Why would you say that cob is impractical for your landscape? Wattle and daub was used in the UK historically, wasn't it? Isn't it at least as damp in some parts there as it is in Oregon?
I
should think that if you have eaves of sufficient overhang, such that no precipitation can directly contact the cob, you shouldn't have issues that way. If methods of moisture-sealing natural plasters were used on the exterior, I anticipate no issues, providing adequate air exchange within the structure.
Rammed earth is another option. I think I prefer it to the similar Compressed Earth Block because you can use equipment off the shelf that can even be rented out of big box stores to get the work done. All that is required are the raw materials in the right ratios, forms exactly like those used for
concrete (or insulated concrete forms that you leave in place to insulate your thermal mass), and a tamper. A cement mixer for mixing the sand, clay, and mineral soil would accelerate matters.
Earthbag construction is also an option. But the observation about what materials you have available is pretty pertinent. What suitable material do you have in sufficient quantities that can be had cheaply? If it's pallets, think about that. If your neighbour has a glut of square
straw bales he can't get rid of, maybe that's what you should look at (I know, moisture issues. Good
boots and a good hat, though. I wonder if anyone has done sections of straw bale rows sealed in earthbags for moisture protection, then cob or natural plasters?).
I would see what alternative building is happening in the area. See what is working, and pay attention to what has failed or been abandoned and why.
Also, think about raw materials processing. If I was building a tiny house out of lumber I engineer from pallets and OSB and/or plywood, I would definitely look at using a Shou Sugi Ban method of
wood charring for every applicable piece.
One project I have in the planning stages is a capped pipe-based temperature-controlled retort (like a giant, fire and woodgas-powered pipe bomb, but with pressure valves and a thermostat) for the making of
biochar, but designed such that I could kiln-dry wood, or turn whole logs (relatively small ones, or branches, or dimensional lumber) into biochar, with multiple pressure valves blowing the woodgas back into the fire fueling the process without destroying the material. I want to find out to what extent I can pyrolise wood and have it retain structural integrity.
I would hope that I could at least take wooden siding with pre-drilled holes, pyrolise it sufficient to preserve it, and use it as the external building envelope, along with wooden shingles pyrolised the same way.
One last thing: do you intend for your tiny home to be mobile? If so, you've immediately limited your construction materials and complicated your whole project. If it doesn't need to be mobile, or if it at least can be built on heavy skids, designed to be moved with a flatbed like those giant dumpsters, you will have more wiggle room for random materials use and creativity.
A skidded or not-at-all mobile building will make it easier to use a heavier heating solution like an
RMH, as well as any heavier or more delicate or brittle finishes.
Good luck. You have many tiny options. Don't restrict yourself unnecessarily, and question everything. Keep us posted.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein