So in trying to meet the updated requirements for an Osburn 1600 (mfg. 1990) that I just got I need an r value of 1 to extend 16" in front. Current hearth is non-combustible but will be short by 2-3". I want to make the hearth extension removeable so I don't have to tear into the hardwood floor. If I screw 3 sheets of durock together (staggering the seams) will I still need an underlayment if I decide to tile it? Other easy option to get me through the year is just screw the 3 sheets together lay them on the floor and throw a hearth rug over the top. This would be the easiest to move around and I could just pull it up in the summer and throw it in the garage without the worry of tile cracking.