Creating a new first-floor

[Royston]

I'm refurbishing a barn (France) and need to install a new first floor over green oak beams and joists. I plan to start with a layer of plasterboard (which will give me a ceiling when viewed from the ground floor) and had planned to cover this with:-

1. flooring grade chipboard (19mm)
2. dpc
3. 30mm of insolation
4. reinforcing metal grid
5. underfloor heating pipework
6. 65mm screed
7. and finally sandstone slabs (35mm thick).

qu. Would it be possible to combine the chipboard/insolation layers with 30-40mm of extruded polystyrene instead. Would this be rigid enough to walk/work on before stages 4-7?

 

[Static]

You planning on floating a screed over a timber floor?! Firstly i doubt the joists could take it, unless they are about 6x the normal size or spacing, next timber floors quite flexible and your screed slab will be liable to crack if it hasnt already fallen down and crushed whatever lives on the ground floor.
You would be far better off putting in a mezzanie floor either concrete onto external walls or steel inner frame with composite flooring. Or just go traditional timber floor..