screed insulation base not flat

[Glanmor]

Hi

new build, 6x5m concrete slab layed over 10 months ago. Since then its has somewhat erroded surface with stones showing up all over the place. Some above 5mm.

I'm now getting ready to screed this floor with 25mm rigid insulation on the slab followed by 75mm traditional screed ontop.

Question is how to deal with un-even base slab surface. Too many stones to grind down. Surface worn and very dusty and may not take latex leveling compound securely. Can a layer of sand do as was done for insulation below the slab? or does it have to be fixed cementous leveling.

Perhaps any other products to go on first, similar to sound proofing mats used in floors?

Thanks in advance.
Glanmor.

 

[Alastairreid]

Put the insulation directly on the slab!

 

[tomfe]

Out of interest why are you putting in 25mm of insulation?

 

[Glanmor]

thanks both.

Can put insulation directly on the slab but was worried it might not be stable and might rock a bit. Will the weight of 75mm screed weigh it down enough to push the raised bits into the insulation?

25mm insulation going there to create a Thermal barrier with slab. I know you are going to say there is 100mm insulation directly below the slab but I keep thinking the 100mm slab plus 100mm screed (if upper insulation omitted) will create a very large thermal mass that could be very cold during winter months. Now is the the chance to improve this so hence the 25mm separating layer considered.

Any further thoughst please.

thnaks.