Insulated base for combustion unit help!

I'm new to RMH and cob and have a few questions:

I think I want to insulate the ground under my burn tunnel because I want the heat going mostly into the battery which in my case is an outdoor bench (which will be kept covered w/tarp when not in use). Also I want the bench/battery cob to be such that it will sustain a lot of thermal shock as it's an outdoor system (I'm in zone 5a and the unit will be in use all winter).

I've built a mock-up successfully and now I'm at the stage of leveling the ground and want to integrate perlite before building the combustion unit on it.

Does anyone have any links to video of the following?
- How to prep soil for mixing w/perlite for the ground below the combustion unit
- How to prep cob mixture that will experience a lot of thermal shock

I've tried googleing/youtubing key words on this but I'm not coming up with anything useful for these. There's this https://www.youtube.com/watch?v=8g8lbL4XVig but it doesn't really show how the soil is prepped before mixing w/perlite.

Thank you!

Just mixing perlite with soil may achieve thermal insulation, but it won't inhibit water from wicking up into the cob. For an outdoor unit in zone 5, that means a lot of moisture getting into an area exposed to freezing and thawing, which will destroy cob or any porous clay structure. You need an effective moisture break below the cob. Several layers of flat stone if you have that available should work. What region do you live in, and what local resources do you have? How damp does the ground tend to be?

I would suggest plenty of straw in the cob for best crack resistance, recognizing that there will be some extreme temperature differentials as the bench heats up and some stress cracking is likely. I might try putting in control joints to direct the cracking to more acceptable locations, like along the sides of the bench so the top has more freedom to stay intact.

Thanks for this perspective. I'm in Lexington KY so very, very moist soil. Here are my thoughts after your thoughts.

For the moisture break materials:
I have plenty of round stone gravel, massive slabs of concrete, brick and cored brick from a demolition/renovation I could use (like this http://www.lowes.com/pd_10298-215-600370_0__?productId=3433312) (can I even use cored brick or is that unsafe due to hot air expansion?). I was saving my limestone and urbanite for the benches.

For the combustion unit base then it would go in layers like this: tamped soil, then gravel, then concrete or brick base to take me above ground level, and then a layer of straw+perlite+soil on the brick just above ground level on the concrete.

For the benches base to get me above ground level I could use gravel, then brick or urbanite (gaps filled w/cob), then limestone/cob to surround the pipe, topped & surrounded by cob heavy on straw with control joints at each turn (my design is an octagonal bench about 12' in diameter using 8" stove pipe with seven 45 degree joints -- the final joint is an opening/entrance and there's an actual fire pit in the middle).

I just did a google search since I wasn't familiar w/the term control joint and got this http://en.wikipedia.org/wiki/Expansion_joint. This makes sense. I wonder how I'd do this in cob without losing massive amounts of heat through these cracks?

I wonder if anyone else has already done this in a cold, wet climate...

One thing which is nice under benches, is air entrained concrete. Like Ytong pannels. They make some 4"x16"x20" tongue and groove which is usefull.

"I wonder how I'd do this in cob without losing massive amounts of heat through these cracks?"

The control joint would not have any width to lose heat through. I might use a layer of paper or cardboard in the outer 2-3 inches of the bench surface, perpendicular to the surface (not close to high heat areas). This will help ensure that any differential expansion has a neat place to happen. Not critical to function, but may be good for esthetics. If you feel like experimenting, you could use the control joint in one area and none in a similar less exposed area, and see what difference there is in performance. I don't know exactly how important this will be for this application.

Thanks for all this. I've got my gravel/urbanite-filled trench dug and I'm ready to pour concrete for the combustion unit to make sure the cob stays above ground. I paused and wondered, will normal concrete be okay for the combustion unit? If I have a 2" layer of perlite/clay will that be enough to not crack the concrete upon heating/cooling?

