Tiles in the hallway, but what is this..?

[Savett]

I live in a modularly built Borohus from '91 with a crawl space. Now I'm going to lay tiles with electric heating (quicknet) in the hallway. The floor plan is open, so in the hallway, living room, and the living area, the floor blends together, oak parquet (15mm). In the hallway section, I've thus cut out the floor where the tiles are going to be (thanks to whoever invented the plunge saw ). Underneath, there was building paper as I thought. But under that, there's plastic. The layers are as follows.
1. Wood floor
2. Building paper
3. Plastic
4. Particle board

What's the plastic doing there? It shouldn't be a moisture barrier, right? I'm going to glue floor gypsum before I start laying it, but it gets tricky on the plastic . Just kidding, could the plastic have been laid to minimize clamping noise from the wood floor, but then why the building paper...? The plastic must go for the gypsum to be glued, but I'm worried I'll change something in a construction that could have unfortunate consequences. Does anyone know if it usually looks like this?

 

[para]

I think the plastic should be there. Compare with exterior walls, where you also have plastic on the warm side.
Why glue the floor gypsum? Isn't it enough to just screw it?

 

[Savett]

Those I've talked to since my post say that the few square meters (5) don't make any difference if the plastic is removed. And you do glue, if I've understood the step-by-step instructions I've seen and also those I've consulted. It should be rigid and if you screw, the plaster can move, and that's what you want to avoid. How else do you do it if you can't/shouldn't remove the plastic? I'm probably not the only one who's encountered this