwork had been damaged - it's not near the floor, or the window or any pipework, but the plaster surface is very cold and damp - is this simply a case of letting the wall dry out a bit more (maybe until after the summer when hopefully it will be warmer and drier), or could it indicate something more sinister??
From your description I assume your house has solid walls.
You need to have a look at the new rendering and see if there are any imperfections in it ie cracks. The rendering also should be painted, again with no cracks.
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