I am in the process of renovating my bathroom, which was built in 1978 according to the building practices of the time. Partly I was tired of the '70s style, and partly I knew the grout in the shower had been leaking water. When I tore out the old bathroom, the wooden studs in one corner of the shower were indeed rotten, but otherwise, everything else was dry and undamaged. The floor consisted of a cast slab on two layers of ground boards over a fill of washed macadam. On the casting was a bitumen mat (moisture barrier), and on this, a 7 cm casting with an embedded coil of "green" (iron) heating water pipes as underfloor heating. On top lay terracotta tiles. When the tiles, drywall, and wooden studs were removed, the room was bare to the adobe walls, which were also in good condition. I chiseled away the floor tiles and casting down to the bitumen mat - bone dry. The underfloor heating coil was in good condition, and I assess it can last another 40 years (I zinc-sprayed it for safety). Now that I am building a new bathroom, I want everything to be made of stone material, so I am constructing walls of lightweight concrete with an air gap inside the adobe wall. I am hiring professionals and want a wet room certificate, and they must then install a moisture barrier according to the highest standards. When we poured new concrete on the bitumen mat, which I left in place, it suddenly struck me; How smart is it to have a casting between two moisture barriers - with an embedded pipe coil? The casting is Bemix P3 with "rapid drying." On the casting, which is limited by the old foundation wall (stone and clay), the lightweight concrete walls will stand. This will be the only path for remaining construction moisture to escape once the new floor moisture barrier is applied. Building the lightweight concrete walls might take 3-4 days, which gives the concrete a drying time of about 10 days. Can the construction work? What moisture content should the casting/self-leveling screed reach before I dare to apply the floor moisture barrier? How long can it be expected to take, weeks - months? The casting is 4-5 cm thick. (Cannot find/receive any useful data from the manufacturer) An alternative is to have no moisture barrier on the floor at all (= no certificate) and let it function as before, where the underfloor heating coil dries out remaining construction moisture and any future potential moisture penetration. (Here the problem is that the construction industry has made people terrified of bathrooms without a wet room certificate)
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I think it's a problem that industry regulations are being confused with the building code (BBR). The latter has two requirements for wet rooms: 1 it should not leak water into adjacent spaces, and 2 floor and wall constructions should not be damaged by penetrating water. The industry regulations are designed by companies that want to sell as much tile as possible, even in contexts where it is less suitable. I think it's inappropriate, not to say unethical, to use tiles, which can last for thousands of years, on top of wooden structures in wet rooms. The moisture-proofing only lasts a fraction of the time that is reasonable. Therefore, I believe you are absolutely right to choose stone materials as the base for floors and walls. But you cannot have double moisture barriers.