Floor construction, screed, low height

Hello everyone,

I have been puzzling over our floor construction for weeks and wanted to ask around...

We have an old house built in 1966 and now want to renovate completely, including new underfloor heating on the ground floor, first floor, and attic. There is a basement as well, but it's not heated.
The existing floor buildup is very minimal and with insulation, screed, and covering only about 6.0 - 6.5 cm.
Basically, I wanted to remove everything first and then lay a floating screed including the 16mm floor pipes on a proper insulation layer.

After consulting various screed contractors, I am now more confused than ever, everyone says something different...
One recommends the Schlüter studded panels because they only require 0.8 cm pipe coverage, but I then saw that the panels alone are already 4.4 cm thick, so that puts me back at at least 5.2 cm, I am calculating about 5.5 cm plus insulation around 2 cm? That would be a total of 7.5 cm plus covering.

Another recommends a solution with Retanol additives, which could reduce the coverage to about 2.5-3.0 cm, so with floor pipes about 4.5 cm plus insulation I would be at 6.5 cm plus covering. That doesn't sound bad at first, but it's probably really expensive, since no price has been given to me yet...

The third would leave out the insulation and build up entirely conventionally, with about 6.5 cm total plus covering.

Does anyone also have tips on whether to use cement screed or calcium sulfate? I know cement screed is good for wet rooms and outside, whereas calcium sulfate conducts heat better but should not get wet. But how about the Retanol additives? Or are they only for cement screeds anyway?

And what do you say about 2 cm thermal insulation? Can you get something decent with that? According to which classification or product should I look?
Or how about leaving out the insulation? Or insulating from below on the basement ceiling? Does that even help? Basement height is almost 2.2 m.

I hope I have included as much information as possible for now, I am happy to answer further questions.
I would be grateful for any advice first of all!
Best regards

Does anyone also have tips on whether to use cement screed or calcium sulfate? I know cement screed is suitable for wet rooms and outdoor use, while calcium sulfate conducts better,

Both work practically equally well – regardless of whether it’s a wet room or not.

I had a similar setup. "Proper" insulation on the ceiling is nonsense if you can properly insulate the basement ceiling from below (e.g., 10-12cm PUR).
Suggestion: Use 20mm PUR aluminum-coated for the insulation on the ceiling. On top of that, staple the underfloor heating pipes directly (there are suitable grid films for this) and have a thin screed laid – whether with Retanol or other products. The surcharge is manageable, and if you add an accelerator, you will recover the extra costs through, for example, a screed that dries 4 weeks faster.

if you can insulate the basement ceiling well from below (e.g. 10-12cm PUR).
Suggestion: Take 20mm PUR with aluminum foil for the insulation on the ceiling.

So, first of all, thank you very much for your incredibly quick feedback!

Would you actually do both on the basement ceiling, meaning insulate from below and above? Or would that be redundant and basically unnecessary?
Or is the 20mm insulation layer on the ceiling more to be seen as impact sound insulation?
I have basically ignored the impact sound insulation topic so far because I thought I wouldn’t have any building height for that anyway, and a 20mm insulation hopefully brings a bit of sound improvement too!?

Would you then actually wrap both sides of the cellar ceiling, meaning from below and above? Or would that be redundant and basically unnecessary?

If you later hold a thermometer to the cellar ceiling, then you will know that you should have insulated it better. So yes, if you want to heat the living spaces and not the cellar with the underfloor heating, then proper insulation is absolutely essential (you will also notice this in the heating costs).

Or is the 20mm insulation layer on the ceiling more to be seen as impact sound insulation?

See it as light insulation and as a base on which the pipes can be stapled. You can almost disregard impact sound insulation since you only have the cellar beneath the rooms. Nobody is bothered by impact noise there.

If you hold a thermometer to the basement ceiling later on, you'll know that you should have insulated it better. So yes, if you want to heat the living spaces and not the basement with the underfloor heating, proper insulation is absolutely essential (you will also notice this in the heating costs).

Consider it a small insulation layer and a base on which the pipes can be stapled. You can almost neglect impact sound insulation since you only have the basement below the rooms. No one is bothered by impact noise there.

Thank you very much! That already sounds really good!
Let me quickly summarize:
Insulate the basement ceiling from below with 10-12cm
Then, on the ceiling, first 2cm insulation board plus foil, then staple the underfloor heating pipes on top, and then cover with as thin a screed as possible. If possible with an additive (Retanol) to reduce the thickness and drying time.
I then probably come to about 6.5cm build-up height plus covering means about 7.5cm. That's about 1.5cm more build-up height than existing, but I think I can live with that.

Regarding impact sound, I just hope the 2cm insulation layer helps a bit. For the basement it's of course irrelevant, but on the upper floor and attic the same floor construction should apply. So does that mean I just shouldn't complain and should put up with the kids?
Or is there maybe a tip for that? I guess a few millimeters of impact sound insulation on the screed is nonsense with underfloor heating, right?
Oh, and we planned laminate flooring, can I then safely lay it directly on the screed or do I need an underlay?

And do you actually need a vapor barrier between the raw ceiling and insulation, see picture? Or what exactly is that?
Best regards

I believe that with 2 cm PIR/PUR insulation laminated with aluminum foil, it becomes a bit tight to use staples. In any case, make sure that the boards (of this thickness) are approved for stapling, and pay attention to the length of the staples. However, there are also hook-and-loop systems that work almost just as well.

PIR/PUR alone is not suitable for impact sound insulation. However, there are composite boards that combine PIR/PUR with EPS and, in a way, bring together the advantages of both materials. But then the thickness is correspondingly greater.

But as long as the ground floor (EG) and attic (DG) are in the same "climate zone" regarding underfloor heating, it is not necessary to install expensive PUR in the attic—there you do not basically need insulation if the rooms are heated relatively evenly.