Question about Insulation and brick thickness for Rocket Mass Heater

Hello and thank you in advance for your help.
I’m trying to design my Rocket Mass Heater for my greenhouse based on the information in the book “The Rocket Mass Heater’s Builder’s Guide”.
In building the heat riser, I’m a little confused about the options for the thickness of the brick and the thickness of the insulation. In the drawing that (I hope) is attached, I’m showing the barrel as glass so you can see the thickness of the brick and the insulation. The bricks are full thickness fire brick (2 3/8”), and the insulation shown is 2” mineral wool. These thicknesses seem to make the space inside the barrel really tight, and that doesn’t seem so good. In the corners, the insulation is almost right up against the barrel.
I’d like to make something that is very long lasting, and, from the wording in the book, it seems like this is an option, but I may be misinterpreting what was said.
It also seemed like they were implying that if you use full thickness firebrick, that maybe the insulation isn’t needed?  (Chapter 4 - Insulation Section).

Hi Mc,   The heat riser in particular is the most important place where insulation is needed to allow the temperatures to get real hot and burn up as much of the gasses from combustion as possible. Any mass will just suck out the heat and never really allow this to happen. If those are heavy full firebricks that you are using, that wouldn't be a good thing.
Here are some other options:
1) Split firebricks are a bit better (as I believe the Wisner's often used) then the outside lined with wool or ceramic fiber blanket.
2) Better yet would be to use insulated full firebricks by themselves with no insulation wrap needed.
3) The cheapest way would be to make a perlite/clay riser which is perlite mixed with enough clay slip until it just sticks together in a ball when squeezed, then compacted between two pipes.  
4) The newest and fastest way would be to make what is called a "5 minute riser" which is a piece of ceramic fiber blanket lining the inside of a pipe.

Lots of videos and info on all of these if you need further instruction.

Thank you for the clarification. I’m trying to pursue your option 2, using full thickness insulating firebricks without additional insulation. I’m having a really hard time finding the brick at a reasonable price. And, it seems that all of the manufacturers are very far away.
All of the masonry supply companies that I’ve called are saying that their firebrick is “fine for pizza ovens”, so I want to clarify that standard firebrick is not acceptable, correct?
The use of insulating brick is crucial so that the temperature inside the heat riser is much higher than the temperature outside the heat riser (but inside the barrel), right?

Hi Mc;
Yes, you are correct in that you want to contain all the heat inside the riser to get compete combustion.
High temp insulating materials are costly no getting around that.
Finding insulating bricks can be an issue.
My masonry supply has full and split heavy firebricks by the pallet!   Less than $2.00 each  #50 sacks of fireclay are $7.50...
Ask them for insulated bricks and you get a deadpan answer of don't have them can't get them!

Gerry's  #3 or #4  would be much easier to locate and build.
I used a perlite clay riser for over 7 years!
I replaced it by making a 5 minute riser.
Easy to make and not very expensive!
here is a link to my thread about it.
https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket

Thank you so much for that answer.

I’m not sure if I should start a new thread for this question, but I’ve finished drawing up the RMH and find that it eats up so much floor space in the planned greenhouse. I’m wondering if it’s possible to put most of it below grade?  The only thing that troubles me is the idea of water infiltration, and that’s a big can of worms.

Continuing the thinking on trying to minimize the footprint ... has anyone ever stacked the pipes vertically?

The attached image shows the rmh moved down in elevation so that the top of the feed tube is level with the finished floor, and the pipes are moving vertically rather than horizontally.

Is this a bad idea?

Below grade or pipes along the wall with both work. If you wanted to use the warm surface as a seedling starter or butt warmer, maybe would be good to go with the bellow grade option. Pipes along the wall takes away the conductive function of a heated mass but take up less floor space.
I would also consider not using pipe but go with building a bell chamber that will stratify the heat naturally and not induce drag on the exhaust. They can be built almost any shape or size and often easier to clean.

One example of an underground build: Advice-RMH-build-Hokkaido-Japan

Edit: Your also going to want to have a vertical insulated exhaust pipe that extends just past the ridgeline of the structure to ensure proper draft. Going straight out the sidewall could be asking for problems, particularly if the wind shifts and starts to blow your dragon exhaust backwards.

Thank you, Gerry.

Bell chamber?  This is new to me. I’ll look around for more info on that.

I guess I should elaborate on a few things. The greenhouse will be in zone 8, and our winters are relatively mild, but we average maybe 10 nights per winter below freezing. I’m looking for a good way to keep the greenhouse above 40 degrees F during those cold periods.
I heat my house with a regular masonry heater using stored radiant heat, so this is not entirely foreign to me. My farm is off grid solar (8kw), so electric heat is not an option. I could maybe get by with a propane heater, but I much prefer using scrap wood as fuel rather than fossil fuels.
I definitely want to heat what will become a concrete counter top above the pipes or bell chamber to use as a seedling warmer. I’m hoping especially to grow onions and garlic from seed.  Lots of other things, too, but I’ve never been successful with those.
I attempted to research the bell chamber, but ended up sort of confused. It reminds me of a smoker, which made me think I should make it double as that, but I guess with this unit the completeness of the combustion would not have enough smoke?
A secondary function of the greenhouse will be for processing wool (washing, drying, combing, carding, dyeing, maybe spinning) in the winter months (hence the big sink).  I’m a shepherd and have lots of wool. Last, but not least, since I don’t have a dryer, I can hang clothes to dry even if it’s raining.
So, I’m not shooting for 70 degrees, but rather taking the edge off of cold nights.  I think there will be a good amount of solar thermal gain in the 3 foot tall stone walls and precast countertops and stone floor, and that may be enough, but I think it won’t hurt to have this backup.

In simple terms, a 'bell' is really nothing more than an open chamber where the hot flue gasses have time to stratify and then be released to the chimney much cooler than when they entered.

A couple of links describing bells and ballast gasses:

wood-heat-storage-flues-vs-bells
Free gas movement

Thomas Rubino has a new rocket oven that he is currently building and is also looking at using it as a smoker:
Build-Black-White-Rocket-Oven