That depends in part on what materials you are using, both for the combustion core and for the insulation.
If you are using dense firebrick (the common variety), you will want at least an inch of refractory blanket insulation, or a couple inches of rockwool, or 2"+ of perlite/clay mix... there are many possibilities, and someone can give more specific recommendations with more information from you. If you have insulating firebrick (it's very light and easy to cut or break), you may have enough insulation already.
The feed tube needs less insulation though you want some, but the burn tunnel wants plenty, as does the heat riser.
Besides insulation to improve your Rocket mass Heaters RMHs ability to rapidly come up to temperature and promote a super efficient and clean burn, it is important
that the RMH work at safe operating temperatures for hours. Part of the job of the insulation around the combustion core is to reduce the Danger to local exposures,
walls and this can include wood 2x4s behind sheetrock ! and curtains, drapes, rugs, and wooden floors.
We also need to talk about insulation between the RMHs Burner Base and a damp concrete floor, Without any insulation under the combustion core it is possible to
radiate off enough heat through to the concrete to cause steam explosions that chip or spall of chucks of concrete !
Starting at the floor it may necessary depending on the type of floor below Your RMHs burner base to have air channels, a full sized layer of bricks in the shape of the
outline of your Rocket Followed by up to 4'' of packed perlite insulation and then on top of that the first bricks that make up your Combustion Core.
Starting again from the burn tunnel you have your fire brick which should be wrapped in 2''-4'' of perlite mixed with clay slip for more insulation then the 1st shell of
Cob or Cob and facing stones, some areas will take further layers of structural cob and a finish coat !
The feed tube area is a special case, due to rapid in-draw of air this area usually stays cool when only the lower tips of your vertically stacked wood are burning,
most of the heat that you will get is radiated heat from the burn tunnel. An inch of perlite clay slip is sufficient in this location !
So we have many jobs for our insulation to do. Hope this was helpful and timely ! Big AL !
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