I have a toilet on the floor of my house, of which 1 wall gives to the outside (with window that we open quite often), one is contiguous to a shower cubicle whose wall is completely tiled and another wall is contiguous to the corridor but is the extension of another wall of the shower cubicle, also tiled (imagine a T whose right space is occupied by the shower and the left by the WC). Large humidity on both walls (in plaster) concerned by the shower enclosure (left space of the T) about 50 cm high starting from the ground and the angle of the two walls affected, and concerns especially the wall located in the extension of the shower enclosure up to about 50 cm from the ridge (inside corner of the two walls); this humidity is stronger on the WC side than on the corridor side of the wall concerned. Shower tray level slightly raised above the WC floor. No trace of water on the lower floor (but the two floors are separated by a concrete slab it seems). I redid the silicone seals of the shower tray and the floor-wall angle of the shower enclosure and padded Starwax waterproofing over the tile joints of the shower up to 80 cm from the floor, but in three weeks, I do not feel that my wall dries even a little. What can be the cause?
It is always difficult to give an opinion without going on the scene to see the problem. From what you describe the concern could come from a problem of waterproofness of the shower tray. If that were the case, it will have to be changed. Over time the joints of your tiles could become porous. Repeat the joints with a waterproof mortar. You specify that humidity is higher on the side of the toilet, a water pipe in the wall may be defective. I advise you to take as soon as possible the opinion of a plumber, who can establish a diagnosis and find a solution to the problem before it grows.
However, you do not specify what is the ventilation of this toilet and shower room. Opening the window "often enough" seems to me very insufficient to ensure good ventilation of these parts... Put an extractor in each room or, better yet, install a VMC.
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