I am going to replace chuck out the floorboards in my living room and replace them with rigid board insulation sandwiched between osb or plywood or hardboard. I'll be using 30mm or maybe 40mm polyiso boards (Kingspan, Celotex, or one of a hundred other brands). Joist spacing is
47cms (18.5 inches).Polyiso isn't strong enough by itself of course so I need wooden boards below and on top. My questions are:
OSB, plywood or hardboard ?
What thickness ?
I want to keep thicknesses to an absolute minimum so as not to raise the level of the floor any more than necessary. At present I'm thinking 6mm ply below and 9mm or 11mm on top.