Glenn Herbert

Here is the Popular Mechanics article: https://www.popularmechanics.com/science/archaeology/a70277820/architectural-marvel-worlds-driest-cities/

Pavers would just be a structural base, same with laid-up bricks. Cob makes beautiful organic forms, but a cob finish on a bench would be constantly shedding dust and getting pants dirty. You need a shield or sealer, like lime plaster or tiles or an oil that strengthens cob surfaces. Bricks can be made organic as a seat, stone slabs not so much.

I wouldn't make the roof from just cob. You want solid slabs of something to cover with several inches of cob. It could be flat stones, concrete pavers, or anything strong and noncombustible.

The rate at which you can build up cob depends on its composition and wetness. More straw makes it stiffer, as does less water. More straw also makes it more insulating and less massive, so there is a tradeoff. I have found that a mix stiff enough to stand but not so stiff as to have a poor bond with a preceding dry course can rise 8"/20cm a day. For best structure and airtightness it is best to keep building so the previous course has not dried past solid stiffness when new cob is added.

Leaving some ash on the floor would certainly buffer and protect the bottom of the feed tube and burn tunnel, but I don't see how it would affect the top of the feed tube. I would be interested in finding a mechanism that would explain your experience. I only clean the ash from my J-tube every week or two and find it helpful, though I can't speak for the top as I have a steel insert that routes secondary air to the P-channel.

Very nice! Simple and effective technique. You just want all the mortised parts of a piece of furniture steamed at once and the tenoned pieces ready to go into the right places.

I believe the principal reason for insulating the plunger tube in a bell is to avoid wasting heat out the chimney - a bare metal tube would get as hot as the top of the bell. Depending on details of layout and design, it might be helpful in protecting the tube also.

From the look of your plan, with the chimney rising straight through the central peak of the roof, I would expect a 6" chimney to work. This assumes that you don't have weird atmospheric or environmental conditions.

An important thing to be aware of is that a J-tube system has much less power for the size than a batch box. It has been found that an 8" J-tube is roughly equivalent to a 6" batch box in rate of heat delivered, so using the bell size for a 6" batch box needs an 8" J-tube to heat it adequately.

My experience is that in a space/chimney with good natural draft, an 8" J-tube is well served with a 6" final chimney leaving the bell.

Benjamin - I would make it a priority (if you don't already know) to get the current chimney draft situation clarified, as that will affect how functional the different options are.