My biggest problem is probably the weight distribution of the mass. Being a post and pier foundation it limits me a bit without going through a not so inexpensive 'upbuild' of the foundation.
In the house I current have set up a dummy room that is the only part I keep heated during the winter months. Roughly 6x8 or maybe a slight bit bigger. Still pretty much at or just under 50 sq ft. I put up some 2x3's or 2x4's, don't remember which, to give me something to nail the 1/4 inch plywood to for the inner room surface covering. With a gap between the original walls and the new walls I left a couple of inches for breathing space to keep moisture build up under control. Otherwise between the plywood and original walls I put up 2 pieces of R-30 fiberglass rolled insulation all the way around the room. Added in the dummy door and the room was finished. Yeah, not to building code by a long shot but since there is hardly ever anyone in the house but me no one ever sees what I've did. Not an issue.
If I were to put in an RMH I would definitely be taking the room back down leaving me with plenty of plywood and R30 insulation to have to get rid of.
Last night I started seriously question why not use the insulation instead of cob for making the bench. In the research online I still can't get a good answer that really explains the difference between mass and insulation. I know I'm thinking the insulation may not hold the heat quite as well as the mass does and yes, no insulation is fireproof. From what I've seen online cob doesn't have the R value of insulation by a long shot so I tend to think the difference comes from mass versus insulation.
What I'm thinking right now is to do a regular RIH, rocket insulated heater. Everything is the same as with a normal RMH until you get to the exhaust. From the floor up instead of using cob use the R30 insulation and then build a regular wooden bench on top. It would significantly reduce weight and get rid of some of the insulation and plywood/2x's I currently have set up for the dummy room.
Again, what is the difference between mass and insulation and why is this a stupid idea that I have?
In general terms, mass is dense, and thus it conducts and transfers heat/cold through it, absorbing and radiating in a slow flux. Insulation on the other hand, is generally less dense, and thus has more air or space in it, and as a result, does not transfer heat/cold like mass does. Solid rock, for instance, is denser than sand (sand has air between it's aggregate individual grains). Sand will hold heat, but not as well as solid rock. Rammed earth, is much more effective of a thermal mass, than is dry loose soil. Clay is better than silt, etc because it is denser. Fibreglass insulation is not nearly as effective as an insulator if it has been compressed, because it becomes denser and is more towards the mass end of the spectrum, though compressed insulation is still much less massive/dense than sand, stone, rammed earth, clay or silt.
Mass absorbs and then releases heat while insulation acts to slow the transfer of heat.
Like John said, and as Glenn explains in RMH terms, the reason that the burn tunnel of a RMH is often recommended to be insulated, is so that the internal working chambers of the burn tunnel and riser are kept as hot as possible, creating the best draw of thermodynamic flow, and hotter combustion which makes the burn cleaner/more efficient. The efficiency is due to the maximum release of energy from your wood source as quickly as possible, with high heat and rocket draw.
In contrast to that, the mass of the bench, wrapping around the exhaust from the burn chamber barrel onward, is great for absorbing whatever heat is not thrown into the room by the barrel, and slowing the flow of energy from leaving your building, thus maximizing the amount of energy that is stored in your home, rather than released into the outside air. The heat is drawn out of the 'chimney' that is in the bed through conduction and radiation before the actual chimney vertically releases warmish 'air' out of your building. This warm air has been freed of most of it's volatile gasses due to efficient combustion.
As Glenn mentioned, super insulating under your floor would be a good use of some of the insulation. Also, if you have access to your attic/ceiling space, insulating there can be even more effective. The insulation in your walls, floor, and ceiling, slow the heat from leaving your home because there is trapped air space in the insulation matrix.
Placing more mass in your room near your burn chamber/barrel will also increase the thermal storage and moderate heat/cooling in your room.
I hope that's helpful.
I was going to insulate above the ceiling until I took a quick look outside the house and saw just how far down the toward the ceiling the roof vents were and then I knew I pretty much couldn't do anything unless stick it underneath the ceiling in the room itself, just not enough room before I start covering over the roof vents.
As for the floors, in the part of the house I always use, now and before adding the dummy walls in, the floor joists show the original part of the house was built quite a long time ago, admittedly only back in the 1950s from what I can see looking back at old deeds. The floor joists are anything and everything, nothing is 16" on center in that part of the house. Everything from 12-24 inches, very inconsistent spacing. Probably another bit of the problem with using a heavy weight mass for a heater. Hence why keep looking for something that I already have, and more importantly something that is light weight. Yeah, I know...good luck on finding that.
So the floor joists from below show that there is no insulation? In that case, cutting & fitting the insulation to fill the spaces, and fastening the plywood on to hold it up, would be the best use of those materials.
I agree with this... However, I would consider where the mass heater is going to go, and do whatever you can to reinforce the crappy construction, with the possible addition of a couple or a few more pilings and a beam spanning them to support the section in question. The cost of this would be negligible when compared to dealing with a collapse of a poorly constructed foundation.
You can also go with the minimum mass thickness, trading days-long heat storage for just overnight storage, with the benefit to you of much less weight.
Do you have an idea how deep the existing support piers go? Ideally, you would add a couple-few new piers that match the existing ones so the loading characteristics and frost resistance are the same.
A bell can have a small footprintabd expand vertically,minimizing the area that needs reinforcement.
It can also be relatively thin walled and it relies on the stratification of the hot gasses inside for heat transfer,not a winding pathway through a dense mass of cob.
An idea from something I read here or somewhere has me questioning another idea.
Put the mass on the ground under the floor and wrap the extra insulation around the mass so all the heat has to go up into the house. The heater itself would sit in the house with the exhaust going down through a hole in the floor into the crawl space. I think I have heard, I'm guessing due to the rocket naturedness of an RMH, that you can 'pipe' the heat down underneath the heater...but how far? I would only be talking 3 foot, it that for headroom beneath the floor before you hit dirt. Feasible or just more stupid thinking on my part.
Our house was built in the 50s and is an old design for that period. It was built with a brick hearth for the open fire and another for a wood fired stove in the kitchen. These were built on their own foundations separate from the rest of the house which is all wood.
You could cut through your floor and build your heater on it's own foundation. You could support any weight you like this way and it should last forever. It's a lot of work though
Taking out a section of floor to build a new floor incorporating the mass could be effective as long as the sides and bottom are well insulated. You would need quite a bit of rigid, waterproof insulation beside the fiberglass for this.