ᐅ Insulating an Existing Basement Floor and Installing Underfloor Heating

In my newly purchased detached house from 1985, I want to retrofit insulation under the basement floor and install underfloor heating.
There is a screed without insulation present. The screed was poured directly onto the concrete floor.
I now plan to break up the screed. The concrete basement floor underneath is uneven.
Now my questions regarding the new construction...
1. Do I need to level and seal the uneven floor? The floor and the existing basement are dry. However, I have heard that when installing underfloor heating retroactively, the floor can then draw moisture (or condensation)?!!
2. My plan is to place 10 cm (4 inches) of rigid polyurethane insulation on the existing floor, install the underfloor heating system on top, and then pour screed over everything. What is the correct layering or procedure here?

As part of a renovation, we installed underfloor heating. Since there isn’t a basement everywhere, the layers above the slab were as follows:
Leveling compound
6cm (2.4 inches) insulation
Dry screed panels
Retrofitted underfloor heating
Screed/leveling compound
Tiles

Do you have enough ceiling height?

Good morning,
yes, a ceiling height of about 2.60 m (8 feet 6 inches) is sufficient. Is it not possible to install the underfloor heating directly on the insulation?
That would reduce the overall build-up height...
Thanks and best regards

As far as I know, no, because the insulation cannot withstand the pressure. There needs to be an additional layer for load distribution in between. Our construction added almost 15 cm (6 inches) in height.

Vinzenz schrieb:

2. My plan was to lay 10cm (4 inches) of PU insulation on the existing floor – then install the underfloor heating system – and finally pour screed over everything. What is the correct assembly or procedure here?

I would install a vapor barrier membrane on the existing floor (this also helps against radon; whether this is necessary or advisable depends on the residential area – checking the "radon map" can provide guidance). Then the PU insulation → thin fixing/plastic plates → underfloor heating system with screed → floor covering.

Whether a leveling compound is needed on the concrete floor beforehand can only be determined on site – small unevenness can be compensated by the insulation, but a 3cm (1.2 inches) height difference from one corner of the house to the other cannot.

Pinky0301 schrieb:

To my knowledge, no, because the insulation cannot withstand the pressure. There needs to be an additional layer for load distribution.

I suspect there was a different reason, as standard floor insulation boards and underfloor heating fixing panels are already designed to distribute the load of the screed applied on top. Additional dry screed is not necessary in this standard construction setup.