As you found out, terra cotta tiles can't stand the thermal shock of a
RMH in operation. They can take slow, uniform heating, but that is not what we are dealing with here. Your 8" tile will not leave
enough space inside for sufficient insulation, and the stovepipe inner liner will burn out before long, leaving the insulation to support itself (or not).
A better riser construction method is to have two stovepipes, one 4" or more greater in diameter than the other, and fill the space between with a mix of perlite and fireclay (just enough clay to stick the perlite together). When the inner liner burns out, the fireclay will be strong enough to hold up. The outer pipe will be protected from degradation by the insulation.
It looks like you have a very long horizontal burn tunnel, with a normal-sized firebrick burn tunnel followed by a whole flue tile. That flue tile will be exposed to severe heat load and is not likely to last, and the whole run is much longer than recommended. The current recommended proportions are 1:2:4 for
feed tube, burn tunnel and heat riser. (You can measure along centerlines or outside edges as you like; the differences are not significant enough to matter.)
So if your feed tube is 16" tall from the floor, the burn tunnel can be 32" overall, and the riser 64" tall from the floor. A much longer burn tunnel will reduce the draft from the vertical riser.
You mention that the
RMH will be backed up to a hardibacker wall surface, which is okay, but if there is any
wood framing behind it where the barrel sits, that will over time char and eventually be likely to burn. You need a metal heat shield with a 1" airspace, as well as standard woodstove clearances, to protect any combustible materials near the RMH hot zone.
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