ᐅ Floor construction with screed and low installation 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 are now planning a complete renovation, including new underfloor heating on the ground floor, first floor, and attic. There is also a basement, but it is not heated.
The existing floor structure is quite simple, with insulation, screed, and covering adding up to only about 6.0 - 6.5cm (2.4 - 2.6 inches).
My initial plan was to remove everything and then install a floating screed along with 16mm (5/8 inch) underfloor heating pipes on a proper insulation layer.

After consulting various screed installers, I’m more confused than ever, as everyone recommends something different...
One suggests using Schlüter dimpled plates, since they only require 0.8cm (0.3 inch) of pipe covering, but I then saw that the plates alone are already 4.4cm (1.7 inches) thick, so that brings me to at least 5.2cm (2 inches). Adding some extra for pipe covering, I estimate about 5.5cm (2.2 inches) plus around 2cm (0.8 inch) insulation, totaling about 7.5cm (3 inches) plus the floor covering.

Another proposes a solution with Retanol additives, allowing pipe covering to be reduced to about 2.5 - 3.0cm (1 - 1.2 inches), resulting in about 4.5cm (1.8 inches) with pipes plus insulation, totaling 6.5cm (2.6 inches) before the floor covering. That sounds promising, but it is probably quite expensive; no price has been given yet...

A third option is to leave out the insulation completely and build in a conventional way, with a total height of about 6.5cm (2.6 inches) plus covering.

Does anyone have advice on whether to use cement-based screed or calcium sulfate screed? I know cement screed works for wet rooms and outdoors, whereas calcium sulfate conducts heat better but should not get wet. What about the Retanol additives—are they only compatible with cement screeds?

Also, what do you think about 2cm (0.8 inch) of thermal insulation? Is it enough to make a difference? What classification or product would you recommend? Or would it be better to leave out the insulation completely? Alternatively, insulating from below at the basement ceiling—is that worthwhile? The basement ceiling height is almost 2.2m (7.2 feet).

I hope I’ve included as much information as possible for now. I’m happy to answer further questions if needed.
I’d be grateful for any advice!
Best regards

Gotop09 schrieb:

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

Both work almost equally well—regardless of whether it’s a wet area or not.

I had a similar setup. Proper insulation on the ceiling is pointless if you can insulate the basement ceiling well from below (for example, 10–12cm (4–5 inches) of rigid polyurethane foam).

Suggestion: Use 20mm (1 inch) foil-faced rigid polyurethane foam for the insulation on the ceiling. You can fasten the underfloor heating pipes directly on it (there are suitable grid films for this) and then have a thin screed installed—whether with Retanol or other products. The additional cost is manageable, and if you include an accelerator in the screed mix, you can make up the extra expense by having the screed dry, for example, 4 weeks faster.

nordanney schrieb:

If you can properly insulate the basement ceiling from below (for example, 10-12cm (4-5 inches) of PUR foam).
Suggestion: Use 20mm (0.8 inches) foil-faced PUR foam for insulation on top of the ceiling.

First of all, thank you very much for your incredibly quick response!

Would you actually insulate both the basement ceiling from below and above? Or would that be redundant and basically unnecessary?
Or is the 20mm (0.8 inches) insulation layer on top of the ceiling mainly intended as impact sound insulation?
So far, I have basically ignored impact sound insulation because I thought I wouldn’t have the build-up height for it anyway, and a 20mm (0.8 inches) layer of insulation hopefully still helps a bit with sound?

Gotop09 schrieb:

Would you actually insulate both the underside and the top of the basement ceiling? Or would that be redundant and basically unnecessary?

If you later place a thermometer on the basement ceiling, you’ll realize it should have been better insulated. So yes, if you plan to heat the living spaces—not the basement—with underfloor heating, proper insulation is absolutely essential (you’ll also notice it in your heating costs).

Gotop09 schrieb:

Or is the 20mm (0.8 inch) insulation layer on the ceiling more for impact sound insulation?

Think of it as light insulation and a base for stapling the pipes. You can almost ignore impact sound insulation since below the rooms is just the basement, where no one is bothered by impact noise.

nordanney schrieb:

If you hold a thermometer against the basement ceiling later, you’ll know you should have insulated it better. So yes, if you want to heat the living areas and not the basement with underfloor heating, proper insulation is absolutely essential (you’ll also see this in your heating costs).

Think of it as a thin layer of insulation and as a base to which the pipes can be fastened. You can almost ignore impact sound insulation because you only have the basement below these rooms. No one will be disturbed by footsteps noise there.

Thank you very much! That already sounds quite good!
Let me summarize briefly:
Insulate the basement ceiling from below with 10–12 cm (4–5 inches),
then place a 2 cm (around 0.8 inch) insulation board plus a vapor barrier or foil on top of the ceiling,
then staple the underfloor heating pipes onto that,
and then cover with the thinnest possible screed. If possible, add an additive (Retanol) to reduce both the thickness and drying time.
I’m probably going to end up with a total build-up height of about 6.5 cm (2.5 inches) plus the flooring, making around 7.5 cm (3 inches). That is about 1.5 cm (0.6 inch) more build-up height than before, but I think I can live with that.

Regarding impact sound insulation, I hope the 2 cm (around 0.8 inch) insulation layer already provides some reduction. It doesn’t matter in the basement, of course, but on the upper and attic floors the same floor build-up will be used. So should I just not worry about it and accept the noise from the kids? Or is there any tip you could share?
I guess a few millimeters of impact sound insulation on top of the screed make no sense with underfloor heating, right?

Oh, and we are planning to use laminate flooring. Can I lay it directly on the screed without concern, or do I need an underlay?

And do you actually need a vapor barrier between the raw ceiling and the insulation, as shown in the picture? What exactly is that supposed to be?
Best regards

I believe that with 2 cm (1 inch) foil-faced PIR/PUR insulation, stapling might be a bit tight. Make sure to confirm that boards of this thickness are approved for stapling, and pay attention to the length of the staples. There are also hook-and-loop systems that work almost 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 some ways, offer the advantages of both materials together. These boards are correspondingly thicker.

If the ground floor and attic are within the same “climate zone” regarding underfloor heating, it is not necessary to install expensive PUR insulation in the attic – you generally don’t need insulation there if the rooms are evenly climate-controlled.