I think it would work, if you coat the interior with poly-modified cement over your cob foundation and frame. Seems like the cob would be a good insulator, but you would have to transition the interior from the silt structure (that water will destroy) to the cement structure and coating. Not sure exactly how that would work, but if the cob frame were sturdy enough, it shouldn't be too much of a problem to do a tough and flexible concrete coating inside the tub. Probably keep it simple on the plumbing, and it should work like any built-in spa. I'm in Boise, too, so if you want to give it a shot, let me know and maybe I can help!
there is a book written by becky bee that tells how to do it. I have never made one but i own the book and have pondered making one for a long time. I have been pondering making one with a metal water trough instead of a bathtub.
OH, that is so good. Cowboy tub with class. and efficiency.
"You must be the change you want to see in the world." "First they ignore you, then they laugh at you, then they fight you, then you win." --Mahatma Gandhi
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"Family farms work when the whole family works the farm." -- Adam Klaus
That's a cool oven/tub combo! Thanks for posting the link.
I really liked the idea doing one from cob....but, by the time I decided to get off my rear and make hot water happen, winter was upon us and I was too impatient to wait for warmer days to attempt something from cob. So, I ended up buying a basic 5'x3' wood tub kit from Snorkel which came with the stove, tub, and fence. I redid the fence, and built the benches (3 heights), pipe and stove lid heat protectors, steps, cover, and lids.
I set it outside our bedroom under a patio roof that extends over my daughters 'earth bag/wood chip' playhouse. The tub is against the playhouse. To protect the earthen plaster on the playhouse from splashes, I installed a wainscot from a salvaged redwood fence. I used the same fencing as a privacy wall on one edge of the patio. I left the other side open which looks into our forested yard.
Been heating it with left over firewood I had from our house stove, tree trimmings from the yard, and lumber scraps. I'm really happy with it.