Hello. It's been a very very long time sine I've been here. Work commitments have cause home improvements to dry up for ages.
I'm thinking about a "cork on cork" flooring fix.

Situation is 1970's house, mostly IWI throughout (plus cavity fill).
Ground floors solid. Most of the area will remain as-is due to existing hard floors, but we have a "snug" of 16M^2 with P/A ratio 0.7 (three external walls all IWI). This abuts an area of the remainder of the ground floor which has 20-25 mm thick reclaimed pitch pine parquet throughout.
We have removed a 25 year old revolting carpet (not a fan of carpets at all, that was the only bit in the house). Thinking about solutions. I have up to 25mm to play with once I remove ex Marley tile. I could cope with a small slope up, so maybe up to 30mm would be acceptable. Its dry.

No UFH or plans for, (don't like it anyway and silly idea on solid floor).

I was considering more of the quite easy to get 25 mm reclaimed pine parquet over a high-tech backing. Looking at Thermablock (10mm aerogel on 3mm stiffener), but the handling, the dust, not to mention the cost of that aerogel material is off putting.

U value of the floor (concrete) with P/A 0.7 is about 0.77 by BRE method. Parquet will scarcely touch that, but adding many £ worth of 10mm aerogel plus the parquet gets me about 0.35.

Then I thought about thick cork.. 25mm of cork plus 4mm WPS ply would be approaching the performance of 10mm aerogel+25mm hardwood parquet.
It looks like I could get from U 0.77 down to about 0.4. There might or might not be further rugs.

I'm looking at budget EWI grade 20mm cork board (economical, maybe £10 per M^2, perhaps less), with 4mm WPS ply on top and another layer of flooring grade cork tile (say 5mm) on top. There may need to be a fair bit of glue :(
Two considerations I guess. Ongoing compressibility of the EWI grade cork under the 4mm ply (I feel unlikely a problem in a light loaded domestic, but opinions welcome). Secondly VCL The sealant to the top cork may be adequate, but that seems doubtful to me, I don't want to encourage mould on the concrete sub floor or within the cork at the underside of the lower layer.

I could of course go isocyanurate or phenolic board but without a fireproof skin (plasterboard) as on the IWI, this makes me a little more uneasy, and I'd be less confident about it's compression characteristics under only a thin plywood layer.