Sealing raw floor slab on XPS layer

Hello everyone,

with us, electrical and sanitary lines lie directly on the raw floor of the floor slab.

Proposal of the screed installer in the discussion before the rough installation: attaching the Alujet sealing membrane on the insulation (XPS). According to him, this is not a problem for the XPS boards. In the manufacturer's information of the Alujet membrane, the possibility of installation on the insulation layer is explicitly mentioned.

Can anyone say something about whether this is actually okay? Or should we better loosen the lines again to slide the sealing membrane underneath?

It is basically possible, but it depends on what exactly you want to waterproof against (rising moisture, something else, etc.). *And* whether and how the waterproofing membrane should be connected to the (possibly existing) cavity wall barrier. *And* whether you are planning underfloor heating – if yes, there are additional insulation layers where the pipes can be fixed.

It is generally disadvantageous to use XPS insulation above the floor slab – with PUR/PIR insulation, possibly aluminum-coated, either better insulation values or lower build-up heights can be achieved. However, these must then definitely be placed over the waterproofing membrane.

How should the cavities around/above the pipes be filled? If the membrane is installed underneath, you are relatively free there, but if it is above, for example, loose-fill insulation must also be moisture-resistant.