Firstly, can anyone explain why a damp proof membrane is necessary under a water-piped underfloor heating system? My architect is insisting it is not necessary but all other websites I have consulted on construction tecniques seem to include one as standard, above the subfloor but below the insulation surrounding the pipes.

Secondly, whats the alternative to PVC - is there a more ecological option or is this just one of those things we just have to accept for practical reasons? And what about the material for the pipes? (oh dear, this being green can be very challenging for us novices).

For interest, my system will be powered by biomass from locally sourced waste material, the insulation is natural compressed cork board topped by a layer of concrete (oops) and the finish either local stone or FSC-certified engineered oak board. It would seem a shame to have to use polluting plastic for the DPM, although I understand there isn't an alternative to the concrete layer over the pipes.

Any advice would be much appreciated.

It's certainly not necessary in a solid-wall construction. I've laid plenty of limecrete floors - with UFH - in rubble-stone houses and not yet fitted a DPC, or used any concrete. You can just as easily use limcrete over the pipes.

In floors I always think that there should be a vapour barrier on the warm side of the insulation. But with solid floors and UHF this may not be a problem however with suspended floors it may well be?