Floating insulated floor.

[Paulymase21]

We are mid build on a single story rear and side extension. Progress has stalled due to COVID. I’m an X plasterer with a small amount of knowledge on different trades.
With the down time I’ve been doing what I can to push things on but noticed a lot of poor workmanship.
The over-site was laid terribly and the builders haven’t used a sand blinding before laying the dpm so the insulation boards are not sitting flat. I’ve had to lift and cut some of them and filled voids with building sand to try and get them to sit more stable. There is still some slight moment and spring, as much as 5-8mm.
My question is, do the insulation boards have to be absolutely solid before a liquid screed is applied or will the screed tolerate that slight 8-10mm movement. Many Thanks

 

[Bodgedbuild]

I wondered this too when I had some movement in my PIR boards. I opted for thicker insulation than most would and used 100mm boards laid over 100mm boards to 200mm in total. I then had 60-80mm of anhydrite screed on to of that and the weight pushed it all down nice and tight. I also let the screed guys pour their excess into a slab in the front drive and once cured it had made an incredibly heavy slab (about the size of a large table) that was very difficult indeed to break.
In short I wouldn't worry if you have a reasonable thickness going on top.

This probably shouldn't have been posted here by the way!

 

[Paulymase21]

Thanks for the reply, first time using one of these forums so wasn’t sure where to post it ??
200mm of insulation!! Wow!! ££££. I only have room for 120mm plus approx 50-60mm of screed.
I’m now toying with the Idea of lifting it all and starting again by laying a proper sand/blinding layer and ruling it off level so the boards are properly stable.
Btw Where should I have posted this question??

 

[Bodgedbuild]

The blinding layer is important as much to prevent the damp proofing membrane getting holes poked into it as anything else so if it is lacking or missing, it might be as well to lay it again. I used some large and some fairly small boards and the small boards settled much easier and conformed to undulations in the sand layer quite well.
I used so much insulation because I removed a voided wooden floor (over a concrete sub floor) and had to make up close to 300mm to get it up to the same level as the new extension. The insulation cost nothing because a local building site had a considerable amount of pre-cut and pre-packed boards that had been sat around since the site had started. In the end they opted to cut the boards to fit in between joists on site rather than try to space the joists to fit the pre-cut boards. They had to get rid quickly because a company director planned a site visit!
The link below might have been a better place to ask questions btw.

https://www.diynot.com/diy/forums/floors-stairs-lofts/