Low Profile Floor for Water Underfloor Heating retrofit

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I've installed a wet underfloor heating system over my ground floor subfloor and have just been told by a supplier that I can no longer buy the stiff boards that would have gone under the finish layer. My builder doesn't have experience with UFH so I'm asking for any recommendations for what floor to lay.

The original plan was for the floor to end up like this:

3mm LVT. 0.02 m2K/W (0.2 tog)
1.5mm thermally conductive underlay eg. Duralay MultiFit 0.012 m2K/W (0.12 tog)
3mm thermally conductive levelling compound 0.012 m2K/W (0.12 tog)
4mm MultiTop polyurethane boards by Omnie 0.05 m2K/W (0.5 tog)
Already installed:
- 18mm UFH polystyrene overlay boards
- 20mm SUBFLOOR: chipboard / weyroc
- 25mm of EPS insulation
- Concrete slabs

So total height above the UFH would've been 11.5mm and the thermal resistance 1.2tog. Hard to beat.

Omnie's MultiTop boards for the "intermediate" subfloor were ideal because they have roughly the strength and thermal resistance of 6mm Plywood, but are thinner, cheaper and flatter. Omnie decided they would no longer sell them to me if I didn't buy a full UFH kit from them so it's back to the drawing board because if I just replace with plywood I need a thicker levelling layer, increasing height, thermal resistance and cost.

I went with the XPS UFH overlay boards because the subfloor isn't very insulated (1990's build). But I didn't go with Omnie because Continal's SlimFix 18 boards are stronger (500KPa compressive strength) and have 100mm pipe spacing giving more heat output and allowing the heat spreading layer to be thinner without risking hotspots.

So I'm wondering: What flooring should I install to minimise thermal barrier, cost and height?

Thoughts so far:
1. Thermal conductivity is the top priority, then cost (taking into account install cost and lifespan), then thickness is important but less than the other 2 aspects.
2. I think wood and engineered wood are out of the question due to cost.
3. If I lay subfloor boards over the UFH (6mm ply seems to be the thinnest recommended), can I get away with putting a thicker underlay down and avoid levelling compound? The subfloor is pretty flat and the UFH panels are too, I dare say the plywood boards would be the main cause of undulations. I guess that any height reduction will be lost because the vinyl planks will have to be click type so thicker.
4. I've seen 6mm fibre cement boards for use under tiles. Their thermals are great but they seem to take less load. Would they be appropriate over 18mm of XPS in, say, a living room, and with a non-tile floor on top?
7. Is laying planks straight over UFH pipes, without the intermediate subfloor layer of eg. 6mm plywood a bad idea due to hotspots?
5. Can I lay a thin laminate (eg. 10mm) straight over the XPS boards?
6. Could I place thick vinyl planks (8mm?) directly on the boards?
8. Can I get away with very thin (eg. 1.2mm) underlay if using click-system planks, and let the planks take up most of the undulation? I guess in that case there would have to be air pockets under the planks, reducing heat transfer.

Thanks very much! If you can recommend any specific products, that will also help loads as well :)
 
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