Hi again guys. thanks for your input regarding BCO and needing to fit an openable window with my french doors in an earlier thread. Now I have possibly another problem.... ....I mentioned to my BCO that i planed to re-fit the under floor heating in my kitchen, as 18 years ago I first fitted it but drectly onto the concrete sub floor and screeded over the pipes, thinking heat rises and there was no room for insulation as the area to work with is only 5cm, (it never went warm enough to heat the room so i left it switched off and used radiators instead) I probably made a mistake to mention this to my BCO as he said that he'll need to see the insulation before screeding over it. I told him that i have seen many low profile wet under floor heating systems. I have noticed online that 6mm and 10mm insulation board are very common to use for low profile under floor heating systems, so i'm guessing they comply with building regs? but on the other hand, i really can't see how such thin boards can achive a u value of 0.25w/m2K as part L building regs states that a floor has to be. So your input would be greatly appreciated regarding retro fitting under floor heating onto a concrete sub floor with a maximum 5cm height, and satisfying building regs.