ᐅ Dry screed installation in the basement of an old building under renovation

Hello everyone,

I want to convert a basement room into living space.
1. The raw, uninsulated concrete slab will be sealed with a Börner Erika vapor-tight sealing tape, which will be slightly raised at the edges.
2. After that, a dry screed with a maximum height of about 2cm (1 inch) will be applied, and on top of that, 22mm or 25mm (7/8 inch or 1 inch) OSB 3 panels will be installed. (The room is dry—outside it has been newly waterproofed and the exterior walls are plastered with cement plaster.)
3. Should wood fiber insulation boards be placed on top of the screed before installing the OSB panels?
4. The screed must not be thicker because there is limited ceiling height available.

What type of screed would you recommend for this? I was thinking of Knauf Bituperl or possibly Fermacell’s dry leveling screed.

Should an additional vapor barrier be installed on top of the OSB panels before laying laminate flooring?

Thank you very much in advance!

Hello "chrissi88".

No matter how you choose to build it, the damage pattern is already predetermined because you have an uninsulated, ground-contacting slab, and indoor humidity will definitely condense inside the construction during the cold season.

By placing a vapor barrier, you can basically only control where the water condenses.
Note: for a ground-contacting slab, 10 cm (4 inches) of insulation with a thermal conductivity of WL035 should be used.

Besides condensation problems, you will have quite cold floors; do not underestimate this.
The more you heat later to compensate, the more moisture the warmed indoor air can absorb and then release within the floor structure again.

No, it doesn’t work like that!

Regards, KlaRa

That’s not quite what I wanted to hear...
but unfortunately, I have no other option since the room is adjacent to other already finished rooms, which were constructed in the late 1960s using mastic asphalt screed.
So, I can’t change any heights due to the ceiling height as well as the adjoining floor.
It worked before, so why not anymore?
The mastic asphalt, like the Boern­er Erika, has an SD value greater than 1500.
Therefore, similar conditions exist starting from the concrete slab. And in the old bathroom, the tiles were even glued directly onto the slab with mortar....

Hello "chrissi88".

I understand well that such feedback is not encouraging. It may be true that other floor constructions in the building have worked. I personally rely only on building physics and listen carefully to what it tells me.

I’ll gladly explain: It says that at an assumed room temperature of a cozy 22°C (72°F) and a relative humidity of 65%, the air contains about 17.1 g of water per cubic meter (m³). We can just ignore this or listen further! Building physics goes on to say that for components in contact with the ground, such as floor slabs, whether sealed with "Börner Erika," "Katja," or something else, the floor temperature in winter months can be expected to be around 8°C (46°F). I already mentioned the “cold feet” at the beginning.

Building physics further states that at these temperatures, the saturation point for air is 6.65 g of water per cubic meter (m³). If we calculate the difference, the amount of water vapor in the air (17.1 g) will cause about 10.5 g of water to condense per cubic meter (m³) of your room’s volume onto your nice laminate flooring.

This is exactly what I was referring to in my first post on this topic. Usually, this situation is addressed by designing the construction and material thicknesses (especially insulation) so that the dew point shifts below the waterproofing layer.

If this is not possible, there are only two options: accept that it will get wet inside the room, or use the space for a different (secondary) purpose.

In many cases, building physics can be “outsmarted” with modern materials, but there are limits we have to accept.

You are certainly free to build as you envision. However, it will be little help to listen to voices who have become so-called experts through self-teaching following the motto “I once had the same...”

Any damage that might occur will be your sole responsibility, without sympathy from public forums, and the self-proclaimed experts will quietly stay out of it.

--------------------------------------------

Good luck: KlaRa

Thank you very much for your detailed response!

Does it make a difference for the other rooms, or should I expect something similar? The poured asphalt has poor heat/cold transfer compared to cement-based floors.

That's why I was considering OSB, as it feels warmer underfoot.
Am I shifting the condensation into the OSB board or the filling layer, or is it only on the surface of the finished floor?
By the way, the room is going to be a bedroom.

Is there a way to minimize this with other building materials without having to accept high insulation thicknesses?

Best regards,
Chrissi88

Alternatively, I came across this board
LIN LINITHERM PGF
20(+18) mm (0.8 (+0.7) inches), 1235 x 635 mm (49 x 25 inches), 024
possibly also available in 30 mm (1.2 inches) insulation thickness, but preferably 20 mm (0.8 inches)