Hearth Extension Substarte

I am in the process of replacing my old wood insert with a new one and let me tell you, it has been quite the learning experience! Anyway, one of the things I have learned is that I need to install a hearth extension for my fireplace. My fireplace has a 17" brick hearth that is raised 10 inches above the plywood subfloor. The user manual of the stove I intend to buy indicates a minimum thickness of 0.5 inches and a thermal conductivity K factor of 0.84. If I did my research and math correctly, that translates to a R factor of 0.05 using the formula R = (1/K x 12) x T, where T = thickness in inches. Given that Durock cement board has an R of 0.26 per half inch, I am thinking that I will be OK with a 0.5 inch Durock substrate. Is that correct? Also, is there a special kind of thinset that should be used?

Welcome to the forum pine! Durock Nexgen has an R-value of 0.39 per 1/2" the original Durock was 0.26.. I used thinset cement between the floor and the cement board and between all other layers.. Have fun!

Ray