We have a problem damp-proofing an old bathroom wall above the bath to prevent the moisture penetrating through the plaster wall to the paper insulation of the attic roof.
Above the vertical layer of tiles, this wall/ceiling slopes upwards with the angle of the roof (rising above the end of the bath), and this insulation lies behind it. I have placed special plastic sheeting over the wall and tacked it into place with a tacker. Over that I am placing a very thin, lightweight 6mm plaster board. I have heard that one should not place the board directly on the surface of the plastic sheeting as otherwise the board will not dry out but run with water from the steam. If it is kept off the plastic sheeting, the board will gradually give back it dampness to the room as it dries out. But what is the optimum distance from the sheeting for it to do this? The sheeting is not completely taut and so sags a bit in places. I was thinking of just places little bits of the 7mm board behind the board proper to hold it away from the wall, but would it be better to construct a wooden framework and join the boards on that? Also how should the joints between the boards be filled, and should one do one's best to prevent steam getting through the cracks as far as the plastic sheeting? How? And at the edges of the board wall where the end of the sheet emerges into the open how should one seal it and finish off? (The sheeting turns the corner and we end up with about six inches left on the other vertical house walls which have only brick, not attic insulation behind them.)
Another problem is that the sloping wall joins the ceiling in a kind of curve lasting about 9 inches. I have had to make indent cuts on the far side of the board so it will bend to follow this curve. But the bending operation makes some splits in places. How best seal such splits once the board is in place?
We need to use this very light board to prevent the weight bringing the whole slope down, and have not much room in the bathroom so the smaller the space we lose the better.