Floor construction & height adjustment in old buildings (underfloor heating)

Good day dear community,

we have been renovating our house (old building) for about 1 year and have created a granny flat. This is located entirely on the upper floor and extends over the main house (marked in green, see attachment) and an extension (marked in yellow, see attachment). In the yellow-marked extension there is no screed but only the raw concrete ceiling. In the main house there is already screed. Therefore, there is already a height difference of about 2-4 cm between the two areas. In the extension, we have a buildup of 16 cm at the lowest point (so the meter mark is currently at 116 cm), other spots in the extension are at 14 cm. In the main house, the highest point is at 12 cm (so meter mark at 112 cm).

It is now planned to install underfloor heating in both areas and new screed on top of it. We are now wondering how to best proceed. Basically, I have two questions, one about height compensation between the two areas and the other about the general buildup.

Height compensation: Since the floor in the old building is very uneven (partly 2-3 cm difference), we wanted to use a leveling fill first in order to have a flat surface between the extension and main house. That way we have a consistent height everywhere.

Buildup: We now have about 12 cm floor buildup. The studded plates of the underfloor heating are 32 mm, plus screed and the floor covering. The floor covering will probably be vinyl; we still need to find out the thickness. Either way, something must still be installed under the studded plates of the underfloor heating to reach the required 12 cm. In the extension, we wanted to lay EPS on the fill and then the studded plates (which also have about 10 mm insulation). It is also somewhat easier to walk on the EPS boards when laying the underfloor heating than directly on the fill.

For the main house we then have two options. Either we lay EPS boards on the old screed here as well and then the underfloor heating, or we put the underfloor heating directly on the old screed and therefore need a thicker screed layer.

Summary:
1. Is the leveling fill (e.g. Fermacell) suitable for height compensation?

2. Can we lay EPS boards under the studded plates of the underfloor heating in the extension? Which boards are recommended or what must be considered? I remember that you have to be careful that only one kind of insulation or only one layer of impact sound insulation is allowed under the underfloor heating (keyword: give). The studded plates also have a kind of insulation, which EPS boards (or something completely different?) can we use here?

3. Should we also lay EPS boards under the underfloor heating in the main house or better more screed?

We are under a bit of time pressure; perhaps this influences the options (e.g. because more screed takes much longer to dry). However, regarding the screed I will start a separate thread anyway. Time pressure or not, it should be done properly.

Also, below the granny flat (which is on the upper floor) is our house. Therefore, (impact) sound insulation rather more than too little.

Thank you very much!

You have summarized it quite well! The screed goes, for insulation EPS 35 with a compressive strength of 150 kPa!!! Making the screed thicker increases the inertia of the heating.

Do I understand correctly that you want to lay another (heated) screed on the existing screed upstairs, possibly even thicker? Does the ceiling allow for that structurally?

Otherwise, 3 notes:

1) Use a bound fill, the loose ones always end up where you don't want them. Fermacell is quite expensive (80 liters for 30€), we used "Cyclepor rapid" (200 liters for ~15€).

2) You can find impact sound insulation as "EPS DES" and the boards usually have a WLG of 040 or 045. Watch out when buying! You can also notice it by touch, EPS DES is soft and pliable, EPS DEO rather hard.

3) There are also "solo" stud plates, i.e., without integrated insulation. This has the advantage that you can make the substructure yourself and usually save money.

Hi,

yes, understood correctly. New screed is to be laid on the existing screed. We asked the structural engineer back then and he said that would be okay. I can't assess that, but since he was otherwise quite meticulous...

We have already received the studded panels from the heating technician, as mentioned there is an approximately 1cm thick polystyrene board (?) underneath. The note was that you are not allowed to install impact sound insulation (i.e. cushioning) twice. I assume that we are therefore only allowed to use EPS DEO (without impact sound insulation), right?

If there are actually only 10mm EPS attached, I would assume that it is DEO and consequently a layer of DES could be possible underneath. No labels attached, or have you tried the tactile test?

Hello. I was at the house yesterday and took a look. These are Kermi x-net c11 pebble boards, article number: SFDNSP02000. It seems to be EPS DEO insulation. Does that mean you could lay DES boards underneath? I still don't quite understand what the "rules" are for how the insulation should be installed...