I have a first floor extension which is nearing completion – it is a bedroom over the existing lounge which extends slightly beyond the ground floor footprint and has a balcony.
I have attached a section from the architects drawing (which has Building Reg approval). The balcony has a single ply covering (in place of the decking shown) and it now proposed to clad the underside entirely with T+G boarding on cross battens (largely to level the boarding – but it will also give an air gap between the boarding and the insulation which isn’t on the original detail)
It has been suggested to me that celotex should also be installed on the underside of the joists beneath the bedroom floor (which seems sensible) but also that (largely due to pipework serving a radiator in the bedroom above) 100mm of celotex should be fitted between the joists, (level with the underside) with the gap between it and the underside of the floorboarding above (glued T+G chipboard which is already down) filled with mineral fibre insulation – which I am less sure about.
I realised much too late that there was a lot wrong with the original detail in terms of airtightness and the absence of a VCL on top of the bedroom floor joists could be an issue but had concluded that the best I could do was to seal as many of the air pathways as possible with foam (but to leave the gap between the T+G boarding and insulation open) However having read more about condensation risk (mainly on suspended timber floors) I am becoming concerned that it may not be wise. On the other hand, with the exception of the underside of the IG lintel, the structure may be warm enough to ensure that condensation isn’t a problem?
I would be grateful for any help in deciding how to proceed with this.

Thanks – I’m sure your right. The walls are insulated cavity and the joists are built into the inner leaf - I don't think there was ever any prospect of the insulation under the bedroom floor being joined up with the wall insulation.
Unfortunately at this stage I have to make the best of a poor basic design – accepting that the end result will be a long way from optimal.
From a practical perspective the only option available to me is to insulate under the bedroom floor from the underneath – without a vapour barrier on top of the joists.
This looks to me to be similar to insulating beneath an existing suspended floor – and previous threads emphasise the need to ventilate beneath the insulation. With this in mind could I:
1) Pack between the joists with quilt (200mm)
2) Hold it in place with a breathable membrane secured by 25mm battens running along the underside of the joists
3) Secure the T+G boarding to 25mm cross battens at say 600m centres.
This would give a 50mm gap between the insulation and the T+G boarding. The gap could be ventilated by leaving the outside edge open to the uninsulated void under the balcony and putting soffit vents in the boarding under both the balcony and bedroom floor.
I have attached a very rough sketch showing what I have in mind.
I accept that the insulation level will be substandard but I am more concerned now to prevent long term decay due to condensation and am working on the basis that this cannot be achieved in this situation using celotex.
Is this likely to work?

Setting aside the linking of the insulation with the cavity wall insulation I think you are suggesting that the space between the joists be filled with celotex (or maybe a combination of celotex and quilt) with a layer of celotex under the joists. This is close to the variation suggested by my builder. I am concerned that as moist air will inevitably get into the structure from above and below, condensation will occur and as celotex is impermeable there would be insufficient ventilation to dry it out and in time the joists would rot. I thought your earlier suggestion to use quilt (which I took to be vapour permeable) was to reduce this risk. Am I worrying unnecessarily?