I am laying floating porcelain tile that is a half inch thick. I also have to lay down an underlayment under the tile. After accounting for both of these items I come up about a half inch too short under the door jambs, trim, etc.
I am wondering what is the proper way to add roughly half inch of height to the subfloor. Can I just use any type of OSB or plywood? Should I use cement board even though the tile I am laying doesn't require any type of mortar?
I should note that this is in an unconditioned entryway to a 100 year old apartment building in the Midwest. So there is not heat or AC in this entryway and is subject to moisture from peoples shoes coming in an out.