Hello,
As far as I know, with concrete it is not vapor diffusion but transport through capillary action that is crucial. By the way, WU concrete is not immune to this either.
That is correct ... I once again realize that I shouldn’t always keep it brief
WU Concrete:
*Waterproof (WU) structures are concrete constructions that are built without additional external membrane-like sealing and prevent water ingress in liquid form solely due to the material and special structural measures such as joint sealing and crack width limitation.
Moisture Transport
Moisture transport and diffusion within an undisturbed cross-section of waterproof concrete can be summarized according to recent studies as follows. Capillary water uptake on the water-facing side occurs independently of hydrostatic water pressure to a maximum depth of about 7 cm. On the air-facing side, the component slowly dries out. This diffusion zone describes the area where the excess water from the concrete is released to the indoor air (escape of construction moisture). The drying depth in the diffusion zone is a maximum of about 8 cm because the diffusion resistance increases with depth in the concrete. Furthermore, the moisture amount released by occupants on the room side counteracts drying through diffusion. As long as the capillary zone and diffusion zone do not overlap (e.g., component thickness > 20 cm), no moisture transport apparently takes place in the core area. This means that indoor moisture release behaves almost independently of the boundary conditions and the moisture situation on the outside, provided the thickness is sufficient. In buildings with high-quality use, diffusion slows down over time in the drying zone and reaches an equilibrium state because the temperature gradient from inside (approx. 20 °C) to outside (approx. 8 °C) counteracts water vapor release inward.
*Source: Regulations and recommendations for WU concrete
That then does protect, because with proper execution the cracks are small, against pressing water, but moisture is nonetheless transported through.
However, the term pressing water is not correctly used in this case, as special sealing according to DIN "18-haumichtot" must be applied as protection against this type of load case.
It is gradually becoming clear to me why WU concrete is nowadays constantly referred to as "white tub," even though regulations use this term erroneously at every turn from my subjective point of view. I learned this quite differently a long time ago...
**On the subject of "insulating the top floor ceiling":
Basically, two variants must be distinguished regarding insulation of the top floor ceiling. On the one hand, an open (non-walkable) attic insulation can be implemented if the attic cannot be used as a storage space, or on the other hand, one can decide on a walkable attic insulation in order to have storage space or room for a drying rack in the future.
Procedure and construction for a concrete ceiling:
1. As a vapor retarder, a vapor retarder foil should be applied over the entire existing concrete floor and carefully fastened and sealed all around. At beam footings and wall connections, leave at least 10 cm overlap and seal with special sealing adhesive. Absolutely ensure stress-free installation of the foil to avoid cracks and defects. Thoroughly clean the existing storage ceiling before applying the foil (sweep, vacuum, and preferably also wet wipe).
2. Now the insulation can be laid. Glass wool mats, stone wool, or natural insulation materials such as wood fiber insulation boards can be used. Begin laying always in the farthest rear corner and then work forward towards the stairs / access. When insulating the top floor ceiling, make sure that the insulation is sufficiently thick. Ideally, 240 mm+ WLG035 insulation should be applied. To prevent thermal bridges between the insulation mats, it makes sense to lay the mats in two layers (e.g., 2x 120 mm) with staggered joints.
Walkable attic insulation (concrete ceiling)
If the attic insulation is to be walkable, there is little difference in insulating a concrete ceiling compared to non-walkable attic insulation. A load-bearing insulation material must be used instead of insulation mats. Load-bearing XPS or PUR boards with circumferential tongue and groove are suitable here. These insulation boards often have better insulation values (WLG 032 or even 028) and can therefore be dimensioned somewhat thinner (about 160-180 mm) than the mats according to the above instructions.
To compensate for unevenness in the attic ceiling, a thin (10-20 mm) impact sound insulation mat can be placed under the actual insulation layer to avoid difficulties when laying the rigid XPS / PUR boards. To reduce thermal bridges at the connection areas to the beams, two layers can also be used here.
As a floor covering for walkable attic insulation, OSB boards or chipboards are laid floating directly on the insulation. The boards are glued in the tongue. The result is a load-bearing floor that now also retains heat inside the house.
Alternatively, the edge areas of the floor, which are not walkable due to the roof slope, can be insulated simply and non-walkably with glass wool mats as described above. This saves some money and facilitates connection to the roof beams to avoid thermal bridges.
**Source: from the depths of my information on house construction
Rhenish greetings