I'm now going back to my original thinking and considering a Northstar zero clearance fireplace, http://hearthnhome.com/downloads/installManuals/480_1081.pdf . The install manual says the hearth pad needs a 2.18 R value, which its says (subject to check) is equal to 1" of micore, or .5" of "Armstrongâ„¢ Privacy Guard Plus". I'd like to have a flush hearth, like there is now (but which I doubt has the required R value). The floor the extension would abut is 3/4" wood. There would continue to be a subfloor. My questions are: 1) What is Armstrongâ„¢ Privacy Guard Plus, and where does one get it? 2) Can one tile right over the Micore? Wouldn't the material be strong enough without putting backer board on top, especially since no one will be walking on it. 3) Can the joists be notched to make up for the thickness? Would 2x4s or plywood have to be sistered? Thanks for any help.