Kind of hard to see in my old photo, but I built a layer of bricks up against a 3/8" plywood board and then removed the board leaving an air gap behind.
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Isn't there a rock wool type product that's suitable? They fill the middle of twin flue pipes with something? I was thinking of a few sheets of that ideally but wasn't sure what, exactly, it was.
Rockwool takes some beating. I incinerated a large amount of young weed plants for a friend in a spot of bother, they sat in Rockwool cubes and I folded the lot up and put it all into a well established blaze in an 11kW stove. In the morning the Rockwool cubes were perfectly intact and did not crumble or degrade when rived about.
If I understand correctly the binder used in manufacturing fails at 250c but the structure remains undisturbed.
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Just stick this to the wall first to keep in place then screw the metal plate over it into the wall each corner and one in the middle if you want
I've used Glassroc or Master board many times. Glass fibre compressed with cement. Only 6mm thick, slightly flexible and easy to score and snap. That provides a barrier and the steel in front acts as a heat diffuser preventing super hot spots.
for me id insulate into that area with normal insulation
use a thin piece of steel plate to go over it and add a seecond piece of steel with a gap at the bottom as that front sheet will get warm it will start to convect all on its own therefore adding more heat into the room
As above I'd go with some kind of rock wool product...pretty sure the stuff is good for 600C+ when I last looked into it. The name rock wool is basically the description of what it is. I watched a "how do they do it" where they showed them making the stuff....quite impressive.