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Planning a Non-Mobile Tiny House Build, Passive Geo-thermal Stonework Structure Insulated On Outside

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Basic idea =

1. - Dig a big hole (okay, not so big).

2. - Lay in a natural stonework foundation laid deep below the frost-line, down to where the earth in my area maintains a stable upper 50's F temperature year around.

3. - Insulated around the foundation with high density bury-able ground contact foam, probably a double layer of 2" thick sheets.

4. - Back-fill around the foundation leaving very top surface of the foam exposed (for seamless transition to outside wall foam insulation above ground), partially fill in the inside, lay in-floor plumbing and electrical conduit, cap off inside with natural stonework floor.

5. - Continue the natural stonework walls upwards leaving openings for doors/windows and installing in-wall plumbing and electrical conduit as I work up.

6. - Continue the external foam sheeting insulation on outside securely gluing it to the outside of the stonework.

7. - Cap off with wooden beam and rafter roof once again with foam board insulation both in-between the rafters and then on top of the rafters as continuous sheets between the rafters and the roofing material.

8. - Stucco plaster the outside of the exposed foam on the walls and paint leaving an "adobe looking" weather proof exterior coating over the foam insulation.

Desired results =

1. - A nearly continuous thermal insulation barrier on the outside of the home with little to no thermal bridging that bypasses the insulation (like what happens with stick frame construction with most of the thermal loss being conducted through the wall studs bypassing the insulation).

2. - The Resulting home stays reasonably warm (always above freezing point) even in the dead of winter when its below zero outside due to the internal stonework structure passively conducting the ground warmth upwards into the home.  Or in other words, I can leave the house in the winter for weeks with no artificial heat source without fear of the plumbing freezing and in a survival situation all you need is warmer-clothes/extra-blankets and that is enough even without any other heat sources besides the passive geothermal and your own body heat.

3. - The Resulting home stays reasonably cool even in the triple digit heat of summer due to the internal stonework structure passively conducting the excess heat down into the cooler ground sink.

4. - When additional heating is required in the winter to bring the temperature up to a more comfortable level the large thermal mass of the internal stonework structure of the home stores the heat and makes it last long after the fire has burned out when you go to sleep at night.

5. - Since the interior of the home is natural stonework there is no problems (or at least considerably minimized) with exposure to man-made chemical toxins contaminating the inside of the home from construction materials.

Part of the reason I'm considering building using this kind of methodology with natural stonework is because there is one local gravel pit in my area that is not normal round gravel but instead offers various sizes of strong hard shale stone that they are mining from the bottom of a massive rock slide coming down off the side of one of the local mountains.  They crush it to specific sizes but their raw "pit run" straight off the slide without any processing are nice big chunks in the 4-to-6 inch size and I can order entire dump truck loads of the stuff for very cheap delivered right to my site so the cost of the stone is practically nothing compared to quarried stone and it will work just fine if your building with stone for structural, thermal conduction, and heat sink qualities rather then architectural beauty reasons.  I would be building up the stonework walls accordingly just using the simple method of short slip-forms and just slopping in the mortar mix (50K+ high strength, rich mix portland cement, flyash, and sand mixed to proportion on site not normal weak brick mortar mix that comes int he bags) and packing in the rocks and tamping the whole mixture of mortar and rock down with a sledge and tamping board old Roman style.  Structural stonework, not architectural beauty stonework.

Anyway, would appreciate any thoughts on the idea.

P.S. = As to legality, no minimum size restrictions either for total house size or room sizes in the area I'm considering building so no I don't need to build on a trailer to make a tiny house legal.  I've just got to watch my minimum door widths, hallway widths, door heights, ceiling heights, electrical, plumbing, etc. . . And I'm in "negotiation" right now with the local fire martial in regard to whether I can use alternating tread stairs so long as the stairs meet the codes otherwise as to total width of stairway and tread depth and riser height between right and left treads rather then between sequential treads on each side.  Looks like so long as the main sleeping area is downstairs he's going to okay it.  Don't need a building permit in my rural area but your supposed to have the fire martial either pre-approve and/or inspect for safe egress and EMS access if you want to be able to rent the place; and not just live in it yourself only as the builder if its a self build (non-self build don't have option).  Provided that works out I'll be doing a two story with 7-foot ceiling heights top and bottom (slightly more then 6'-8" local code minimum) done the reverse of most tiny houses with kitchen/living-space upstairs and sleeping space downstairs rather then the usual loft sleeping space over living space.  Being able to put the bottom floor at ground level rather then trailer deck height and staying close to minimum ceiling height in both lower and upper levels does mean though I won't be much taller then other tiny houses.
Posts: 4948
Location: Cincinnati, Ohio,Price Hill 45205
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I am no builder, but my reading indicates that your continuous foam barrier will not have breathability,a function sought out and valued in natural building.
So maybe only use the foam board below ground.
How about roof with large overhangs,stainless steel wire as ties on the outside of every course of stone,rock wool insulation affixed to the ties, steel mesh on top of that, and then the stucco.
Steve Smitherson
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I have considered using straw-bales and plaster/stucco for the above ground insulation portion.  Would have to still use at least a foot of foam at the bottom though above ground because early spring thaws in the intended build location result in a few inches of standing water on top of still frozen ground and ice at least every once in a while which could soak into the bottom of most natural and/or bat type insulation and wick upwards. (Planning on making the exterior door thresholds one step up for this reason as well so the stone foundation provides a dam against any such thaw water getting into the house under the bottom of the doors.)

The other reason I was mainly considering foam board is because I already know how to glue it to exterior walls and stucco finish over it (done it as a construction laborer years ago on many remodels of stick frame buildings where they used the method to add extra insulation) and because I'm trying to keep it still a "tiny house" sized structure and I'll already be making the walls a little thicker then normal and increasing the footprint due to going with an internal stone structure.  Start adding the thickness of natural insulation options such as straw bales to that and you start seeing noticeable increase in external footprint size for the same internal square footage.

Regardless as you state I'm pretty sure I'm stuck with foam for the below ground insulation, only natural option that I know that would work for that is a couple foot thick boundary of lava rock around the outside of the foundation with full base drainage and portland-cement/earth slury mix pored as a boundary around that to prevent the lava rock from getting "clay impregnated" over time due to ground water seepage pulling it into the lava rock and destroying its insulation value.  Similar effect could be achieved with using old tires as non-natural but recycled material as thermal barrier underground around the foundation but both of those options are a huge amount more work and in the case of the lava rock option a whole lot more money since there are no natural sources for such material in my area and it has to be trucked in from far away and costs like $30 a cubic yard.
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