Question may seem obvious at first - answer being level with the screed.

 

Ideally I'd like to aim for 50-60mm of screed, and the rest PIR insulation.  However he slab is currently anywhere between 160mm and 200mm below FFL at the moment.  There are large portions of the slab (total area is 48m2) where I could feasibly get up to 120mm of PIR under the screed.

 

Current plan is to mark out areas that will only accept 100mm, and decide how many extra 20mm and 40mm boards I need to level out the PIR over the entire area so that the UHF and screed is a fairly uniform depth.

 

I can level smaller areas with sand.

 

Any suggestions / comments / reccomendations?

 

Thanks!

 

GB

I would say your suggestions is pretty good

I did something similar but I used 120mm boards which were cheaper than 100+20 at the time. Levelled some areas with a dry mix. Screed ended up 40-50mm thick which is fine with UFH.

Insulated screed.