Dirt for the riser insulation

Hey guys, I am building my first RMH. I live in a part of the world where refractory brick and perilite are really hard to get. So, I built my riser out of normal clay brick, then made a form and packed dirt about 2 inches thick around the outside of the brick. When I did a test burn, my fire tends to draft good enough that no smoke comes out the feed box, but I am not getting nearly enough heat, and it seems the fire isn't burning as clean as it should. Any ideas what should change?

Hi Nate;  Welcome to Permies!

I' m curious Nate, what part of the world are you in ?  

You can use red clay brick in your riser they just won't last long term.
 Dirt as a filler is better than none but... it isn't much of an insulator. Can you get volcanic rock ? It would be a better insulator than dirt.
Are you in the US ?  Junk yard glass top ovens have ceramic fiber as insulation, scrapping one out could get you  the best riser material currently available for free.
With using the dirt, after burning awhile the inside and outside will become the same temperature, this could lead to stalling and smoke back. Key word there is COULD... it might be fine.
As to why your stove seems, not to be hot enough, nor burning clean enough.  I suspect it has to dry out and heat up fully before it will burn clean and hot.

Thanks Thomas,
I am actually in a remote, mountain region of China. The locals here use dirt to build everything, including significant portion of their houses. Some still use dirt to build their cook stoves though most have gone to metal stoves which are very inefficient.

It seems like you are right on about the back draft. It seems that after an hour or so of burning, a lot more smoke comes back into the room. Any other suggestions for insulation?

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Nate Stoltzfus wrote: Any other suggestions for insulation?

Hi Nate,   If you could acquire enough wood ash, even if it were only a temporary solution, would help out until you can find something else more long term. Stabilizing it with a small amount of clay slip would help keep it from settling but it would make it less insulative.

Can you get sand ? It would be better than dirt.

thomas rubino wrote:Can you get sand ? It would be better than dirt.

Yeah, sand is no problem. Should I mix it with the dirt? What ratio? Not a lot of wood ash, but a bit. I might be able to get some low quality fiberglass insulation, to. Should I wrap an inch or so of that on the outside of the riser?

I'm curious, too. Is the length of the burn chamber critical in relation to the height of the riser? My heat riser is around 50 inches, burn chamber is around 24 total length

Hi Nate,

For heat riser insulation, you could try fiberglass. I.e. to protect the fiberglass, perhaps make a two layer insulation by using fiberglass for the outer most 25mm thickness layer, and plain sand for the innermost layer, to let the sand take the full brunt of the heat from the riser's brick.

What's your heat riser to barrel top gap like? I.e. the distance from the top of the heat riser to the barrel's inside top horizontal portion? From your photos it looks like the top gap may be a little tight. I typically shoot for a minimum riser top gap of 75mm. Making it a little larger won't hurt unless you intend to use the top of the barrel for cooking etc. But an advantage of a slightly larger gap is that it will allow the stove to draft stronger on marginally short chimney systems.

Brick of any sort should moderate the heat enough that a simple wrapping of fiberglass will work just fine. You can use sheetmetal like a piece of duct, or even chicken wire, to hold it in place.

Hi Nate;
Sand will work better for you than the dirt has.  No dirt mixed in, unless you need it at the bottom to hold your sand in place. Wrapping plain fiberglass around the outside of that will help against stalling. Sort of a double protection. 1" of fiberglass alone might not be enough, so I recommend using the sand as well.  We want that heat to stay inside the riser.  
As Byron asked , what is your top gap ? Are you planning on cooking on the barrel ?  If not then a top gap of 4" would be better than 2"
This looks to be a 6" system ?  16" deep feed tube ? 5.5" square thru the whole core? 24" wall to wall burn tunnel ? Then a 50" riser ?  Those numbers should work fine.

Thanks guys. I will take the dirt it this weekend and put in sand, then a bit of fiberglass around that. I will increase the top gap, too. I think I was under 2 inches for it before. I'm excited to try it and see how it works. I will update after I try it again and let you know. I have had some locals already tell me that they want me to help them build one.

One other question... They burn a lot of dried cow chips here. How will that work in one of these heaters? Anyone have any experience trying that?

Thanks again

Hi Nate ;  Good luck, I think you will be happy with the results.
 I have not heard of anyone burning dry dung, but folks have been burning it for century's so I'm sure you can add some. Might make extra ash though ?  Big deal if it does , scoop it out and carry on.
When you go to building your mass , do not use the sand, use lots of rock with clay if you have any or just plain dirt.

Quick question, are 55 gal barrels plentiful, and cheaply available  ?  If the answer is yes then you might consider building a 1/2 barrel bench rather than a piped bench. Quicker , less cob, faster heat, the down side is it will cool off sooner than a solid mass.

One last thing Nate;  Your outlet to the mass , might should be larger. At least 8" then neck down to 6".  The horizontal transition area is a big choke point. Many first time builder's make this area too tight and their rocket does not perform to expectations.

Seems like you have some good solutions, but let me throw in some ideas.
Do they grow rice near you?
Rice hulls could be added to clay earth to create a insultaive voids.
The rice hulls are largely made of silica and are very porous.
If they are exposed to high heat, they will pyrolyse in their little cavity, leaving behind an even more insulative cavity.


If you could make a mold from paper , cardboard, steel or aluminum cans, etc. you might be able to eliminate the bricks and build an all insulative riser, which i think would perform better.
Common fiberglass insulation has a melting point of 1300 degrees, or less due to the binders usually used in it's manufacture, so I haven't ever tried it in direct contact with high heat.
I have used it in a soil/cement/perlite mixture to abate cracking, and I think it it works great for this.