A small (hairline)crack appeared a few months ago in the corner of the room above my walk-in shower cubicle. Extends from the top of the tiling (at approx 2m, to ceiling), consistent in width. Tried filling it in, but it has now come back, opening up again, but also slightly pronounced. I can only assume it is a direct result of water vapour from the shower steaming up the walls and affecting the plaster in different ways (one wall is a party wall, the other external). The repetitive absorption of water and subsequent drying (at different rates no doubt as the walls are probably slightly different temperatures). Is there a sealant product I could use to stop the plaster taking up the water vapour...? Or indeed any other solution to the problem?