I've noticed that in Oehler's books and videos, and in Paul's wofati's that there are typically woodstoves or rocket mass heaters. I'm wondering if anyone has tried it or if it would be helpful to add some below-ground thermal mass heating to eliminate the need for any other heat. I'm thinking a greenhouse either on one side of the wofati or on the uphill patio with pipes taking hot summer air below ground under the house and storing the heat into one giant thermal mass.
That is essentially doing a more intense version of what those structures already do. I expect you could increase the mass temperature some with active systems (unless the greenhouse could be lower than the living floor, you wouldn't get passive heat cycling into the mass), but I wonder about the payback for the investment. A greenhouse may pay for itself in cold season growing as well as summer heat storage, though. (It would be inherently counter to one of the "official" wofati goals of being invisible from the air, if that means anything to you.)
Any structure that seeks to stay warm in winter, no matter the mass around it, will have heat loss through windows and doors proportional to the outside temperature. The interior temperature will be the weighted average of thermal storage radiation gain and exterior radiation loss, plus occupancy heat gains. The mass surfaces can only transfer heat at a certain rate, proportional to the temperature difference between the mass and the space. In very cold weather, the loss rate may be more than the gain rate can be without the interior temperature dropping to uncomfortable levels. If the mass is warm enough to offer comfort in extreme cold, it may actually be too warm for comfort in milder weather. I think that dealing with the coldest weather with a supplemental heat source may be more cost-efficient than trying to make the envelope itself able to cope with the worst case.
If you haven't already, you may want to look into PAHS, or Passive Annual Heat Storage. By insulating underground, they are able to retain heat captured in the summer months and raise the average temperature of the surrounding earth (thermal mass). I am thinking of even taking it a step further and using solar concentrators to heat water in the summer and circulate it into the mass to boost it further. You could conversely also cool it this way if need be. PAHS and geothermal are two of the most efficient climate control systems out there because they both use thermal mass as their "battery".
I have done as you have suggested with excellent results. Granted I do not have a rocket stove as a heater, I have a traditional boiler with radiant floor heat but added 400 tons of rock for thermal mass under my home. People are floored (pun intended) for how little I heat my home.
What you are proposing is done all the time in traditional radiant floor heating systems. It used to be that the concrete slab was insulated from the earth by insulation 100% across the base, but this is no longer done. Just the first 4 feet or so is insulated, leaving the middle of the slab to glean some geothermal heat. The outside of a slab is where you lose the majority of your heat and thus why that is insulated.
Now, do not be put off by the fact that I use a traditional boiler and concrete instead of a rocket stove and earth floor. A btu is a btu is a btu no matter how it is derived. Solar heat, rocket stove, compost heat...all are viable candidates for what you propose.
He lost me with the foams and plastics, I fail to see how they fit into "permaculture" or "natural" building techniques. Why these respected pro's fail to address or hide the well known issues with them I don't get. Probably the same reasons he skipped right over the cost of the building and systems pay back periods. There are better choices.
If you are using slab insulation as structural support not a good idea for it not to be continuous, there are MANY issues with the foams (EPS, XPS, Poly's) using the concrete we use today and rebar, that should be under suspended slabs, ants are eating away at. The footing or "wings' is where the heat loss are greatest but that depends on a lot of factors.
To answer your question properly if you noticed the computer models in the video we can hope are CAD and simulations to get the zoning corrected, in most cases it is not that simple as throwing some mass down and hoping for the best. One can do that and take chances they guessed right I guess, but air is not the best performer for the installation cost these days, nor is solar thermal(water tubes), and how effective they transfer to mass very complex.
You insulate or build at air sealed continuous mass and insulation throughout the building, realize what zones are getting passive solar gains or losses, then design active zoned HVAC according to it. If you did it right it should only need less than 5000 btus/sf annually less with a phase change material. The home has no hot/cold spots and has a high sensible IAQ. After all what good does it do it you have low utility bills but live in a sick home that causes cancer, or allergies from mold and bacteria growth? I think we are beginning to start to understand what toxins are emiting into our buildings from open loop mass/soils, like we did our water back in the day with new concerns about IAQ.
Thanks for the replies everyone. I appreciate the experience of anyone who has done alternative heating methods, even if it isn't "Ultimate Permaculture". It is important for us to discuss and try to conceptualize what that Ultimate Permaculture would look like, but then people have to actually go out and test a bunch of stuff and document and share results before we can make real progress.
I definitely wouldn't want to do the air circulation method now that I've read your replies and thought about it... who knows what kind of gunk is building up deep underground in those pipes and then being channeled into your house everyday? As for water radiant heat I'm sure there is a lot of embodied energy in establishing that type of system, but I'd be interested in how long the payback period is and how long these systems typically last.
Thanks for the links Terry, I'll check those out when I have some time!