ᐅ Insulation Beneath Screed in Older Home Renovations with Underfloor Heating – Experiences?

Hello dear home builders,

I live on the outskirts of Munich and am currently renovating my grandparents’ detached house from the 1960s. The entire floor will be redone with screed and underfloor heating. The old screed and insulation have already been removed. The build-up height of the old floor was about 8cm (3 inches). To avoid problems with doors and balcony windows, the new floor should, if possible, not exceed 10-12cm (4-5 inches) in height—that is, only 2-4cm (1-1.5 inches) more than the old one.

I have received a quote from a heating engineer that includes the following insulation materials, which I don’t know how to evaluate. They are supposed to be placed on top of each other below the underfloor heating and seem unusual to me in this combination. This insulation is planned for the ground floor (unheated cellar) as well as the floors above. No additional insulation is included in the offer.

- Rock wool impact sound insulation 5 kN/m², thickness 13-30mm (0.5-1 inch), thermal conductivity 0.63 mK/W
- Mineral fiber impact sound insulation 3.5 kN/m², thickness 20-50mm (0.8-2 inches), thermal conductivity 0.37 mK/W

My online research has not clarified this so far. I am currently stuck here and would really appreciate it if someone could explain what these materials are and whether this combination is common or if there might be better solutions for me. Of course, I will also discuss this with the heating engineer, but I want to gather more information beforehand if possible.

Many thanks in advance!
AndreasBauer

AndreasBauer schrieb:
Stone wool TP impact sound insulation 5kN/m² 13-3mm of the skr 0.63 m²·K/W

Stone wool = Material (harder than glass wool)
TP = Trim sound insulation Panel (manufacturer’s abbreviation, possibly Knauf?)
5kN/m² = Load capacity (approximately 500 kg per square meter, which is higher than a typical screed)
13-3mm = Thickness of the panel and maximum compression. The panel is 13 mm (0.5 inches) thick and can be compressed by up to 3 mm (0.12 inches) under load
DES = Designation of the application area for these insulation panels. Here: interior insulation of ceilings or floor slabs beneath screed with impact sound insulation requirements
SK = Indicates the "compression class." However, to my knowledge, there are only "sg," "sm," and "sh" classes (low, medium, and high). Could the "K" be a typo?
R = R-value, the thermal resistance, which is the inverse of the U-value
0.63 m²·K/W = R-value of this insulation panel (although this value seems questionable to me)

Regarding whether this construction method is common, I cannot answer due to lack of experience. However, I have been told that mineral wool used as insulation beneath screed is not ideal because it is generally too soft. This can cause permanent sagging of the screed, leading to associated problems with baseboards and/or grout joints at the wall-to-floor transition.

Generally, I would first ask why this setup is planned for the ground floor above an unheated basement. On the one hand, there are more effective options (e.g., polyurethane panels) that provide significantly better insulation values, and on the other hand, impact sound insulation is not really necessary toward the basement level.

Regarding the construction height: Have you considered the rise of the first and last steps of your staircase? If the new floor is 4 cm (1.6 inches) higher, the step height will decrease, which may create a tripping hazard. According to DIN standards, a maximum deviation of 15 mm (0.6 inches) is allowed at the start and end of a stair run.

That is hardly any insulation, just impact sound insulation. However, underfloor heating should have proper insulation beneath it. Depending on the available build-up height, this can be easily achieved with a dry construction underfloor heating system (including screed, total build-up height 4.5 cm (1.8 inches)) plus the insulation layer. With 8 cm (3.1 inches) total height, that leaves only 3.5 cm (1.4 inches) for insulation, so the final floor level remains the same.

In this case, the best solution would probably be a 2 cm (0.8 inches) vacuum insulation panel (which I believe has a thermal conductivity of 0.07 W/(m·K)) and the remaining 1.5 cm (0.6 inches) should be reserved for leveling the floor. Since the underfloor heating system panel itself also includes 2.5 cm (1 inch) of EPS insulation, this insulation will be more than sufficient.

Thank you very much for your quick and detailed answers!

I wasn’t sure if it was relevant, but on top of the insulation there will be PE screed foil and a Praski Bavaria biofiber perforated board (which also doesn’t seem to be standard), on which the underfloor heating pipes will be clipped. Above that, there will be an anhydrite screed. The floor covering will be tiles on the ground floor and first floor, and laminate or vinyl in the attic.

I exactly feared the lack of thermal insulation. Would an alternative to insulating the ground floor be insulating the basement ceiling? Unfortunately, I only have limited space there due to the low basement ceiling height (2–3 cm (1–1.2 inches)). I will take a closer look at the suggestion of vacuum insulation panels.

Thank you very much!
AndreasBauer

Of course, you can also install the insulation board from below (on the basement ceiling), but this does not prevent most of the heat from flowing downwards (into the floor slab). The partition walls will also be heated in this case, whereas with insulation applied from above, you can reduce this effect somewhat by using an edge insulation strip. If you have not yet signed the heating company's offer, it may be worth obtaining an alternative quote that includes insulation from above.

The monetary cost of vacuum insulation panels is likely disproportionate to the potential energy savings.