Small DIY backyard sauna - collecting tips and ideas

This will be a phased build as I gather materials, many of which I hope will be recycled/upcycled. Right now what I have is a pile of gravel sitting on a rectangular arrangement of landscape fabric and cardboard in my back yard--a sight that makes me very excited.

Tentatively, I hope to finish Phase I, a ready-to-build on foundation of some sort, near the end of autumn.

Why a sauna:

...Rather than a shed or a greenhouse or something.
A freestanding sauna has more of the same challenges or benchmark requirements as a house (weather-tightness, insulation, interior comfort, fire resistance, ventilation, possibly some basic plumbing and/or electricity, etc.) but scaled down to a single room, which makes this an ideal chance to experiment and learn how to use and combine various natural and conventional building techniques while also testing some concepts in passive design, which brings me closer to my main bucket list item of building my own house one day.
(Also my wife asked me to build her a sauna.)

About the site:

I'm in central Ohio. My sauna will be positioned under a large maple tree in the SW corner of my property, which slopes gradually (about 1:30) downward toward the west. The sauna will be about 10' inset from the right angle where two neighbors' privacy fences meet, and about 75 feet from the nearest exterior door of my house.

I tried to stay between two large parallel roots of the maple tree, so this determined the size and location of the landscape fabric footprint: it is roughly 6' x 11', with one of the long sides facing south. I could probably expand the footprint past the roots but I have a feeling it would be tricky.

Because of the maple tree, the structure will be mostly in shade during summer days and mostly in sun during winter days (when it's not gray).

Wishlist:

1. I've never worked with cob before, so I'd love to do a cob stove inside the sauna, or a cob stub wall on the north side of the sauna, or both. I've never worked with cob.

2. I'm also a bit curious to try shou sugi ban (charring the wood to protect it) or something similar on the exterior, but I don't know yet for sure what my cladding material will be. (Suggestions welcome)

The last phases will be a couple very small-scale off-grid systems, more to get introduced to and learn them than because they are really needed:

3A. Close shot: a solar panel and battery that can reliably power an interior light.
3B. Long shot: a rain catchment & water filtration system (also powered by the solar panel), with the end goal of being able to twist a faucet and get drinkable water that originally fell on the roof as rain.

Known unknowns:

- How should the rest of my foundation go?
I was imagining cinderblocks or ground-contact-treated 4x6s as a perimeter around my gravel, with my bandboard and floor joists going across those. (I do plan to dig a drainage channel around it first.) What alternatives to these materials/methods should I consider?

- I am confident I can salvage a lot of 2x lumber and sheet materials or find them cheap, which contributes to my idea to frame most of the sauna conventionally. What alternatives should I consider?

- What kinds of insulation should I consider, and where should I source it?

- I've never installed a vapor barrier or a roof before. I know how to inspect walls and roofs, and I'm used to running wires in them, but not the nitty gritty of how to construct them. Give me your knowledge!

Unknown unknowns:

I know basically nothing about saunas. It's a room you sit in, and there's something hot which makes the room hot, and you pour water over that hot thing and make the room steamy, and you sit there and it's good for you or something.

What hot thing should I use? Should it have a lot of thermal mass? How do I get all that steam out without rotting my sauna from the inside? Is there anything special about the construction of the interior (bench, walls, etc.) that I should know? Is there special wood you need to use? (Cedar I bet?) I am certain a sauna requires insulation but does it need to be special insulation because of the steam? Same question but for all the other materials too. What else am I not asking about because I don't even know about it yet?

Answers valued from anyone, but especially sought from people who've built saunas, weathertight outbuildings, and who've worked with cob and other "natural building" materials/methods.

sauna times
It has lots of help.
I would be concerned about covering the roots of any tree
Full details of fitting it out, heating , including the need to have drainage within the sauna

I built one with cedar boards milled by my grandpa,  using a small woodstove from his old greenhouse.  It held up to 4 folks.    It was great, but don't end up like my buddy Dan....he was a bit light headed and inadvertently self-administered a second degree burn on his nether regions.  Ouch.  
And to think, most people believe Free Willy is a kids movie

Here are some threads using a RMH:

https://permies.com/t/22435/Check-Rocket-Stove-Sauna

https://permies.com/t/60768/RMH-sauna-cabin-success

https://permies.com/t/88925/Seeking-input-RMH-sauna

John C Daley wrote:
I would be concerned about covering the roots of any tree

Copy that, will stay between the roots and not cover them. 11x6 is plenty of room.

John has a long history of engineering and building, I am guessing he also means don't cover any roots of a big tree, many are underground but huge. Please be careful with placement, possible to build farther away from tree? And have no worries about roots whatsoever

Having an outdoor sauna is becoming a very popular idea.
Why put the sauna under a tree?  Is that the only available spot?

If that is the only available spot the put the sauna outside the drip line.

Sounds like a fun project Ned - good luck!

Some other threads that may help or at least be interesting for you:
T Blankenship's bathroom possibly using mushroom insulation: https://permies.com/t/285619/permaculture/Redoing-bathroom-natural-building-mushrooms
Larry Fletcher's wood fired sauna built from pallets: https://permies.com/t/234875/permaculture/Building-Sauna-Pallets

Copy that, will stay between the roots and not cover them. 11x6 is plenty of room.

As Anne suggested It is best to stay out of the drip line of the tree, there are millions of roots between the larger roots.

Thanks, everyone, for the helpful responses. More than one person gave advice related to where I intend to locate this project, and specifically about tree roots. That sounds important to know!

I'm quoting Rico, but this is intended as a reply to both Rico and John, and Anne who had questions/advice about that:

Rico Loma wrote:John has a long history of engineering and building, I am guessing he also means don't cover any roots of a big tree, many are underground but huge. Please be careful with placement, possible to build farther away from tree? And have no worries about roots whatsoever

In the section of the yard where I want for various reasons to locate the structure, unfortunately I don't think I have a way to get it farther from the tree and still not be on top of buried utilities, within easements, or where the ground stays mushy after it rains.

I am not on top of any big roots that I can see. I know now there are millions of buried roots. What portion of them are under my footprint, do you suppose?

This tree's trunk is probably 3 feet in diameter and its dripline extends something like 30' in all directions, so the whole footprint of my project right now ((6*11)/(pi*30*30)) takes up like 3% of the area under the dripline of the tree, give or take. And maybe like 10% of the area extending between any of the large roots I can see, which all run about 15' radially out from the trunk ((6*11)/(pi*15*15)).

Is the concern that I will deprive too many of the roots of their water? Their air? That the load of the structure weighing on them would be damaging? Something else?

Not clear what the issue with its placement is, so I am not sure how to even start thinking about alternatives.

Thanks again, this is fascinating as well as informative!

What about using stumps and have it above the ground?
that may mean a traditional sauna construction.