ᐅ Sealing of the raw concrete floor slab on the XPS insulation layer

Hello everyone,

In our case, the electrical and plumbing lines are installed directly on the unfinished surface of the concrete slab.

During a discussion before the rough-in phase, the screed installer suggested placing the Alujet sealing membrane on top of the insulation (XPS). According to him, this should not be an issue for the XPS boards. The Alujet product specifications explicitly mention the possibility of installing the membrane over the insulation layer.

Can anyone confirm if this is actually acceptable? Or would it be better to lift the lines again to slide the sealing membrane underneath?

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

Using XPS insulation above the floor slab is generally disadvantageous – with PUR/PIR insulation, possibly aluminum foil-faced, better thermal performance or lower construction heights can be achieved. However, these must then definitely be installed above the waterproofing membrane.

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