Hi everybody ,please be patient with my English,is all I have for the moment.
More I read or watch videos about rocket mass heaters more I like the idea to make one,but of course like any beginner will start with questions.
I have done one rocket stove for cooking ,three or for ears ago,is doing he's job very well .I use wood ash for insulation.
Could be wood ash a good insulation for a heat riser in a mass heater?
Or could be wood ash one component in a homemade refractory mortar recipe?
Wood ash is mostly oxides of potassium, calcium, phosphorus. It will certainly be good insulation. Be careful working with it. It can be very corrosive and burn your skin. Be sure not to breathe any of it.
I've been thinking the same question since I started reading about rocket mass heaters. We wouldn't have access to perlite or vermicultie and would have to do homemade low-tech insulation in the riser. Does plain wood ash work? Or better mixed with something?
A friend wondered if a small amount of sawdust in cob would burn away leaving holes to form insulation?
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
Safest thing is to build it up off the floor. There was a builder in new England a yearor two ago, who thought he could get away with cement board on the floor with cob over that... A few years later he removed his entire rmh and went with conventional heating. Seems his nice hardwood floor was charred and burnt … not the rmh fault but the building method... Please don't make that mistake.
As with most homemade mixes, you have to experiment a bit to get the correct proportions to suit the materials of your area. What I have had good success with is 3 sand: 1 clay soil for the base and then 'some' sifted ash. I used about 1/2 proportion and it seemed to work pretty good as a mortar with no cracking.
In this small amount though, ash won't be insulative enough but rather just give the mix a bit more refractory properties.
I've also tried just sifted ash put around my rmh core with nothing to bind it and it does insulated quite well but tends to settle and find its way out the smallest holes or cracks.