Guidance on RMH build in a light wood construction

Hey Permies! We’re going to be starting the next phase of building on this light wood construction on a cabin that is roughly 532sq ft. One bedroom, one bathroom, open kitchen/dining area and a loft.
It’s raised up 2ft off the ground on poles that are embedded about 2ft+ into the earth, the round poles are about 4in in diameter. The floor joist frame has 2ft x 2ft square spans, plenty to support the 120mm SIP panel floor we’re planning on installing. There will be a ¼” composite floor on top of that.

There’s a few buildings on the property built this way, though with a more traditional plywood + barrier + floor sandwich. The cabins are lightweight, inexpensive, and DIY.
And they are only lightly insulated/sealed and can get quite cold, though never freezing in the winters. We’ll be using light claw straw in the exterior walls of this building to insulate.

Some have conventional wood burner stoves and we don’t want to use that in this house - too inefficient and I’ve been wanting to build a RMH heater for awhile now, but this would be my first.

The climate here goes up to 90F in the summers and down to 40F in the winters, so not so challenging in terms of the cold, though it does have regular strong winds from the South West, the direction in this photo.  Elevation 1,500ft. Flat terrain, mountains to the east and ocean further to the west. It rarely is humid here, so moisture and evaporation are not a challenge. The goal is really to make this lightweight cabin a home that’s cozy and warm all year around. This in in central Chile and there's also frequent earthquakes. The lightweight construction dances and flexes well in these conditions.

I don’t intend to build the RMH or the bedroom wall over the SIP floor/compositing flooring, we would create a different structure over the joists to support the higher mass density of the RMH.

I’ve been reading the forums, read two of the books (RMS Superefficient and RMS Building’s Guide), and checking out videos and I see that some have built onto wood floors with some reinforcement below or in between the joists. I’m thinking something simple and maybe smaller than what’s common since the temperatures don’t get so cold in this climate. My biggest uncertainties about building a RMS in this building are with are weight, isolation and positioning

My first preference is to build the RMH below the stairs to the loft since this space is not particularly useful for anything else and we could use the lower half of the bedroom wall as thermal mass if we build it out of cobb. We could even use the cobb bench as the lower landing for the start of the stairs.
We can frame the rest of the wall over the cobb mass. If it’s at all possible to keep the weight under 1 ton and still have enough mass to have safe temperatures on the outer surface of the cobb to the touch. The maximum space I think I can work with in this location of the building is 9.5ft along the length of the bedroom wall and 3ft along the exterior wall until it comes close up to the exterior door. The stair landing would be built over that corner near the exterior door. The exhaust pipe either needs to go up through the roof, or if I can squeeze in a third row of exhaust, it could go out the side of the building. The eves extend 2ft out from the wall. With only two rows of exhaust, that only leaves about 18ft of pipe.
My concern is this will be too little of mass to absorb the radiant heat before it exits the mass.

None of the RMH in these images are the correct scale and detail yet - at this stage I’m mostly just playing with the overall space consumption. Once I decide where to build I will draw up a scale model with the correct proportions of burn chamber, riser, insulation, etc.

What do you think of that idea? Too small of space for the needed mass? Too risky with the stairs above the RMH?
None of this except the frame/roof has been built yet so we can modify the design and materials of the stairs to fit over the RMH in the safest way available.

If I were more experienced I think I would also go for bringing outside air intake through the floor to counter the negative pressure.
For this build it seems it has enough challenges that I’m thinking a conservative 6” design, J bend, Cobb mass, with an exhaust pipe going straight up through the roof. Unfortunately I don’t see a way to put the exhaust pipe up through the gap in the roof ridge, since the bedroom door is right in the center.

Other ideas I’ve had are extending the mass through the bedroom along the wall to the end of the house and exhausting out there. This would leave a narrow, low bench all along the wall and not interfere too much. I might even be able to get away with very little reinforcement along the single pipe bench.

Or probably the most reasonable option is just to move the RMH to the living room and occupy that space in a way that I don’t prefer.

Or forget the J Bend + Cobb Bench and build a shorty core with LOTS of reinforcement under the floor in that location?

Or a CottageRocket? https://permies.com/t/150536/Uncle-Mud-CottageRocket-Paper


Other ideas?

Also, if it seems totally silly to put a RMH in a house that doesn’t even reach freezing temperatures or if it looks like I’m just using this as an opportunity to build a RMH when it doesn’t belong here, please tell me:)

Thank you for your time and energy, I’m so glad this community is here.

Hi Mathew;
40F with wind is plenty cold enough to have an RMH. Add some rain/ sleet, and you will really appreciate it.
I suggest you consider not building a bench with pipes.
Most new builds utilize a brick bell (stratification chamber).
A much better use of space than a long immovable bench.

Shorty Core is a superb choice; being "short," her bell could fit under your stairs.
Construction is more technical, but still well within most folks' capabilities.
A 6" Shorty utilizes a bell apx 4' x 5', easy to add a few extra supports underneath, weight with bricks should be apx #2000- #2500

An 8" J-Tube is very easy to build, though it requires headroom for the tall riser, and the space under the stairs would not be ideal.

Also, there is a big difference between using a barrel versus using a brick bell.
The barrel will rapidly throw heat in the room, and can quickly become too hot.
A brick bell takes some time to heat the bricks before they can heat your room, but once heated they will hold and share that heat for many hours

I fully agree with using a bell instead of a piped mass. I do think that with your climate and likely intermittent or light heating requirements, a J-tube would be quite practical, and easy for a novice to build. With the core and riser in the location you show, you can build a single-skin brick bell to clear the riser and have enough space to the stairs above to be safe. (I would not feel safe with a barrel under the stairs at all.)

I would consider a narrow tall bell (just big enough for good airflow around the riser) with a bench-height extension to the left. This would all need to be sized for the right internal surface area. Batchrocket.eu gives the complete information on this; an 8" J-tube has been found to be similar to a 6" batch box for bell sizing purposes. If you can exit nearly straight up, I believe a 6" chimney will serve you well. An 8" chimney would be bigger than you need, and you would have lazy exhaust and possible condensation issues. I would not be concerned about oversizing a J-tube, as you can burn just one load a day if your heat needs are slight, and not overheat the house.

The stair location looks ideal for evenly heating your house.

For structural support, I would suggest adding a couple of posts between the existing ones under the bell, to equalize the loading. I also like your idea of making the mass exposed on the bedroom side.