Do you believe this R-Value

[vrytir]

Hi guys, just about to install underfloor heating and trying to get insulation boards. I found 6mm thick versa backer board http://www.building-boards.co.uk/tile-backer-board/#.U5WCKnZ4CDg which has high R-Value 1.25 and it is only 6mm thick. Would you believe this? As 10mm warmup insulation board has u-value 2.7 (r-value 0.37), marmox boards 10mm has R-value 0.28 and Kingspan Thermafloor TF70 20mm has R-value 0.9.
So should I believe clam that 6mm has rvalue 1.25?
What insulation board would you recommend. Installing on to concrete and then laying oak engineered flooring.
Thanks

 

[m3fitter]

http://www.ecowho.com/tools/r_value_calculator.php

 

[vrytir]

http://www.ecowho.com/tools/r_value_calculator.php

Had a look but didnt find it. But it seems that from 6mm thickness is R-Value to good to be true. What do you think?