Remy Olson : Any concrete made with Portland Cement will disassociate the lime found in the mix and the concrete will crumble under the extreme high temperatures
in the core, I would double the thickness and make my base wider and add a layer of house bricks NOT red stained concrete pavers on top of the insulation for
added protection ! For the good of the Craft ! Big AL

Just 2" of perlite/clay under the combustion zone will not be enough to protect the concrete from heat damage in a long burn. You need more, either a bunch of material or an air space above the concrete. Putting a layer of spaced-out bricks with cement board on top to support the insulating layer a couple of inches above the concrete would be sure to work. There should also be some sort of reinforcing in the slab so it doesn't shift when it cracks (anything more than 4-6 feet long or so will eventually crack on exterior ground, sooner if it has a heavy load on it).

Remy Olson : at the bottom of this page find the ::--> permies >> forums >> energy >> Rocket stoves click on rocket stoves the next page is all
rocket stove Treads most resent 1st, scroll down, find, and click on Rocket mass heaters in Greenhousesyou want page 5 Paul Tofflemire, jan 26 2014 ,

The picture shows a Heat Riser Stack and the base of the Burn tunnel sitting on about 2'' of vermiculite setting on dirt, for a combustion core setting on concrete
twice the thickness and then a row of spaced bricks some backer board that will take years to fail and then the base of your combustion core.

Hope you find this handy, For the good of the Craft ! Big AL

allen lumley wrote:Remy Olson : at the bottom of this page find the ::--> permies >> forums >> energy >> Rocket stoves click on rocket stoves the next page is all
rocket stove Treads most resent 1st, scroll down, find, and click on Rocket mass heaters in Greenhousesyou want page 5 Paul Tofflemire, jan 26 2014 ,

The picture shows a Heat Riser Stack and the base of the Burn tunnel sitting on about 2'' of vermiculite setting on dirt, for a combustion core setting on concrete
twice the thickness and then a row of spaced bricks some backer board that will take years to fail and then the base of your combustion core.

Hope you find this handy, For the good of the Craft ! Big AL

Thank you so much for this thread! I will also ask on that thread, but figure I'll ask here, too: is the perlite mixed with clay and tamped level below the combustion unit? Or is the combustion unit sitting on plain perlite held by some kind of crib? It's hard to tell from the picture. Ianto suggests a perlite-clay mix which befuddles me because I have yet to get my hands dirty with the clay/dirt. Any photos/video of someone mixing perlite and dirt?

Glenn Herbert wrote:Just 2" of perlite/clay under the combustion zone will not be enough to protect the concrete from heat damage in a long burn. You need more, either a bunch of material or an air space above the concrete. Putting a layer of spaced-out bricks with cement board on top to support the insulating layer a couple of inches above the concrete would be sure to work. There should also be some sort of reinforcing in the slab so it doesn't shift when it cracks (anything more than 4-6 feet long or so will eventually crack on exterior ground, sooner if it has a heavy load on it).

I'm going to take your word for it. Better safe than sorry (and I really only have time for one shot at this). Question though: would cored brick work? (the ones w/the big 3 holes or the ones w/about 12 holes -- I have both from a demolition left over).

If so then would I need to fill the holes in the brick w/dirt or can I leave the air there? I would think from your description (spaced out brick) that this would be fine to use. If I have to use solid regular house brick then that's fine too but would rather use the cored brick if possible.

I will buy some cement board as you suggest to go below the perlite. As I asked above just now: how do I secure the lightweight perlite? In dirt mixed and tamped? Or do I just pour it into a crib made from cement board and screwed together?

The slab we poured yesterday we laid with 3 scrap galvanized steel pipes in it running perpendicular to where the riser will be. The slab was poured to fit the unit exactly once cobbed -- 31"x48"

Remy O. : This looks like pure 'vermiculite' packed tight, it like a good mix of Clay Slip (mixed to a thin pancake batter) and perlite is mixed with the perlite
just dampened to hold down silica dust, until snow ball sized balls can be formed, these balls when squeezed between a finger and thumb will 'pop' apart, this
is the mix that gets tamped down into the boxed under layer that the RMHs Burner Base gets built on ! When well packed they will support the bricks and
backer board, E & E recommend covering that surface with a heavy duty grade of Aluminum foil shinny side up to further help refract the heat back into the
Combustion core !

