yes its 18mm chipboard. its solid and free from deflection. i had an issue with the plumbers who put in the ufh as they put 18mm chipboard down then 18mm ply so i told them that the heat wouldn't get through to the tiles after advice from here . so they took up the ply and i had a look at the floor and its all new chipboard and its solid. i thought that 6mm hardi with a de coupling membrane on top of the chipboard would be ok but want to check if it is ok