ᐅ New Concrete Slab in Existing Basement

Hello everyone,

I removed the floor slab (5cm (2 inches) of tamped concrete + 3cm (1 inch) screed) in my basement. The house, built in 1963, stands on strip foundations. Now I have installed 20cm (8 inches) XPS insulation boards and plan to pour a new floor slab on top.

The planned structure is as follows:

• 10cm (4 inches) XPS insulation boards
• Vapor barrier foil
• 10cm (4 inches) XPS boards laid crosswise
• 10cm (4 inches) new concrete slab (concrete C25/30 with Q188 reinforcement)
• Another layer of vapor barrier foil, extending about 8cm (3 inches) up the wall above the slab
• Tacker boards with underfloor heating
• 5–8cm (2–3 inches) screed
• Vinyl flooring

My questions are:

• Is this structure suitable?
• Can I pour the slab directly against the concrete wall, or should it be installed as a floating slab like a screed with edge insulation strips?

The vapor barrier foil was recommended to me to be placed between the XPS layers.

Thanks in advance for your feedback.
Best regards

I find the construction method with the Katja terrible. Who comes up with such an idea?

Floor
2x10cm (2x5 inches) XPS
PE film as a separating layer
Concrete slab
Katja

I would install the concrete slab as a floating slab with 10mm (0.4 inches) edge insulation strips. Thermal separation, settlement behavior, sound insulation (yes, I know it’s a basement – but still).

Thank you for your feedback.

At the time, I didn’t really think about it and just believed what I was told. Unfortunately, that’s how it is now. The question is whether it’s enough to simply lay a PE (polyethylene) sheet again under the slab. For edge insulation strips, I would first need to level the bottom of the wall with a concrete filler or something similar, and then attach the edge insulation strip. This is clear for the screed, but for the slab, it would have been preferable to cast it directly against the wall. However, I’m not sure if that’s a good idea since the slab can move. Using edge insulation strips means more work again.

dedieter schrieb:

The question is, is it enough if I just put a PE foil again under the slab.

For what purpose? What would that achieve?

dedieter schrieb:

It's just more work again.

That’s the “problem” with well-done work. It takes effort. Doing things quickly is always possible – but if you rush at the start, you might have to redo it later. Of course, it can also work out fine. But it’s not guaranteed.

This means it is better to install an edge insulation strip so that the new floor slab is laid as a floating slab. Hmm, then I guess I still have to do that work. Thanks for your feedback.

I am also curious why the foundation slab should be decoupled from the basement walls using an edge strip. The screed will later be decoupled with insulation and an edge strip anyway.
From my gut feeling, I would rather pour concrete directly against the walls. At least then it’s sealed.