Gypsum panels r value

Right the first sentence:

that is encased in a moisture resistant, 100 percent recycled green face and back paper,

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Paper is combustible so not possible to use it as hearth. For a r = 2 hearth you probably need micore or similar. That table may help:
https://chimneysweeponline.com/horvalue.htm

begreen said:

Another way to achieve the R value would be air space. If built on heavy metal studs with a couple layers of durock on top, the R value requirement will be met or exceeded.

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That's what I would have although thought but I was just checking the FAQs of the Myriad as the OP was interested in that stove earlier. Please see what Drolet writes about airspace (at the bottom): http://intranet.sbi-international.com/uploads/rcompleteng.png
First time I have heared that. I wonder where that regulation comes from; maybe a Canadian thing?

You could see if your local Home Deport is able to special order the Englander Madison for you. Cost should be $900. It only needs ember protection for a hearth. Here is a thread with lots of info about the stove: https://www.hearth.com/talk/threads/madison-in-my-burn-trailer.128150/

Another stove with a similar price point and ember protection only would be the Pacific Energy True North. That stove would also be good if your chimney is on the short side (12 to 15 ft) as PE stoves usually do well on a short flue.

begreen said:

Not sure where they got an 1/8" air gap from. That is miniscule. 4" is a lot different. For improved performance the air space could be ventilated. Regardless, with 3 layers of Durock and the air gap rated at R=.92, the hearth is over the required R=2.0.

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If you read the notation at the bottom is specifies that you cannot "stack" 1/8" air-gaps to achieve the .92 of a full inch.

The reason is because once you get much higher than 1/8" thickness air-gap the air is no longer considered "still air" and it then becomes convective air. Convective air does NOT insulate like still air, it's actually counter-productive; it helps to move the heat from the source to the sink (from the hearth to the underlying combustible materials). The metal studs/risers do the same thing.

I hate to say it but I think they are right, you cannot just add up air-space to get the right R-value. You need actual insulating materials...

CountryBoy19 said:

The reason is because once you get much higher than 1/8" thickness air-gap the air is no longer considered "still air" and it then becomes convective air. Convective air does NOT insulate like still air, it's actually counter-productive; it helps to move the heat from the source to the sink (from the hearth to the underlying combustible materials).

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That makes a whole lot of sense and explains why fiberglass or cellulose is much better as insulator than an empty wall cavity. Just because of the metal studs which will transfer heat really well I would make sure to have a r-value of 2 on top.