I have a two part question.  I'm renovating my master bath & installing the Warmfloor as the sole heating source for the room.  I have 5/8" plywood subfloor & I placed 1/2" Hardi backer board on top of that & will install 18"x18" marble tile as the finished floor.  I was told the closer I have the heating elements to the tile the better or warmer the tile will be.  Question 1.  Should I put the elements on top of the 1/2" Hardi board & then thinset the tile on top of the elements or should I first thinset another layer of 1/4" Hardi board before the tile?  This way the elements are sandwiched in between the two layers of Hardi board. I"m sure the tile guy will prefer this so he is working on a smooth floor.

Question 2.  What is the best way to fix the elements to the Hardi board?  Staples won't due.  Someone suggested duct tape.  Can I nait it down maybe with galvanized roofing nails?  Any suggestions will be appreciated.