For the Good of the Crafts Big AL

In a 31" x 48" slab, reinforcement should run the long way for sure, and both ways is better. Not sure from your description which way yours run. I hope the pipe is not large diameter relative to the slab, or it will provide smooth lines for the concrete to crack along ("control joints"). Rebar has corrugated surfaces for a reason, it allows the concrete to grip it for slip resistance in structural use and also make a bit less of a parting plane.

The cored brick should work fine away from high heat zones, lika as the base layer on the slab. I just wouldn't use it for fire contact or close downstream. The open cores will actually be good for insulation.

So, I'm kind of in the same boat right now... I'm building an insulated base for my core going into the garage...the bottom layer is going to be a piece of delta rib steel roofing (ribs down). With only the ribs touching the concrete, there's a minimal contact patch and 6" wide air gaps that will run the length of it, so there's pretty much no way for the concrete to get hot....

My issue is what to put on top of that. I want to use some sort of perlite insulation, but it has to be structurally pretty strong since it has to support the weight of the entire core/barrel assembly. I have plenty of fire clay on hand, as well as sand and perlite. Right now I'm leaning toward the recipe for castable refractory at backyardcasting.com...only thing is it does contain some Portland cement (as well as fire clay, perlite, and sand). With the bricks of the core between the fire and this base, I'm not sure if it would get hot enough to matter...but I only plan on building this thing once.

I'm concerned that just using a fireclay/perlite mix won't be strong enough to hold the weight. I'm planning on making the base about 2.5" thick. I'm making sheet metal sides to weld to the bottom steel that will make the "form" that holds it all together. Hopefully that will make the "box" strong enough to also give some support to the goop. If I need to, I can weld in cross braces...which might help act as a sort of rebar for the goop in it.

One more issue--the weather is turning, and it's probably not going to get above 50* F from here on out...and I need this thing to dry as quickly as possible so I can assemble the core on it. If I just use a clay mix, I'm worried it'll take forever to dry. It would dry out quickly with the core running on it, but I can't assemble the core until it's dry...sort a chicken and egg sort of situation.

Any thoughts or suggestions would be greatly appreciated!

allen lumley wrote:Remy O. : This looks like pure 'vermiculite' packed tight, it like a good mix of Clay Slip (mixed to a thin pancake batter) and perlite is mixed with the perlite
just dampened to hold down silica dust, until snow ball sized balls can be formed, these balls when squeezed between a finger and thumb will 'pop' apart, this
is the mix that gets tamped down into the boxed under layer that the RMHs Burner Base gets built on ! When well packed they will support the bricks and
backer board, E & E recommend covering that surface with a heavy duty grade of Aluminum foil shinny side up to further help refract the heat back into the
Combustion core !

For the Good of the Crafts Big AL

Thanks, Big AL. I did this yesterday thanks to yours and others' input: aluminum flashing vapor barrier on top of the poured/reinforced concrete slab, with a layer of cored brick on top of the flashing, with a layer of cement board on top of that with a layer of perlite/clay-slip on top of that contained by a crib of brick. I tamped the perlite/clay-slip mix carefully and as much as I could with a cinder block.

I am dubious that this is solid enough to hold the combustion unit... I'm planning on building the unit on top of this vermiculite layer today... Here goes nothing I would love to hear others who've built their core right onto the tamped perlite/clay-slip mix and how it turned out. FINGERS CROSSED!!!

Okay, here's where I'm at. I'm nervous to build on top of this. Though I've had several competent and large builder friends come and tamp the perlite we are worried it will shift after mortaring and building the riser (though none of us have ever built an RMH or on tamped perlite). Thoughts? Here are photos of the progress:

https://www.flickr.com/photos/remyolson/sets/72157648483903247/

Question: this insulated base w/perlite means the feed tube ash pit will be open to the perlite?? I could build one more layer of house brick up to make this not so but that would mean my exit flue/manifold will be *even higher*... I'd rather not.

You'll also see that we didn't get the fire hot enough w/the stovepipe in the barrel to burn the paint off the papier-mached barrel. try again? Or can I just burn it off when I run the RMH once it's cobbed in?

Finally, the current configuration of firebricks represents my frustration with the jigsaw of trying to get my burn tunnel to 6" tall and 8" wide... My bricks are 2.5"x4.4"x8.9" -- old Savage brand fire brick from a demolition of a ca. 1950 fireplace. I will start anew today with a better attitude. I'm planning on using pottery clay to mortar because I have some (~5 gal bucket).

