El Mariachi

You can do that but you need to make sure you have the correct thermostat for timber floors, Your local stockist should have all the correct info for you. Insulation boards make more sense as they retain more of the heat in the tiled floor rather than loose it into through the floor you could put that to your customer.

As Grumpygrouter said if tiling direct onto timber, correct floor prep is crucial make the floor as rigid as possible.

Have a look for PCI Pecidur