I've been having an interesting time the last couple days checking out various brands of cement boards, I'm finding they are very much not all alike, and probably should not be considered interchangeable. Everything that I mention in the following info comes from either the product websites, or the company "tech support" phone line people. At this point, I would say that the clear winner for hearth use is Durock, with a distant second for Hardibacker. I would be hesitant to use Wonderboard, and absolutely wouldn't consider Permabase. Durock clearly specifies in it's descriptive product info PDF that it is acceptable for use in "UL Listed Wall shield / Floor Protectors", and gives an R-value for the 1/2" sheets of 0.26. The US Gypsum website describes it as "Non-Combustible" - there seems to be no hesitation about labeling it suitable for hearth applications. Hardibacker has that states it is "online documentationthat states it is "Non-Combustible per ASTM E 136", and "1 Hour Fire Resistive per NER 405. It lists an R-value for the 1/4" sheet of 0.13, but gives no value for 1/2". The FAQ mentions it as suitable for "Fireplace Facing" but makes no mention of use for hearth protection. When I called their tech support line, they confirmed that Hardiebacker was definitely "Non-combustible" and said it could be used under a hearth, but that heat would "go right through it" and said it was only about R-0.15. The tech support guy didn't sound really comfortable with the idea of using it for hearth protection. However it sounds like it would be 100% OK to use in "clearance reduction" wall shields. Wonderboard had online documentation that pretty much made no mention of fire resistance, the only thing I could find was the MSDS line on Fire Hazards that said "Will not burn". I called their tech support line, and was told that Wonderboard has an R-value of 0.2, but that the fiberglass facing was "combustible" so the product would only be suitable if it was actually covered in a NON-Latex based cement and tile - then it would "probably" be OK, but didn't seem totally comfortable with the notion. It was an interesting point that he brought up however that it would be a concern about the nature of the cement that was used to put the tiles down with - it needs to be non-combustible, and some formulations contain latex that is. Permabase in their documentation says it can be used for "fire resistant walls" but lists expanded polystyrene in it's ingredients on the label, and has a statement in their online documentation that the product should not be exposed to temps over 220*F (105*C). It was listed with an R-value of 0.2, but their tech support line person said it was "combustible" because of the expanded polystyrene (aka styrofoam) beads in the cement mixture.