I thought I asked all these questions before, but I can't find anything: My Harmon Oakwood manual says the stove requires k=.84 which "is equal to typical .75-inch millboard". Call to the local lumberyard, they have never heard of it. I thought it was a fairly common substrate for tile, or special wall covering, etc. There is a link I found on this site for mail-order Micore 300, but it's in 1/2" thick 20-inch widths, so I'd need 2 layers, and with my hearth pad size, I'd need to either scab together patches of 20" pieces, or buy way excess. Rather use the millboard in 1 layer. Any info on the stuff?