Use a double layer of 15 lb., or a single layer of 30 lb. Over a heating plant the insulation used should be non-flammable. Hardwood flooring can be installed successfully over a slab which is on-grade or above grade. Below-grade installations are not recommended. Watch out for water.
New concrete is heavy with moisture, an inherent enemy of wood.
Proper on-grade slab construction requires a vapor retarder such as 6 mil polyethylene film between the gravel fill and the slab. While this prevents moisture entry through the slab, this membrane also retards curing of the slab. So test for dryness, even if the slab has been in place over two years. Slabs younger than 60-days are generally too wet for flooring installation. Make tests in several areas of each room on both old and new slabs.