insulation

quick question for you. When constructing a RMH am I right in thinking that I should have a complete layer of insulation eg perlite or the equivalent under BOTH J tube burn tunnel etc AND all pipes/cob bench structure to prevent heat loss into concrete floor? Also, for cob mixture can you give me the best recipe.
Thanks in advance

Marcus.

Hi Marcus;  Welcome to the wonderful world of rocket science!
Yes, you are correct.
Your J tube should have a perlite clay base to keep the heat in the burn tunnel. 2.5" minimum- 4" is better.
Under your horizontal pipes need insulating as well but there you use apx. 4" of  cob with bunches of cut straw mixed in.  Hay will work, but not nearly as good, straw is hollow. It insulates the heat from your slab and allows it to rise around your pipes.

COB)  When I built my very first rmh, 7 or so years ago I was very concerned with a proper cob mix. I can tell you now, don't stress about it. Only the final few inches matter if your building an exposed cob bench. If you are building a brick surround than the cob can be as dirty, rocky as it wants, it will not matter ... its only filling air gaps.

Here is the common recipe,  3 part sand to 1 part clay. This is assuming your clay is "good" , "pure", "clean"... straight clay will crack when it dries, sand as a heat sink helps stabilize it.  Dirt and silt can be there ,small rocks are the least desirable.  Ocean sand is not good its to fine.  River / creek sand will work fine, bagged builders sand is a store option.
You will want a #50 sack of fireclay to mix with sand when you are assembling your firebrick core.  No rocks allowed/wanted when using clay slip to build your core.

Question)  Are you wanting an all cob bench? Or were you thinking of surrounding your mass with ? brick , rocks, wood, metal , sheetrock?
If you surround your cob with, lets say clay brick. You have added extra mass and by containing your cob its mixture then becomes much more forgiving.
After you lay your (hvac) horizontal pipes on top of the "insulated straw cob". You carefully encase your pipes with cob. You are creating a clay tunnel, for after your thin wall pipes rot away over time.
Once your pipes are covered, now you want rock lots of rock, the larger the better.  At this point the rocks, not the cob are your real  heat holders.  Your cob (or mud) simply is used to fill any air gaps around the rocks... we call it cob lasagna, layer of rock layer of cob...till the top.

Hope I answered your questions Marcus, rather than bombard you with information...
Read things over! We have a happy crew of rocket scientists eagerly awaiting your questions.

So, come on back anytime you hear?  We will leave the light on for you!

Hey Thomas,
Thanks so much for your tips and instructions. Here in the French Pyrenees we are under lockdown because of the virus but as soon as we are released RMH is number 1 on my to do list so just wanted to make sure I have a good grasp of what's required. Obviously materials differ over here so that can be a challenge but thanks to you guys I do feel a wee bit more confident so again a big thank you and yes I probably will take up your kind offer and be back demanding answers to perplexing problems!

Until then, Be healthy and safe.

Best Marcus.

The composition of your local clay makes a big difference as to what you need to add. My clay (direct glacial deposit after grinding up shale and mudstone bedrock) is somewhere around 20% clay, 30% silt, 30% sand and 20% gravel and rocks. It is a perfect mix as is for mass cob, though if there is any fitting or surfacing to do it needs to have the larger stuff removed.

Hi rocket enthusiasts,
I am prototyping my first jtube rmh in northwestern Italy. I bought Ianto Evans book a long time ago but also the Eisners's recently and I am using the latter as the bible.
I would like to receive advice on the insulation of a 3 feet high brick wall, against which I am building the heated bench
Would a 4 inches layer of clay stabilised perlite be a good choice ?
I have access to powdery perlite so I was thinking of mixing it with clay and stacking it along the wall as cob, like a dirt wall.
Would that sustain itself or should I rather pour a formwork or layering it as an "intonaco" ?
Thank you
Carlo

Hi Carlo. You want to insulate the bench from a wall? Is it an outside wall?

What you want in the insulation material is trapped air. So I don’t know if powdery perlite will work.
I’ll use 5 inch of expanded clay against the neighbors wall as it is the easiest to come by here in Spain.

Carlo Guerra wrote:Hi rocket enthusiasts,
I am prototyping my first jtube rmh in northwestern Italy. I bought Ianto Evans book a long time ago but also the Eisners's recently and I am using the latter as the bible.
I would like to receive advice on the insulation of a 3 feet high brick wall, against which I am building the heated bench
Would a 4 inches layer of clay stabilised perlite be a good choice ?
I have access to powdery perlite so I was thinking of mixing it with clay and stacking it along the wall as cob, like a dirt wall.
Would that sustain itself or should I rather pour a formwork or layering it as an "intonaco" ?
Thank you


Carlo

Ciao, so it's true that there are other rocket builders in the north west of Italy, I live in Aosta... Anyway to answer the question, in my last build I isolated the bench from the outside wall with a layer of expanded clay and a layer of perforated bricks, it seems to work well


Otherwise, if it is an internal wall, I would not insulate it, I would use the wall as additional thermal mass.

Thank you Cedric and Benjamin,
it is an outside wall, yes.
ok, I will try to block the heat transfer with what I have and I am going to tell you the result.
I have another question which is not ralated to insolation but to the dimensions of the burn channel. If it is not in the right thread please help me to find the right one.
Question:
If my bricks are enabling me to have a 19 cm wide and 60 cm long tunnel (200 cm system CSA of 330 cm square, why am I stuck to an internal height (I mean between the floor and the "bridge") of only 17 cm instead of  18 cm? What can I do to reach the 18 cm of the book ?
I thought of carving an arch in the four bricks to make up for the missing flow area but I might have made some mistake.
I have not put clay slip on the bricks but it seems odd that i would gain a cm thickness with it
Thank you
Carlo

To get a desired burn tunnel height, you may be able to combine different orientations of bricks. What are the exact dimensions of the firebricks you have available? Is there more than one size available?

As the Wisners show in their book, some US J-tube sizes are made with a layer of firebricks on their sides (2 1/2" high) plus a layer on edge (4 1/2" high) to get a 7" total. If your bricks are not exactly half as thick as they are wide, you have options.

Thank you Glenn.
in the end I have thickened the clay slip with sand in the first  brick courses, so the the measurement came right. I am not sure of the consequences, though.
Thank you all for the support
Carlo

Fireclay mortar thickened with sand should resist all the heat you give it (getting stronger the more you heat it), and be reasonably resistant to abrasion if the joints are not wider than ordinary masonry joints. Just don't depend on it to act as "glue", because it will not stick hard to the bricks. You need the firebricks braced from outside so logs bumping bricks will not shift them.