Air Source Heat Pump cost?

[Simtronic]

I just checked and my underfloor has only 70mm celotex boards over existing slab then 18mm composite boards on top. I went with fan coils downstairs as I couldn't afford to raise floor height from that. Has anyone tried one of the low profile fibre board underfloor heating or is that a lot less effective than concrete ? I know you can get 18mm or even 15mm systems now.

I guess the lack of thermal mass would mean it cools a lot quicker but would also warm quicker too.