R Value?????

Seems to be some disagreement here. Some of the confusion might be because materials have changed. Asbestos millboard is no longer used. One user reported that looking at the underside of their commercial hearthpad revealed Micore as the bottom layer. When I researched this a while back I found that millboard has a higher insulation value than cement board, depending on its composition. For example McMAster-Carr sells millboard with an R value of .5-.7/ 1/2" http://www.mcmaster.com/#millboard-sheets/=8u1ce2.

here's an old thread on the topic:
http://hearth.com/econtent/index.php/forums/viewthread/8650/

here is another discussion about the R value of asbestos millboard:
http://boards.straightdope.com/sdmb/archive/index.php/t-499220.html

When in doubt I would err towards the higher value. It's really hard to change later on and who knows what stove may be on this hearth in a few years. In the least, I'd put a sheet of metal in between a couple layers of cement board for better protection.

Like the linked discussions revealed, it depends on what the millboard is made from. In the second link it sounds like the consensus for asbestos was R = ~1.4/in.. If so 3/8" would be R .525 or two sheets of durock (R .52/in"). If you want a measure of additional protection, add the sheet of metal or maybe go with 3 sheets of cement board. That equals R .78.

Micore is R 2.33/in so if you want to take that calculation then 3/8" would = R .87. Your pick.

One thing that might guide choice is whether the stove has the ashpan or at least the bottom heat shield. If neither I would go for the higher R value.

Yes, I've posted a link to it a few times. Good info for wallshields, but it doesn't really cover hearth R value, just the basics.