When you are planning to build the thing out of insulative fire bricks there's no need to insulate. That's to say, in case you mean the white insulative bricks, very lightweight. More insulation could be added around using perlite, vermiculite, lightweight expanded clay aggregate like Leca, the kind which is used for agriculture among others. Anything combustible in one way or another isn't suitable.
Curious what you ended up using?? I just did a search for "natural insulation for rocket mass heater" and your thread popped up. I see you didn't get much help here but since it was a year ago I thought you probably figured something out by now! We are building one this week and we're hoping to use only materials from our land...saw dust, clay straw etc. but haven't found that it's too successful. Would love to hear from you about any tips you learned in your process and if you ever found a local insulation source?
Peter gave about the best advice for durable insulation for the heat riser.
If you really are adamant about using only local materials, I would suggest a straw-clay mixture. I would make the inner half inch or so of clay with enough dried grass instead of straw to make it strong when damp. Since the fiber is going to burn out, I have found that the fine texture of dried grass clippings allows precise shaping and a lot of very small voids. For the bulk of the tube, I would mix as much dried grass into the clay as you can without getting solid clumps of grass. This will leave a foamy sort of structure that will have decent insulative quality after the grass burns out. The clay at least on the inner part of the riser will fire into pottery and be quite strong if left in place.
Location: Toutle WA (an hour north of portland)
posted 1 year ago
Thanks for your response glenn. So in your description you talk of making the inner half inch mostly clay and the rest mostly grass, now do you mean actually make the riser out of this or are you saying to surround the bricks with this? Either way sounds like a great thing to try, we've got lots of clay and Grass around. Have you or anyone you know tried this? I am really interested in making my own riser out clay. What do you think about subbing grass for horse manure? A 50/50 mix manure/clay for the first 1/2 inch, then more like 85% manure for the insulation. Seems like the Fermintation would make it stronger as a riser.
3) I think any shredded organic material like straw, sawdust, paper, leaves, grass, would leave an insulating void, even if it outgasses.
4) aerated refractory cement/concrete. Make a foaming solution (6 cups of water , 1 cup of dish soap, 1 tablespoon of glycerin or 1/4 cup of corn syrup). Wip it into a foam. Mix it into the cement/concrete. Or use a concrete mix with aluminum powder which reacts with lime to make hydrogen bubbles (the hydrogen later diffuses out of the set concrete and is replaced with air). Bits of this could be used as a substitute for perlite. This also makes an interesting building material.