Heat will move to any area that is colder. Yes, in free air, the hotter air will rise, but it will conduct easily in any direction. The ground is a great heat sink and can suck much of the heat away - it can be expensive to heat the soild under your slab. There are a copule of things that might work for insulation, the EasyMat you mentioned and possibly Wediboard. It's a tileable foam panel, available in various thicknesses. You'd want to check with the manufacturers of each product to see how they feel about electric heat on them.