I'm about to lay new radiant heat tubing on my old slab and cover it with gyp-crete. I'm planning to put some kind of vapor barrier down first, and am wondering whether I should put some kid of thermal insulating layer down as well. Is there anything anyone would recommend? It has to be very thin, as I'm already raising the floor 2 inches. It seems to me that if it is just some kind of bubble-wrap type material, this would disintegrate over time and be useless.
While it is commendable that you are trying to take this opportunity to make your house energy efficient, I think you will have structural problems with your new floor if you insulate under it. Any insulation that is worth anything will be "soft" (think styrofoam vs concrete). As soon as you put a load on the thin layer of qyp-crete (walking on it), it will bend and not be supported well by the insulation. Cement materials don't like bending and will crack. This would be bad. Unless you can deal with the structural side, I would stay away from insulation in this case.
If you were pouring an entirely new slab, insulation would be a great idea. The slab is thicker, has rebar, and would sit on piers, making it capable of taking loads without bending. Insulation would be great in this case.
we put down insulation before replacing our radiant heat on top of the slab. we used a product I can share with you via e-mail: helen_silverberg at yahoo dot com
However, we did not use gyp crete- we used plywood with hardi backer on top. have been very happy with our new heat, though.