Hi Bob,
Firstly if you ar eplanning on going straight onto the plywood, European
standard IEC335/EN60335
recommends for safety that no more than 100W/m2 is installed directly on a timber floor and only giving you a 'Warm Floor' effect.
If you are wanting 'primary heating' i would recommend overboarding the ply with Insulation boards, have a look at
Underfloor Heating | Floor Insulation | Uheat or if you are restricted with height use Thermopanel insulation
Underfloor Heating | Floor Insulation | Uheat.
As already been recommended a flexible
tile adhesive must be used. the guys on here do like Mapei stuff.
The insulation boards will be stuck down using the Flex
tile adh and mechanically fixed (for security) the loose cable or cable mat should then be used. If you have not used UFH i would go with the loose cable as you can take you time with it and lay the cable where you want to.
Once the cable is down then use a self levelling compound to cover the wires, this helps protect the wires and gives a nicer tiling surface. Then use flexible
tile adhesive to fix the tiles down with. Not forgetting the flexible grout!!
Please have a look at our site
www.uheat.co.uk
If you let me know the details of the room i will be happy to work out all the UFH products you require.
Hope this helps.