radiant heat concrete floor on second level

Have put radiant heat in concrete on the first floor (bottom/ground floor in the States?) and want to put same on second floor. Am I going to have a problem with stability? What can I add to the concrete to make it lighter? How thick does it need to be? I'm putting an apartment in a barn so the basic structure is very solid. Pehaps a 'floating' floor over the present floor/ceiling would help. Any ideas would be greatly appreciated. The locals think I'm nuts (I'm also female, which doesn't help). Thanks.