Glenn Herbert

Ernie and Erica's book is still the best reference for J-tubes and what surrounds them, but the piped mass concept has been superseded by the bell concept.

You can make a bench-style bell in about the same footprint of a piped mass bench. We can give you specifics if you have your core design solidified, and construction materials basically figured out.

2" thick white spruce may well be able to carry that load at 4'-2" span, but deflection is the killer here. You really don't want the boards flexing against each other as you walk on them, not to mention the difficulty it could make in sealing the floor against drafts. If you don't add the center joist, I would plan to dowel the boards together so they can't flex differently.

Internal surface area, yes. Only walls and roof above the height of the exit flue are counted in this.

Here is a good description of an adjustable plunger tube (they can be non-adjustable also, where the adjustment feature is not needed). Putting the sliding part inside the fixed part would allow the moving joint to be inside the bell and only a fixed pipe going through the bell roof. It may be tricky for you to do this, unless you can put a cleanout/access point in the base of the bell to reach the tube for adjustment. You should have a cleanout somewhere in the bottom side of the bell to pull fly ash buildup every few years.

I would move the feed opening farther from the wall, as that might in extreme circumstances have flames coming back out the top. It should not happen with a well built RMH, but better safe than sorry. You could probably twist the core so there is enough space behind the feed tube for exhaust gases to freely reach the exit. 4" clear would probably work fine in your case. Your core's floor would only be 16" below cabin floor level, and even accounting for 4-5" of burn tunnel floor thickness there would be around a foot of clearance below that floor. You can build piers to support the core and allow free airflow beneath it as well as some around the sides.

A 4" thick cob jacket behind the barrel (say a half cylinder an inch clear of the barrel and 8" taller) would protect the wall whether the LSC is technically "non-combustible" or not, and would allow easy barrel removal. A barrel with a lid can be placed with the solid floor largely cut out and the lid on top, so inspection is a very simple matter. Then the barrel does not need to be removable.

How thick are you planning to make the bell walls? I would expect your 7" burn tunnel floor to be about 24" long, plus a few all around for brick thickness. Maybe 30-32" long overall. How wide do you expect the bell interior to be? 3'-6" outside with 4" walls would leave 34" - workable if the core is square with the bell and pushed against one side. Or you could turn the core about 45 degrees with the feed out into the room (4" clear of the bell wall) and the exhaust behind it. With a good cob jacket behind the barrel and some airspace to the wall, you should be safe and convenient. You would obviously want the exhaust to rise straight from the bell interior for the adjustable plunger tube.

If you need to shift the exhaust at the roof to clear a rafter, I would suggest putting one or two lengths of stovepipe at a slight angle rather than a pair of 45 degree fittings. It's a good thing RMHs don't leave creosote, as I would not want to get on that roof peak in winter to sweep it

About the spacing of the floor joists, consider that the omitted central joist would only bear half the load of the side joists, so would not be such a problem for the tie beam to carry. I think the best bet here is to have a header which ties into the two other joists and also sits on the tie beam, to distribute the load over several elements. This would give no point load at the center of the tie beam, which is typically the most stressful position.

That looks like a good plan to me. The barrel also counts as bell ISA, so say 20 square feet for the barrel and 20 sf for the top of the bell, plus at least a foot down on all sides gives around 60 sf. An 8" J-tube is approximately equivalent to a 6" batch box in rate of heat output, wanting around 55-60 sf ISA, so a 7" J-tube could be served by a slightly smaller bell than you propose. I would keep the design, and have an adjustable plunger tube so the sunken bell active volume could be adjusted with experience to give good draft and good heat extraction.

Such a small room could be quickly overheated by a full exposed barrel, so I would allow for enclosing part of the barrel in cob. If you don't have a removable lid for the barrel, you can configure it so you can remove the whole barrel for inspection without damaging the cob.

I would move the feed/barrel unit a bit farther from the wall and tuck the chimney flue behind the barrel. Aside from greater convenience for feeding, the barrel needs woodstove clearances to combustibles.

True, though the buildings that have diagonal subflooring are generally on masonry foundations where racking is not likely to be an issue.

I was puzzling over the rationale for diagonal subflooring. Racking prevention is conceivable for upper floors, but not for the floor directly on the foundation. The ability to lay finish flooring in either direction, as well as joints and ends never lining up with subfloor joints, seems like a strong impetus for the practice.

There have been experiments with compact batch box cores, notably the Double Shoebox Rocket. There have been several distinct variations over the years (DSR1, DSR2 and DSR3). Fox James who posts here has also developed compact cores.