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
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.
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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!
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.
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