Below-grade installation is not recommended. The slab must be flat and level with a trowel finish, free of grease, oil, stains and dust. New concrete is heavy with moisture, so test for dryness before beginning the subfloor. Allow to set for two hours. Over this, apply a second similar coating of mastic and roll out a second layer of asphalt or paper in the same direction as the first, staggering the overlaps to achieve an even thickness.
Puncture bubbles to release trapped air.
Install the plywood after the vapor barrier is in place. Lay plywood diagonally across the direction of the finished floor to help prevent cracks along panel edges. Fasten plywood to the slab with power-actuated fasteners, securing the center of the panel first, then the edges, using nine or more fasteners. Do not use power-actuated fasteners or concrete nails when radiant heat pipes are embedded in the slab. Lay panels in a staggered pattern with at least 2' long planks along starting and finishing walls.