Hi Phil; I used hardy board on the back side of my mass and left a 4" space to the outside wall ( stone) then filled with perlite.
For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
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Look! I laid an egg! Why does it smell like that? Tiny ad, does this smell weird to you?
grow your own garden and build your own home, in Montana, for free-ish