Levelling a subfloor slab in preparation for PIR/UFH/Screed

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17 Jun 2025
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Looking for some advice. I am preparing to install insulation. Actually, I installed it but I am not happy with how it sits so it's coming back up. Floor area is about 90m2. The attached sketch shows the make up of the subfloor. There is (insulation) movement all over the floor, about 10mm bounce and I don't want to press ahead and end up eventually having to revisit this problem.
I have a couple of options. Buy costly self levelling screed which is moisture tolerant (reason for this will become obvious), or buy a couple of tons soft builders sand and some cement and dry screed a 10:1 mix at a maximum depth of around 10mm in some places. If I use the self levelling, I'd have to cut the DPM on top of the dwarfs and use something like ARDEX DPM1C, when that sets, lay ARDITEX NA self levelling compound over the entire subfloor.
The sand/cement option would be much cheaper but what way would you go? Thank You!
sub-floor.jpg
 

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