Natural insulation for stick frame walls - sawdust, straw, dustcrete?

Ill be using 2 x 6 wall studs 16" on center, exterior wall will be 1/2" plywood, interior wall will be either plywood, wood tongue and groove planks, or just plain planks. No vapor barrier or moisture barrier. Will be in the Tennessee (Sub-Tropical, but still has winter weather with below freezing). There wont be any wiring in the walls (its an offgrid cabin) and wont be any AC.

I want to use a natural and cheap insulation material, I do not want to use modern insulation (fiberglass). Id like to use sawdust (mixed with lime or borax "anti mold/insect") which I could pack in to the wall cavities as I build up the wall. This will get tricky when I get to the "fire blocks" in the stud framing. If I am using plywood for the interior wall, I would nail in the plywood, pour in the sawdust mix, pack it down until its level, and so on. I will be packing it down otherwise it will eventually settle leaving air gaps. However, sawdust can cause headaches later on if I wanted to get in the wall or inspect or modify, etc, especially if the wall is a 4 x 8' plywood, taking that piece off the wall will make a giant mountain of sawdust inside your house. I considered using some sort of cardboard bag like a leaf bag and fill that up with sawdust, tape the top, then pack it in to the wall cavities, but I am unsure on the practicality of this - not to mention insulating the ceiling rafters (45 degree angle).

Another consideration is tightly packing straw in the wall cavities, this has the benefit of not pouring out like sand as sawdust would. I powder the straw with lime/borax, build a portion of the wall, then pack the straw in to the cavity, and so fourth.

A last consideration is some sort of clay/straw "wattle & daub" type mix which will harden or some sort of "dustctrete"  which is sawdust mixed with concrete. It will have to be lime mortar and not portland cement, portland does not breathe, retains moisture and will rot wood its in contact with (the wall studs).

Any suggestions on a good natural insulation for walls? Im currently leaning towards straw. Other suggestions like wool will be too costly and prohibitive.

Consider light straw (slipstraw) packed into the voids. You can use plywood forms or jacks and tamp it in, let it dry for a day or two and go up a layer. Once the clay slip dries it's very stable and immune to settling, insect and mold proof, and not a hassle to drill into.

You can use shredded paper much the same ways as straw. Some people call it papercrete.

Excellent ideas! I like light straw idea a lot. Im hoping the property I decide to get has clay on it, or I guess clay is always present if you dig deep enough, right?

Now for the rafter insulation not sure how I would do that, more of a brainstorm in progress, with it hanging from the rafters. Ill have to use plywood or stick supports between the rafters, or I could just use plain straw and no clay for this portion.

I'm in the process of making some light straw SIPs (Structural Insulated Panels). A local earth builder did a workshop at our place a few years back and we made a doghouse with this method. Just make some frames to match your rafter or truss spacing and short enough that you can easily maneuver them into position (120 cm max, I would think). The thickness will be a function of how much insulating quality you want. On the canine casa we used 30x60mm macrocarpa and batten screws. One thing I added after the fact was some willow rods to provide a reinforcing key and keep the top bit of fill from pulling away and flopping around.

The cool thing about light straw SIPs is that you make them in batches and can dry them outdoors in sun and full air circulation, then carry them into the building and screw them into place. Then it's time to plaster...I put lime on the doghouse and it's holding up nicely. The dog is unimpressed, so if anyone in the lower North Island is interested, it needs a home.

Would you consider Earth wool, made from recycled poly bottles?

What about code and fire resistant requirements for insulation? Modern building methods have made some intelligent upgrades to fire suppression/resistance.

I have enjoyed working with mycelium insulation. It is more of a process, but it stacks functions and yields stunning results when properly executed.

David Manchester wrote:What about code and fire resistant requirements for insulation? Modern building methods have made some intelligent upgrades to fire suppression/resistance.

Actually so called modern building proponents in USA try to evade the truth that these buildings are all built out of flammable materials and toxic - so creating toxic fumes. They want to focus on some magical active/complex resistance where passive solutions are known for at least 100000 years.
Putting light straw panels between the flammable studs and sheathing is actually the best what could be done in such a case. The tight fit panels will block oxygen availability, are not flammable,  and will add insulation and some thermal mass to the existing building. They will also improve acoustics, lower the volume of cavities for the vermin and are also natural. And dirt cheap if made by the owner. Genius!

For someone, that is looking for something natural:

https://permies.com/t/mycelium-insulation

I have also read that this insulation is mold resistant, which is another plus.