I've bought (but not yet installed) a Hearthstone Homestead, which requires a hearth with an R-value of 2.5. My house has a rather handsome DIY slate hearth for a freestanding stove (iow, no fireplace) the previous owners had built. My Tribute only needs ember protection, so what the hearth was constructed of is only now an issue. (The owners are deceased, so can't ask them.) It's a raised hearth about 4" high and opening up one end to have a look revealed about a half inch of concrete on top of 1-inch plywood supported by a grid of 2x2's. Obviously, that's not good enough by itself, but I'm wondering what 1/2 inch of slate on top of a 1/2 of concrete adds up to in R value. Anybody know? I really don't want to tear up the existing hearth (which is a good deal larger than what's required for the Homestad) and start from scratch, so I need to figure out what I'd need to put on top of it to get to that 2.5. What would you do?