How to insulate electric heating floor system placed over concrete w/o joists?

How to insulate electric heating floor system placed over concrete w/o joists?
I would like to install an electeric radiant floor heating system over concrete floor without any joists. I am afraid that the heat will warm up the floor underneath along with the wood floor in my room. Which material should I put between (and how) the heating mats and the concrete subfloor to insulate?