Thin film underfloor heating experiences

Hello everyone, does anyone here have experience with thin-layer underfloor heating systems? I have a limited floor construction height (8 cm from concrete, including floor covering). This results in about 3-4 cm for the underfloor heating in my earth-contact basement, where additionally about 3 cm of insulation is applied, in the ground floor it is not that tight since no insulation is needed, only impact sound insulation (if at all). I have requested quotes from 2-3 companies. 2 quotes are for thin-layer systems with special screed (both require only 2 cm above the underfloor heating pipes), 1 for a dry construction system (in total 2.8 cm floor heating height – but here I would have to level the poor concrete slab quite well), 1 for liquid EPS screed. Plan B would be to do without underfloor heating and continue heating with radiators, which would be replaced with low-temperature models. Plan B costs half, but it would be annoying to renew screed everywhere and not install underfloor heating. Thanks. P.S. if anyone has insulated/renovated with PUR/PIR on the earth-contact floor slab, it would also be interesting to learn how well it worked.

P.S. if someone has insulated/renovated with PUR/PIR on the ground-adjacent slab, it would also be interesting to know how well it went.

Not on a ground-adjacent slab, but on a basement ceiling (and about 20cm in the basement). But the floor was not really even – didn’t matter to me, PUR on it, underfloor heating stapled directly onto the PUR, and then a very thin screed on top. Worked wonderfully. Are 20mm of insulation in the basement perfect? No, but both my energy consultant and I don’t see a real problem.

Plan B costs half as much, but it would be annoying to have to renew screed everywhere and not install underfloor heating.

Exactly right. Better to forego a centimeter of insulation and instead install the underfloor heating everywhere with a 5cm pipe spacing if necessary, so you can run with low flow temperature. That can be calculated.

Swisspor PIR Premium 0.018 or Bauder WLG022 are currently unavailable anywhere, which worries me. What also causes concern are the blue drops in the Ubakus calculator, probably due to the water vapor diffusion resistance value. An alternative would be the Foamglas boards, but they do not insulate as well. I even looked at Spaceloft aerogel, but I am not sure if it is suitable for floors (strength). What is appealing about the Turbolight screed systems is that you could theoretically lay normal 16mm underfloor heating pipes.

What for 10mm aluminum foil? You have aluminum-coated PUR.

I thought a vapor barrier (aluminium-bitumen) is needed so that no moisture (condensate/dew water) is released. It is currently also under the old screed. With Foamglas, it would be overdue but Foamglas U-values are unfortunately twice as poor in thickness compared to PIR.