best attachment for 3/8" ply spacer under hardwood

[DIWoops!]

Hello there building types,
I've a need to build up my floor by 3/8" before nailing in 5/8" T&G solid Teak down. It's a 2nd floor reno that I have pulled up all the old planking and glued and screwed down 5/8" T&G ply subfloor. My bathroom floor is 1 3/4 finished after two 5/8's heating pads, Detra and Marble tile. I can't believe how expensive that floor has become :-)

Anyway inorder to bring the rest of the floor up to a 'suitable' height (to match the top stair bull noise and less of a transition to the bathroom) I need to space it up. So what to use as my material and how to fasten it to the 5/8's.

Now I'll sit back and watch the pro's fight... OSB or Ply...... :lol:

Thanks
Barry

[montytx]

Ill say ply is a little more even but cost more. We do lots with OSB. 3/8 is much of a jump for a threshold. If it were me I would just drop a couple in and be done with it.