I just realized I used horticulture grade perlite in my base insulation below the burn tunnel.... Do I need to redo it you think? I called 4 different construction/masonry supply places and none carry construction/industrial grade perlite or rock wool. What do I do!?!?

I looked up this topic and found varying opinions, but the best actual information was that horticultural grade has varying particle sizes, which one commenter called good because it allows for tighter packing of particles and a stronger mix. I have also read somewhere (don't recall the place) that one variety, possibly construction grade, is treated to be water resistant and thus will not bind with clay as well.
I think you will be fine and certainly don't need to replace what you have, even if it's not the best possible variety after all. Do you have any left out in the open? Test it with the same mix you used, and dry it thoroughly. How strong is it?

Glenn Herbert wrote:I looked up this topic and found varying opinions, but the best actual information was that horticultural grade has varying particle sizes, which one commenter called good because it allows for tighter packing of particles and a stronger mix. I have also read somewhere (don't recall the place) that one variety, possibly construction grade, is treated to be water resistant and thus will not bind with clay as well.
I think you will be fine and certainly don't need to replace what you have, even if it's not the best possible variety after all. Do you have any left out in the open? Test it with the same mix you used, and dry it thoroughly. How strong is it?

Thanks for this. I can't find construction grade anywhere. I've made probably 50 phone calls. I'd have to order from some place in TN and not on my timeline or in budget. The perlite/clay mix base is strong but slow to dry. Five days and it's not dry. I'm sure running the rmh will help. I'm going to move on and build on this base when the rains are over tomorrow.

I've been trying to find sheet metal or electric water heater scrapped for the insulation container around the riser but no dice. This is one of the few downsides to living in a city with lower income levels imo... all the scrap is snatched up quick. so I had to resort to ordering 1/8" hardware cloth online (not a single hardware store or building center had this fine mesh to contain the perlite). But I'm still not sure I'll go with that over buying some sheet metal. This raises 2 questions:

Question if I use hardware cloth around the riser to contain perlite: Won't I need to make a layer of clayslip over the mesh to contain the perlite and reduce the turbulence of heat rising against the bumpy mesh? If I use sheet metal: won't the wire around it snap after high heat exposure? The sheet metal won't form into a cylinder unless I weld it. There's more $ to pay a welder as I don't have that equipment/skill.

Finally, the perlite in the riser insulation, is it supposed to be loose and not mixed with clayslip? This seems smarter as its insulation value would be higher...?

Also here's progress on the geometry. I'm ready to mortar it tomorrow and then attach a 8" stovepipe as the riser. Any last words? https://www.flickr.com/photos/remyolson/sets/72157648483903247/

The heat riser, if you use perlite, should be the same kind of perlite-clay slip as you already made. Loose perlite will only work if you have a refractory liner like firebrick, with an outer shell of whatever noncombustible sort. If you want to use loose perlite, the clay slip on the hardware cloth would be to contain it, not really for aerodynamics.
I would not try to use any substantial metal on the inside of the heat riser, as that will burn up quickly and disintegrate. thin cheap metal that will last long enough to hold the perlite-clay while it dries and stiffens is ok. My preference would be a cardboard tube, if you can find it at the right diameter. For the outside, any piece of steel duct or stovepipe you can find with the right diameter for about 2" insulation thickness will work. If you want to end up with a 12" outside diameter, you can take two pieces of snap-together 6" stovepipe, flatten them a bit so they each make a half circle, and connect them to make a perfect 12" diameter pipe. This kind of metal on the outside of the heat riser is reported to last well, as it doesn't face the 1000 degree + heat that is inside.

If you come across sheetmetal and want to use that for the outer riser shell, I would suggest giving it curvature rather than just try to roll and hold it. Three pieces of 2x4 say 3' long can work. Fasten two of them down 6" or so apart (experiment to find the best spacing) with the metal laid over and the third 2x4 placed on top and hammered down to bend the whole sheet rather than hitting a small spot at a time... start at one edge and give it a bit of curve. Experience will guide you in refining it.