Hello! We are going to install underfloor heating and tiles in the laundry room and both bathrooms. The house, built in 1990, has a concrete slab, on top of which lies insulation followed by chipboard and then a wet room mat. The chipboard is laid with a slope towards the floor drains. The question is: How do we proceed? 
Have you considered waterborne heating or electric coils? Do you know how thick the insulation is?
The wet room mat must be removed first of all... then it depends on how good the chipboard is. But you have to add a new waterproofing layer, so I would think that leveling compound is the only option... It will also raise the floor a bit...
Yes, waterborne heating is the plan and the styrofoam is between 5-7 cm I would guess. I wouldn't want to raise the floors any more than they already are. Is there really no other alternative? Does the styrofoam have to remain?
Yes, it is a bit too thin for underfloor heating. There are systems with fairly thin slotted insulation boards, similar to styrofoam. Google Rooth. But I'm very doubtful if it can really be laid directly on an uninsulated slab.
The risk with poor insulation downwards is that you will send more than half of the heat straight into the ground, which will be expensive in terms of operation costs. This, in turn, increases the risk of moisture damage in parts of the house that do not have underfloor heating. The heated ground under the house emits more water vapor, the so-called vapor pressure increases, so that moisture can rise up and then condense against the cold side of the insulation under the floors in the rest of the house.
The risk with poor insulation downwards is that you will send more than half of the heat straight into the ground, which will be expensive in terms of operation costs. This, in turn, increases the risk of moisture damage in parts of the house that do not have underfloor heating. The heated ground under the house emits more water vapor, the so-called vapor pressure increases, so that moisture can rise up and then condense against the cold side of the insulation under the floors in the rest of the house.
If we assume that the current thickness of the Styrofoam is sufficient, which I am doubtful of. Then one might consider removing the chipboard and possibly the battens. Lay, for example, 16mm underfloor heating pipes on the Styrofoam, use self-leveling concrete; I believe that 25mm of self-leveling concrete might be enough to embed the pipes. On top of that, tile. Then you have essentially only increased the floor height by the thickness of the tiles (the chipboard replaced with a load-bearing layer of reinforced self-leveling concrete).
Yes, waterborne heating is the idea and the styrofoam is between 5-7 cm I would guess. I wouldn't want to raise the floors any more than they already are. Is there really no other alternative? Must the styrofoam remain?