Dry screed for office room in the basement

Hello everyone,

my house from the 60s had a party basement, which was later also used as a bedroom. Now it is supposed to become an office. There are two large windows and heating is provided by a radiator on the wall, so no underfloor heating.

Previously, there was a broken wooden floor. It has been torn out and I have about 7 cm (+1 cm new floor covering) free, to arrive level with the tiled hallway. A few cables in empty conduits (electricity and network) are laid on the floor.

I am currently very undecided about how to best rebuild the floor. At the moment I tend towards a dry screed, because I will probably also lay dry screed in the attic due to the even lower installation height. But that is another topic. In the basement I could practice that for the attic.

My current plan would be
- PE foil
- 20 mm (bound?) screed, then the electric cables laid over the floor are also covered
- 50 mm dry screed boards (Fermacell 2*12.5 cm board with 30 mm polystyrene rigid foam)

In another I read that I always have problems with condensing water because the floor slab is cold. So how can I best realize a correct floor construction? What would you recommend?

Your thoughts are not bad at all. The discharge does not have to be bound. For condensation to occur, the water first has to come from somewhere. I would do it that way. And the dry screed is a good system.

Hello unnamed questioner. Since this is a ground-contacting slab, "the enemy comes from 2 sides." This is actually moisture, on the one hand as rising damp from the backside and on the other hand as possible condensation. We could counter the former with a sealing layer or a vapor barrier (PE foil >0.2mm), but this makes no sense for installation pipes running above the slab. So we sweep the surface of the slab clean of all dirt particles and level the entire area with a bonded screed up to the top edge of the installations. Bonded screeds MUST be used according to the state of the art, which does not mean that water is involved. To get the most important point out of the way first: The buildup height is actually not sufficient to meet the requirements of the Energy Saving Ordinance!! This would require 100mm thickness (thermal insulation layer). This means: cold feet are also preprogrammed for the future in winter. Due to a lack of knowledge about the new top layer (with 1cm), I have not been able to precisely calculate whether and where how much condensate will form in the cross-section of the floor. With a superficial calculation, I arrive at a theoretical value of 0.236kg/m² floor area, but this calculation did not take into account that an EPS thermal insulation material (or a PUR rigid foam) is used, which due to its closed-cell structure does not absorb water anyway and must be considered accordingly when assessing the condensation failure limit. As a result, the aforementioned value (condensate) is uncritical insofar as no moisture-sensitive top layer is laid. @ Vicky Pedia, a note that one as a layperson cannot filter out all connections via GOOGLE and recognize their effects. When a question is posted in the forum here, the questioner also expects a usable answer. Because he (!) must bear the consequences later, especially from foreign misassessments. ---------------------- Wishing you success: KlaRa

Thank you .

In summary, this means that it should actually be feasible. I should even be able to get the PE film under the pipes.
As a top layer, I have roughly estimated the 1 cm now; it is supposed to be a vinyl floor, which are generally considered resistant to moisture.
I understand that I cannot comply with the energy saving regulation; that is simply the case in this part of the old building.
Regarding the condensate: How much would theoretically still be acceptable? Ubakus says 0.5 kg/m² within 90 days.

What would it look like with proper screed instead of dry screed? Would that also be feasible? What floor structure would be recommended in this variant?

Yes, in summary that means: it should be doable. Even if you get cold feet. The PE foil on the raw ceiling, you can skip that! It does not bring anything except a lot of “fiddly work” (because of the pipe installations), many foil folds, and the feeling that the sheet joints cannot be sealed cleanly. Much more important is the thicker PE foil (with 20cm overlap at the sheet seams) above the mineral filling! That is also easier to lay (since it is already leveled), and any moisture does not harm the filling. Since the filling also has no thermal insulation tasks, the placement of the PE foil at this spot as I described is already fine.

Zu dem Kondensat: Wie viel wäre denn theoretisch noch ok? Ubakus sagt 0,5kg/m² innerhalb von 90 Tagen.

You can’t say that like this, because this amount of condensate takes into account that it also evaporates again in the warmer seasons. But nothing can evaporate in a usual manner at the spot you planned! My suggestion would be to carry out the structure as described and additionally plan for electric underfloor heating. However, this is not possible with a PVC floor glued on the screed surface, but would be possible, for example, with multi-layer parquet or other, for instance multi-layer modular floor coverings, which are laid floating. With electric underfloor heating, the problem of cold feet in winter would also be solved. --------------------- You may recognize at this point that floor technology requires more knowledge than is available through public forums? Good luck: KlaRa