first time RMH , support please

HI all, new to rocket stoves and RMH. I am going to post this in facebook and permies so I apologize upfront for double reaqding.
I have read the two main books, both have been great and thank you to the writers. I have watched many youtube videos as well, thank you as well.
I have some generic questions and will supply pictures.
So I tried a precast j- tube which ended in failure but still a good learning lesson and I plant to reuse the material so no real issue. I think I did not use enough refractory mortar.

Now I am set on just using fire brick. I am planning to use 8” duct for the exhaust so I think the fire brick will work fine for that.

Questions
1. if using the thicker fire brick should I still do an insulation layer on the exterior of the fire brick within the barrel? I saw on you tube not doing it but also saw one where they did a metal cage and clay (and something else (perlite/vermiculite) ) to wrap the fire brick.  I suppose best method is to wrap the firebrick correct?
2. In one book they suggest keeping the RMH 4” from the wall, I have also read that you can use perlite(2”) or vermiculite(4”) as insulation layer from drywall. So the questions are, do I just leave a 4” air space (not really wanting to), should I use vermiculite(i have this) and if so how do I fit it into the space? (make a cavity and fill it?)
3. Do I need to use brick/block to form in the the RMH or is just hand packing good enough? I do nto have any extra block or brick but can buy it. If hand pack is good enough then I am fine doing this.
4. here are pictures of my test bricks, I like the 2-1 mix which is 1 part clay, and 2 parts sharp sand. I could not break this with my bare hands (same  arm strength I used for the others). Any comments?



5. horizontal exit, I have read this is not good but I am stuck. Any tips? I plan to exit low and have the main stack all outside. Is this an issue? Is it a bigger issue? I cannot go higher because I have a GLULAM beam up high to go vertical. If I try to go straight up it is a much bigger job and issue. I have an attic truss with a 10/12 pitch, so I have space to go through the roof but to get above the roof peak is a long ways.



6. I currently have 16 feet of horizontal run of 8” pipe in the RMH (planned) and then will run 12 ‘of riser outside up the edge of the house. Issues?

7. background – this is for a cabin that is in Michigan, it is heated during the winter so cabin temp is 65F but RMH has two fold( using bench as seating for table, and house uses geothermal so if I lose power I have a secondary source of heat until power is restored). During winter months, there are 3 3 week periods where the place is occupied for a long period. The rest are weekends but intermittent and the heat will just offset the heating bill as it cools back down.
8. I am just using normal HVAC duct work, is the chiminey pipe actually needed? IF hot enough out side I plan to swtich to that if needed but plan to start out with regular metal duct work. Also if hot enough I plan to cover the drywall behind the drum with fireproof material if needed.



Other tips or advice.

If using hard firebrick for the riser, you do need insulation around it. There are numerous effective ways to do this, and any will work if done according to directions.

The 4" airspace is not counting any insulation in/on the mass. You need freely moving air to carry heat away before it gets to the wall. The barrel has the same heat issues as a woodstove and needs the same clearances to combustibles.

I'm not really clear on what you mean by "form in the RMH", but you can use just cob to make the overall shape of the mass, placed around the duct core. You will want a more durable surface for the mass than just cob, so research surface finishes and decide what you want to do.

If you have read Ernie & Erica's RMH Builder's Guide, you know that a plain horizontal exhaust will not consistently work except in very uncommon circumstances. If you need to go through the outside wall instead of up through the roof, make the exit as high as practical. To be safe and effective, the chimney still needs to go up at least two feet higher than any roof within 10', per standard building code. If your roof is as steep as 10/12, I would go higher than that because of the strong eddies the roof will generate in certain wind conditions. If the chimney is on what is usually the upwind side most of the winter, you may be okay with the minimum most of the time. You will get the best chimney performance if you exit straight up near the roof peak, and you will not need a tall metal stack with guy wires. Any exposed chimney would work better if insulated; bare pipe will cool fast and weaken the draft.

Your 16' of horizontal 8" duct appears to have three 90 degree turns in it. At 5' each, these plus the duct only add up to 31' while an 8" system can handle up to 50' equivalent. You could use more duct if you had room for it.

Another point about the galvanized duct: The first 5' more or less after the manifold sees higher heat and more stress than the rest, so it is recommended that that part be black steel stovepipe, both for increased strength and to reduce the possibility of heated galvanizing releasing zinc fumes.

Thank you for the help.

OK brick laid out. the question is will my dimensions work. The shows having 16" height for the feed tube, i have that. the fire riser shows 48 inches but with the drum i have having 43 inches allows for the 2" gap. Will this be OK? the ratio of 16" to 43". I plan to wrap the fire brick and pack with insulation but will only get 1" of insulation to maintain the 4" space to the outside wall. Will this be OK as well?

here is a side shot

It's a bit less than the recommended ideal riser height for that feed tube height, but as long as you have a good chimney to help the draft you will probably be fine. Having less than the maximum horizontal duct run will help too.

You can have insulation that is thicker than 1" everywhere except the corners; there is less heat loss at the corners anyway. You don't need to worry about barrel clearances as long as you keep at least 2" minimum. If the riser is off center in the barrel, the surface will be hotter where the space is bigger.

ok makes sense thanks. hope to build it inside yet today.