Shane Call : Color me happy

, When a new future Rocketeer shows up here we want to make sure that he has the best information possible, its easier on everyone
to get it right the 1st time, and if we haven't pointed out problems in your design you might not mention the change you made, leaving you and us in the dark as how
to fix it later !
As this is your 1st and possibly only build we want to get it right the 1st time. The next sentence below is
my Personal Opinion based on Direct observations !
For a 1st time Builder
the 6'' J-Bend RMH is the smallest you should go. It is forgiving of minor errors, and will certainly heat your space ( depending on your layout )
With that style of Rocket a 1.5 inch gap Between the top of the heat riser and the underside of your Barrel should be 1.5 '' to 2.0'' ; 1.5 '' is a minimum dimension !!
You still have many more choices in the final metal type of Radiating metal 'drum' you place over top of your Heat Riser -to include using a 30 (U.S.) gallon water heater
tanks , this is something found as scrap at most Heating Appliance Dealers !
I am dwelling on this because in spite of the U-tube video you have seen -the use of metal in the Feed Tube and the Burn Tunnel will give you problems with smoke back
into Your house, and will eventually fail from metal fatigue due to " High Temperature Hydrogen Attack'' and ''Steam Embrittlement'' !
Remember that while the Heat Riser is inside Your metal heat radiating drum, the drum will set on top of the burn tunnel*, the burn tunnel can then use more Cob and more
insulation in its base !
In order to match the performance of the 5 year old RMH from the earlier video you must use Firebrick for the Combustion Core, that is the Feed Tube, and Burn Tunnel
and Heat Riser.
You are getting into personal opinions with the difference in Perlite and Vermiculite. If you are using a powdered fire clay ( Lincoln 60 ) You will use just enough clay- to
-perlite to to ridgidize that structure say 1 part Clay to 3 parts perlite.( and water to wet, not saturate ?) The vermiculite will soak up more clay needing more to stabilize
and thus will be less insulating ! Close to 1 to 1 .
I do not like the RMH so close to plywood walls and recommend an air gap then heat barrier then 2nd air gap to protect all flammable materials that close.
For the Crafts ! Big AL
* refer back to the ''Kirk'' video where the barrel was removed only the top of the bricks that made up the burn tunnel were in view the barrel sets on top of the Heat
Riser rather than surrounding it ! A.L.