I have a new construction where 1/2 pex pipes for a hydronic radiant system are stapled down to 3/4″ tongue-and-groove, glued and screwed OSB.  The OSB is supported by TJI 230 11 7/8 @ 16″ O.C. floor joists.

I’d like to put solid, 3/4″, narrow plank, oak, hardwood floors over this but hold the total height off the floor to 1.5″

So I basically want a 3/4″ substrate to cover the pex pipes and provide the subflooring for the hardwood.

What product and techniques would be advised?

I’m currently contemplating:

1. concrete with a small aggregate size, or

2. a sand and mortar mix

In either case, I thought I’d place down pine 1×2’s to act as 3/4″ sleepers to help screet the substrate to the right height.  I would also use these to occaisionally nail the hardwood to, but the primary bond I’m contemplating for the hardwood is Sikabond T55 adhesive.

A related question: Should I seek to bond the substrate to the OSB, or let it slip?

Any advice on any of this would be appreciated.