Is it 100% necessary to have insulation boards with ufh when laying on a concrete floor.

Would always advise a backer board but if height is an absolute issue then some
systems recommend a thermal insulating acrylic primer like therma-coat but even with
this the running costs will be more than that of using insulation boards.

I'd always have them, the running costs of electric UFH is alot higher than the manufacturers will have you believe, with the boards the warm up time is greatly reduced and the floor holds the temperature for longer. Insulation boards will pay for themselves 100 times over throughout the life of an install.

As Bri has said, heat up times and running costs are greatly reduced with 10mm insulation backer boards. I wouldn't waste my money with any type of thermal primer as it doesn't work IMO. Can I ask why you don't want to use it....?

I agree with the gentlemen above.

Thanks for the replys.The reason i ask is that i was worried about the height issue.This is for my own lounge and was going to use marble tiles 10mm thick.

Always use boards 10mm / 12mm unless you intend to heat up the screeded slab first and when hot enough then the tile / room. Height issues can be solved, 60% waste of heat is more difficult IMO

10mm is thin for marble..?

Your right dave just checked and there actually 12mm
Walls and Floors Tiles -Natural Marble Tiles & Mosaics - Cappucino Marble tiles - Dark 45x45 tiles - 450x450x12mm

We would always say use boards.
If height is the biggest problem, you could always use our 200w/m2 mats, but you would still be heating some on the concrete first.
The boards put the heat where you want it, in your room not in the concrete first.