I have an old Bungalow in Bozeman MT (very arid) that I'm about to rehab including a new addition off the back to gain some much needed sq. footage. The current floors are solid 3/4 x 2.5" oak over 1x6 diagonally laid subfloor with a sandwich of tar paper. The plan is to add staple up radiant with extruded plates after the floors are completed.

I've been told that engineered is the way to go (based on the hydronic heat) and have found a couple of styles in the 3/8" and 1/2" x 5" by Armstrong or Johnson.

I've read on this site that my old subfloors are the "Best" for nailing/stapling into. The plan is to leave one room alone with the old floors, but the balance of the home would get the old floors ripped up off the old subfloor then new engineered hw added on top.

What would be the desired material to use to "build up" the old subfloor to meet the height of the old floors to be left in the one room? Also the new addition would be 3/4" plywood t&g and the same build up would have to be done there. Or am I nuts to go through all of that for the quaint desire to keep one 12x14' room in the old 3/4 solid hw? Also concerned about off gassing from the tar paper sandwich in the room to be kept original.

I thought I'd have to cover the diagonal with some 3/8'' t&g plywood, but after reading on this site about the old subfloor being "Best", perhaps I'll just try to butt up to it with new 3/4" t&g ply for the new addition.

And what if any paper or barrier to be reinstalled after I remove the old tar paper, and before the new engineered?

Ideas?

I'm not sure I making any sense of this.

1) tar paper wont gas off..you will be fine.

if you add an under-layment to the existing 1x subfloor there will be no ill effect whatsoever.

It will be structurally sounds, rigid and ready for any floor installation.

Angled sub-floors are desirable for one thing,...they are superior in performing against deflection and expansion issues form floor to floor.

Hope this helps.

Thanks, I'll add 1/4'' ply if I use 1/2'' engineered. Then I'll look for a 1'' t&g subfloor for the new addition.

Don't bother with 1" T&G just use 2 layers of ply to get the same level.