ᐅ Knauf Perlite as a leveling layer beneath insulation and screed

Hello everyone,
We have bought an old miner’s house and are now trying to turn it into our personal retreat through a complete renovation. This renovation is quite advanced, and we are almost at the foundation stage. Unfortunately, old houses have the problem of uneven floors, walls, and everything else. It seems the masons may have been drinking more than working back then.

Now to my problem. We want to install underfloor heating throughout. We removed the old wooden floor, which was laid on a concrete slab (structurally definitely sufficient and checked by a structural engineer). All the interior walls are still standing, and the toilet is enclosed. Unfortunately, the floor sags in the middle by about 6-7 cm (2.4-2.8 inches). I would like to level this out with a dry screed material (Knauf Perlite). After that, we plan to lay the fixing boards for the underfloor heating on top, followed by the underfloor heating system and screed.

My question is whether this approach is really feasible: leveling with the perlite dry screed, then adding insulation boards + underfloor heating + screed on top. I don’t see any issues at the moment, but I just want to avoid any unpleasant surprises later on.

Thank you in advance for your help.

Regards,
nitrox1337

I’m not entirely sure about this either. If you have the loose fill insulation in place and everything installed, where does the screed flow to? Won’t most of it end up in the loose fill?

I also have a dry loose fill insulation, but without underfloor heating. On top of mine, a dry screed was installed, and I believe that is what perlite is designed for.

The screed cannot penetrate into the loose fill. On top of that, the fixing panels are installed, which are taped together. At the edges, there are perimeter insulation strips whose foil is also taped to the fixing panels. So, the screed will remain entirely on the panel.

Does nobody here have any idea?

First of all, why is the floor sagging, and how can it still be considered structurally sound?
If it’s sagging, something is wrong. Unevenness is acceptable, or rather, sometimes common in older buildings.

A filler layer must be solid once installed, meaning not a loose fill but a binding material, for example, Thermozell.
Underfloor heating on loose fill is also a very bad idea because you can’t walk on it without leaving dents and footprints. The stability of boards on loose fill is roughly comparable to walking on a 2mm (0.08 inches) glass plate resting on a mattress.
Even directly on rigid PUR insulation boards, which are designed for floor insulation, we didn’t lay our dry underfloor heating system directly because it was too “flexible” and prone to damage. Instead, we installed a continuous 15mm (0.6 inches) OSB layer over it first. That made it safely walkable during construction. Then a polyethylene (PE) sheet was added to ensure water resistance.

So, if it’s really going to be the stapled system, which I probably wouldn’t recommend in an older building due to weight (plus the screed will still go on top) and also because of the build-up height, I would definitely create a stable and firm base (see Thermozell or similar) and, above all, clarify why the floor is sagging.

By the way, we also used loose perlite as a leveling layer under cast asphalt screed, and removing the “cat litter” was a nightmare. I would never use that material again anywhere.