I’m in the research phase to building a rocket stove mass heater for a 200 sq ft cabin in Northern Minnesota. Many times winter overnight temps can drop to 30 below zero Fahrenheit (sometimes more) which is fine for viewing the northern lights but not for much else.
I understand about the need to use firebrick for the J tube and heat riser instead of metal tubing for longevity so I plan to use fire brick instead of metal tubing.
Will a firebrick heat riser, covered with a 55 gal drum, require an insulating layer around it like one does with a double walled metal tube filled with insulation heat riser? Or will the fire brick have enough of an insulating factor? Perhaps an inner firebrick heat riser with a second one surrounding it?
If you can find insulating firebrick, they will work excellently as the whole heat riser. Otherwise, hard firebrick (preferably "splits" which are only 1 1/4" thick) with insulation wrapped around it will do fine. You can use rockwool batts tied with iron wire or wire mesh, or a sheetmetal tube with perlite poured in between the riser and shell, or a number of other methods. A good and relatively cheap method is to cast your riser using a sacrificial inner liner of sonotube or duct of system diameter, an outer shell of sheetmetal 4" larger in diameter, and packed with a perlite/fireclay mix having just enough clay to hold the perlite grains together. This will present a low-mass face to the fire which will come up to operating temperature very quickly.
The reason Max recommends a batch firebox is that when built according to specs they deliver their heat very fast, maybe twice as fast as a J-tube, and don't have to be tended during the hour that the burn lasts. Thus you can pump more heat into your mass in an evening if you need to.
This is how we most often do the insulation around fire brick heat risers.
Without insulation, the temperature tends to equalize between the "up" and "down" draft areas (inside heat riser, and outside under the barrel). This can create bad draft failures unless you have a very powerful exit chimney (and are sacrificing a lot of heat to keep it 100% reliable in all weather). The most common symptom of this type of draft failure is for the stove to draw OK for the first 30-40 minutes, then start to smoke back out of the feed as it chokes on its own exhaust after the bricks heat through.