I am building a hearth for a circa 1986 Jotul Alpha stove. I have the manual and my county will be inspecting the installation. The manual says 3/8 millboard or equivalent for the hearth. I called Jotul and they don't like to talk to consumers so I've traded messages with them, but I still don't have the answers. Maybe someone here knows: 1) What is the R-value of 3/8 millboard? 2) Is Durock an equivalent to millboard? I will be building up the hearth on top of a plywood sub-floor to the height 3/4 of an inch to meet a 3/4 inch finished wood floor. I'd like to use a continuous sheet of 16 gauge rusted metal as the finish. 1) If I start with a sheet of 1/4 Durock on top of the sub-floor, and then add a plus of minus 3/16 layer of metal strips to create a thin layer of air space, then top that with another layer of 1/4 Durock, and finish with the 16 gauge rusted metal (1/16 inch), would that be equivalent to 3/8 millboard?