In screed cables are designed to be fixed to re-bar which has been laid into foil faced insulation (kingspan/celotex) and directly under a minimum 60mm screed, these cables are only normally used in a new extension or conservatory project.
The cable is spaced out according to the desired wattage, normally a minimum of 200w.
In screed cables work differently to the other 2 types as they take much longer to warm the large screed base in which they are installed, for this reason the cable is much thicker at 6mm and produces an output of 17 watts per linear meter.
These cables can be used directly under thick tiles and flagstones if they are bedded in with a base of 20mm sand + cement with an SBR additive, they are also used in new builds if you are installing amtico or wooden floors on top.
as with any underfloor heating installation it is essential that you have a good level of insulation beneath it to maximise heat output and reduce any downward heat loss. It’s always best to cover with levelling compound if possible.