As stated earlier in Floor Phase 1, the reset of the floor above the gravel is OSB, 2x4s, pex for the radiant heat system (which turned out to be unnecessary, as the small 1 ton Art Cool Mini-split can easily heat and cool the place for very little money (worst power bill during our hottest July on record with a person also living and cooking there was $55 with it cooled to a comfy 72 degrees), sand and more OSB and pine flooring.